Water resource assessment for the Roper catchment Australia’s National Science Agency A report from the CSIRO Roper River Water Resource Assessment for the National Water Grid Editors: Ian Watson, Cuan Petheram, Caroline Bruce and Chris Chilcott ISBN 978-1-4863-1905-3 (print) ISBN 978-1-4863-1906-0 (online) Citation Watson I, Petheram C, Bruce C and Chilcott C (eds) (2023) Water resource assessment for the Roper catchment. A report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Chapters should be cited in the format of the following example: Petheram C, Bruce C and Watson I (2023) Chapter 1: Preamble: The Roper River Water Resource Assessment. In: Watson I, Petheram C, Bruce C and Chilcott C (eds) (2023) Water resource assessment for the Roper catchment. A report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Copyright © Commonwealth Scientific and Industrial Research Organisation 2023. To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO. Important disclaimer CSIRO advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it. CSIRO is committed to providing web accessible content wherever possible. If you are having difficulties with accessing this document, please contact Email CSIRO Enquiries . CSIRO Roper River Water Resource Assessment acknowledgements This report was funded through the National Water Grid’s Science Program, which sits within the Australian Government’s Department of Climate Change, Energy, the Environment and Water. Aspects of the Assessment have been undertaken in conjunction with the Northern Territory Government. The Assessment was guided by two committees: i. The Assessment’s Governance Committee: CRC for Northern Australia/James Cook University; CSIRO; National Water Grid (Department of Climate Change, Energy, the Environment and Water); NT Department of Environment, Parks and Water Security; NT Department of Industry, Tourism and Trade; Office of Northern Australia; Qld Department of Agriculture and Fisheries; Qld Department of Regional Development, Manufacturing and Water ii. The Assessment’s joint Roper and Victoria River catchments Steering Committee: Amateur Fishermen’s Association of the NT; Austrade; Centrefarm; CSIRO, National Water Grid (Department of Climate Change, Energy, the Environment and Water); Northern Land Council; NT Cattlemen’s Association; NT Department of Environment, Parks Australia; Parks and Water Security; NT Department of Industry, Tourism and Trade; Regional Development Australia; NT Farmers; NT Seafood Council; Office of Northern Australia; Roper Gulf Regional Council Shire Responsibility for the Assessment’s content lies with CSIRO. The Assessment’s committees did not have an opportunity to review the Assessment results or outputs prior to its release. This report was reviewed by Kevin Devlin (Independent consultant). For further acknowledgements, see page xxii. Acknowledgement of Country CSIRO acknowledges the Traditional Owners of the lands, seas and waters of the area that we live and work on across Australia. We acknowledge their continuing connection to their culture and pay our respects to their Elders past and present. Photo Looking along the Roper River at Red Rock, Northern Territory. Source: CSIRO – Nathan Dyer 5 Opportunities for water resource development in the Roper catchment Authors: Andrew Taylor, Cuan Petheram, Justin Hughes, Anthony Knapton, Ang Yang, Steve Marvanek, Lynn Seo, Lee Rogers, Geoff Hodgson, Fred Baynes Chapter 5 examines the opportunities, risks and costs for water resource development in the catchment of the Roper River. Evaluating the possibilities for water resource and irrigated agriculture requires an understanding of the development-related infrastructure requirements, how much water it can supply and at what reliability, and the associated costs. The key components and concepts of Chapter 5 are shown in Figure 5-1. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-1 Schematic diagram of key engineering and agricultural components to be considered in the establishment of a water resource and greenfield irrigation development Numbers in blue refer to sections in this report. 5.1 Summary This chapter provides information on a variety of potential options to supply water, primarily for irrigated agriculture. The methods used to generate these results involved a mixture of field surveys and desktop analysis. The potential water yields reported in this chapter are based largely on physically plausible volumes, and do not consider economic, social, environmental, legislative or regulatory factors, which will inevitably constrain many developments. In some instances, the water yields are combined with land suitability information from Chapter 4 so as to provide estimates of areas of land that could potentially be irrigated close to the water source or storage. These estimates are similarly based on physically plausible volumes and areas of land. 5.1.1 Key findings Water can be sourced and stored for irrigation in the Roper catchment in a variety of ways. If the water resources of the Roper catchment are developed for consumptive purposes it is likely that a number of the options listed below may have a role to play in maximising the cost effectiveness of water supply in different parts of the Roper catchment. Groundwater extraction Groundwater is already widely used in parts of the Roper catchment for a variety of purposes and offers year-round niche opportunities that are geographically distinct from surface water development opportunities. The two most productive groundwater systems in the Roper catchment are the regional scale Cambrian Limestone Aquifer (CLA) and the intermediate scale Dook Creek formation. Existing licensed groundwater extraction in the CLA totals 32 GL/year, with 26 GL/year allocated in the vicinity of Mataranka. However, actual groundwater use is less. There is currently very little development of groundwater from the DCA other than stock and domestic bores, and no water allocation plan exists. Assuming full use of existing groundwater licences in the CLA, groundwater discharge from the CLA to the Roper River near Mataranka was modelled to reduce by 8% by about the year 2070. With appropriately sited bore fields it is estimated that between 35 and 105 GL/year (~3 to 10% of recharge to the CLA) could potentially be extracted from the CLA in the vicinity of and to the south of Larrimah (i.e. groundwater extraction occurring between 60 and 160 km from Mataranka), and between 6 and 18 GL/year could be extracted from the DCA, depending upon community and government acceptance of potential impacts to groundwater dependent ecosystems (GDE) and existing groundwater users. Due to the long time lags associated with groundwater flow over long distances, the additional hypothetical extractions in the CLA result in only a further 3% reduction in modelled groundwater discharge to the Roper River near Mataranka by about the year 2070. However, the modelled reduction in groundwater levels ranges from about 12 m at the centre of the hypothetical developments to 0.5 m up to 110 km away by about 2070. Under a projected dry future climate (10% reduction in rainfall), localised groundwater recharge to the CLA near Mataranka results in a 22% reduction in modelled groundwater discharge to the Roper River at Elsey Creek by about the year 2060. This is considerably larger than the decrease in modelled groundwater discharge due to the hypothetical 105 GL/year of additional groundwater extraction from the CLA south of Larrimah. This is due to the short groundwater flow paths between the Roper River near Mataranka and the areas of localised recharge that occurs on the outcropping CLA near Mataranka, and highlights the sensitivity of groundwater storage in and discharge from the CLA near Mataranka to natural variations in climate. Major dams Indigenous customary residential and economic sites are usually concentrated along major watercourses and drainage lines. Consequently, potential instream dams are more likely to have an impact on areas of high cultural significance than are most other infrastructure developments of comparable size. This has particular significance to the Roper catchment. The physical potential for large instream in the Roper catchment is low relative to other large catchments in northern Australia. This is due to the dissected nature of the landscape along the Roper River and its major tributaries, which limits the size of contiguous areas of suitable soil — large areas of which are necessary for the efficient development of large irrigation schemes. The relatively low relief and limited areas of contiguous soil suitable for irrigated agriculture mean it would only be feasible to site potential dams on small headwater catchments. The small catchment area of these potential dam sites limits their water yield. The most cost-effective potential large instream dam in the Roper catchment could yield 89 GL in 85% of years and cost $250 million (−20% to +50%) to construct, assuming favourable geological conditions. This equates to a unit capital cost of $2800/ML. A nominal 9560 ha reticulation scheme was estimated to cost an additional $13,230/ha or $126.5 million (excluding farm development and infrastructure). Water harvesting and offstream storage Water harvesting, where water is pumped from a major river into an offstream storage such as a ‘ringtank’, is a cost-effective option of capturing and storing water from the Roper River and its major tributaries. Approximately 7% of the catchment (540,000 ha) was modelled as being likely to be suitable or possibly suitable for ringtanks. However, unlike many large catchments in northern Australia, contiguous areas of soil suitable for irrigation within 5 km of the river are more limiting than surface water along the Roper River and its major tributaries. Nonetheless it is physically possible to extract 660 GL and irrigate 40,000 ha of broadacre crops such as cotton on the clay alluvial soil during the dry season in 75% of years by pumping or diverting water from the Roper River and its major tributaries and storing it in offstream storages such as ringtanks. This results in a modelled reduction in the mean and median annual discharge from the Roper catchment to the Gulf of Carpentaria of about 11% and 15% respectively. Managed aquifer recharge An opportunity assessment indicates there are few potential opportunities for managed aquifer recharge (MAR) in the Roper catchment. The basic requirements for a MAR scheme is the presence of a suitable aquifer with sufficient storage capacity, soils with moderate to high permeability, landscapes with low to moderate slope (i.e. 10% or less) and a source of water. In the majority of those parts of the catchment where the soils, slope and hydrogeology are potentially suitable for MAR (i.e. Sturt Plateau), the rivers and streams are highly intermittent, and consequently there is no reliable source of water for MAR. In those parts of the catchment where streams or reaches of streams are perennially flowing, it is because they receive baseflow from groundwater discharge and hence the watertable is likely to be shallow (i.e. 4 m or less) and there is little storage capacity in the aquifer. Only minor areas were identified in the Roper catchment that could be used to augment recharge to the CLA hosted in the Cambrian Limestone and the DCA hosted in the Proterozoic dolostone. Approximately 480 km2 (0.5% of the catchment) and 75 km2 (0.1% of the catchment) of the Roper catchment was identified as having potential for aquifers, groundwater and landscape characteristics suitable for infiltration MAR techniques within 1 and 5 km of a major river respectively, from which water could potentially be sourced for recharge (though in the headwaters of these rivers the reliability of flow would need to be locally assessed). Gully dams and weirs Suitably sited large farm-scale gully dams are a relatively cost-effective method of supplying water. However, the more favourable sites for gully dams in the Roper catchment, which are predominantly located north of the road between Mataranka and Bulman, are situated where the soil is rocky and shallow and generally less suited to irrigated agriculture. The remaining sources of water and storage options, namely weirs and natural water bodies, are estimated to be capable of reliably supplying considerably smaller volumes of water than major instream dams. Sourcing water from natural water bodies, although the most cost-effective option, is highly contentious. Summary of investigative, capital, and operation and maintenance costs of different water supply options and potential scale of unconstrained development Table 5-1 provides a summary of indicative investigative, capital and operation and maintenance costs of different water supply options and estimates of the potential scale of unconstrained development. The development of any of these options will impact on existing uses, including ecological systems, to varying degrees, and will depend on the level of development. This is examined in Section 7.2. All of the water source options reported in Table 5-1 are considerably cheaper than the cost of desalinisation. The initial cost of constructing four large desalinisation plants (capacity of 90 to 150 GL/year) in Australia between 2010 and 2012 ranged from $17,000/ML to $27,000/ML (AWA, 2018), indexed to 2021. This does not include the cost of on- going operation (e.g. energy) and maintenance or the cost of conveying water to the demand. Table 5-1 Summary of capital costs, yields and costs per ML supply, including operation and maintenance (O&M) Costs and yields are indicative. Values are rounded. Capital costs are the cost of construction of the water storage/source infrastructure. They do not include the cost of constructing associated infrastructure for conveying water or irrigation development. Water supply options are not independent of one another, and the maximum yields and areas of irrigation cannot be added together. Equivalent annual cost assumes a 7% discount rate over the service life of the infrastructure. Total yields and areas are indicative and based on physical plausibility unconstrained by economic, social, environmental, legislative or regulatory factors, which will inevitably constrain many developments. WATER SOURCE/STORAGE GROUND- WATER† MANAGED AQUIFER RECHARGE‡ MAJOR DAM WEIR§ LARGE FARM- SCALE RINGTANK LARGE FARM- SCALE GULLY DAM NATURAL WATER BODY Cost and service life of individual representative unit Capital cost ($ million) 6.73 1.1 250 10–40 2.95 1.65 0.02 Operation and maintenance (O&M) ($ million/y)* 0.34 0.065 1.0 0.2–0.8 0.125 0.045 ~0 Assumed service life (y) 50 50 100 50 40 30 15 WATER SOURCE/STORAGE GROUND- WATER† MANAGED AQUIFER RECHARGE‡ MAJOR DAM WEIR§ LARGE FARM- SCALE RINGTANK LARGE FARM- SCALE GULLY DAM NATURAL WATER BODY Potential yield of individual representative unit at water source Yield at source (GL)†† 8 0.6 90 2–15 2.8 3 0.125–0.5 Unit cost ($/ML)‡‡ 840 1,830 2,800 2,700 1,050 550 40–160 Equivalent annual unit cost ($ million/y) per ML/y§§ 105 240 205 250 125 60 5–20 Potential yield of individual representative unit at paddock Assumed conveyance efficiency to paddock (%)††† 95 90 70 80 90 90 90 Yield at paddock (GL) 7.6 0.54 60 1.6–12 2.5 2.7 0.11–0.45 Unit cost ($/ML)‡‡ 885 2,040 4,150 3,375 1,180 610 45–180 Equivalent annual unit cost ($ million/y) per ML/y††† 110 270 310 335 140 65 6–25 Total potential yield and area (unconstrained) Total potential yield (GL/y) at source ≥85% reliability‡‡‡ 40-130 <50 320 <100 660 <100 <50 Potential area that could be irrigated at ≥85% reliability (ha)§§§ 6,000– 23,000 <5,000 30,000 <10,000 40,000 <10,000 <5,000 †Value assumes extraction Cambrian Limestone aquifer assuming average bore yield of 25 L/s irrigation 500 ha to meet mean peak evaporative demand over 3 day period. Assumes an average depth of 60 m and a drilling failure rate of 50%. ‡Based on recharge weir. §Sheet piling weir. *O&M cost is the annual cost of operating and maintaining infrastructure and includes cost of pumping groundwater assuming groundwater is 10- 20 m below ground level and the cost of pumping water into ringtank. ††Yield at dam wall (taking into consideration net evaporation from surface water storages prior to release) or at groundwater bore. Value assumes large farm-scale ringtanks do not store water past August. ‡‡Capital cost divided by the yield. §§ Equivalent annual cost of storage/bore per ML of yield of water. Includes capital cost and O&M costs. Assumes 7% discount rate. ††† Conveyance efficiency between dam wall/groundwater bore and edge of paddock (does not include field application losses). ‡‡‡Actual yield will depend upon government and community acceptance of impacts to water dependent ecosystems and existing users. Yields are not additive. Likely maximum cumulative yield at the dam wall/groundwater bore. Potential yield of major dams based on yield of dams at Waterhouse River. §§§Likely maximum area that could be irrigated (after conveyance and field application losses) in at least 85% of years. Assumes a single crop. Areas provided for each water source are not independent and hence are not additive. Actual area will depend upon government and community acceptance of impacts to water dependent ecosystems and existing users. NA = data not available 5.2 Introduction 5.2.1 Contextual information Irrigation during the dry season and other periods when soil water is insufficient for crop growth requires sourcing water from a suitable aquifer or from a surface water body. However, decisions regarding groundwater extraction, river regulation and water storage are complex, and the consequences of decisions can be inter-generational, where even relatively small inappropriate releases of water may preclude the development of other, more appropriate (and possibly larger) developments in the future. Consequently, governments and communities benefit by having a wide range of reliable information available prior to making decisions, including the manner of ways water can be sourced and stored, as this can have long-lasting benefits and facilitate an open and transparent debate. Information is presented in a manner to easily enable the comparison of the variety of options. More detailed information can be found in the companion technical reports. Section 5.5 discusses the conveyance of water from the storage and its application to the crop. Transmission and field application efficiencies and associated costs and considerations are examined. Section 5.6 explores the feasibility and likely capital costs of potential broad-scale irrigation development in the Roper catchment. All costs presented in this chapter are indexed to June 2021 and materials and labour are not representative of post-COVID conditions. Concepts The following concepts are used in sections 5.3 and 5.4. • Each of the water source and storage sections are structured around: (i) an opportunity- or reconnaissance-level assessment, and (ii) a pre-feasibility-level assessment, where: – Opportunity-level assessments involved a review of the existing literature and a high-level desktop assessment using methods and datasets that could be consistently applied across the entire Assessment area. The purpose of the opportunity-level assessment is to provide a general indication of the likely scale of opportunity and geographic location of each option. – Pre-feasibility-level assessments involved a more detailed desktop assessment of sites/geographic locations that were considered more promising. This involved a broader and more detailed analysis including the development of bespoke numerical models, site-specific cost estimates and site visits. Considerable field investigations were undertaken for the assessment of groundwater development opportunities (Section 5.3.2). • Yield is a term used to report the performance of a water source or storage. It is the amount of water that can be supplied for consumptive use at a given reliability. For dams, an increase in water yield results in a decrease in reliability. For groundwater, an increase in water yield results in an increase in the ‘zone of influence’ and can result in a decrease in reliability, particularly in local- and intermediate-scale groundwater systems. • Equivalent annual cost is the annual cost of owning, operating and maintaining an asset over its entire life. Equivalent annual cost allows a comparison of the cost effectiveness of various assets that have unequal service lives/life spans. • Levelised cost is the equivalent annual cost divided by the amount of water that can be supplied at a specified reliability. It allows a comparison of the cost effectiveness of various assets that have unequal service lives/life spans and water supply potential. Other economic concepts reported in this chapter, such as discount rates, are outlined in Chapter 6. 5.3 Groundwater and subsurface water storage opportunities 5.3.1 Introduction Groundwater, where the water-bearing formation is relatively shallow and of sufficient yield to support irrigation, is often one of the cheapest sources of water available, particularly where groundwater levels are close to the land surface (thereby reducing pumping costs). Even the cheapest forms of managed aquifer recharge (MAR), infiltration-based techniques, are usually considerably more expensive than developing a groundwater resource. Further to this, in northern Australia many unconfined aquifers, which are best suited to infiltration-based MAR, either have large areas with no ‘free’ storage capacity at the end of the wet season, or where they do have the available storage capacity, these areas are often at uneconomically viable distances (i.e. greater than 5 km) from a reliable source of water to recharge the aquifer. Therefore, MAR will inevitably only be developed following development a groundwater system, where groundwater extraction may create additional storage capacity within the aquifer to allow additional recharge and hydrogeological information is more readily available to evaluate the local potential of MAR. However, if developed, MAR can increase the quantity of water available for extraction and help mitigate impacts to the environment. Note that where water uses have a higher value than irrigation (e.g. mining, energy operations, town water supply), other more expensive but versatile forms of MAR, such as aquifer storage and recovery, can be economically viable and should be considered. The Assessment undertook a catchment-wide reconnaissance assessment and, at selected locations, a pre-feasibility assessment of: • opportunities for groundwater resource development (Section 5.3.2) • MAR opportunities (Section 5.3.3). 5.3.2 Opportunities for groundwater development Introduction Planning future groundwater resource developments and authorising licensed groundwater entitlements require value judgments of what is an acceptable impact to receptors such as environmental assets or existing users at a given location. These decisions can be complex and typically require considerable input from a wide range of stakeholders, particularly government regulators and communities. Scientific information to help inform these decisions include: (i) identifying aquifers that may be potentially suitable for future groundwater resource development; (ii) characterising their depth, spatial extent,saturated thickness, hydraulicproperties andwaterquality; (iii)conceptualising thenature of their flowsystems;(iv) estimatingaquifer water balances;and(v) providing initialestimates of potential extractable volumes and associated drawdown in groundwater level over time and distancerelative to existing water usersand groundwater-dependent ecosystems(GDEs). The changes in drawdown over time at different locations provide information on thepotentialrisks of changes in aquifer storage and thereforewater availabilitytoexisting users or theenvironment.Unless stated otherwise, thematerial presented in Section 5.3.2has been summarised from the companiontechnical report ongroundwatermodelling (Knapton et al., 2023). Opportunity-levelassessment ofgroundwater resource development opportunities in theRopercatchment The hydrogeological unitsof theRopercatchment (Figure5-2) contain a variety of local, intermediate andregional-scale aquifersthat host localisedto regional-scale groundwater flow systems. The intermediate-to regional-scalelimestoneand dolostoneaquifers are present inthesubsurface across largeareas, collectivelyoccurring beneath about 50% of the catchment. Giventheir large spatial extent, they alsounderlie and coincidefrequentlywith larger areas of soilsuitable for irrigatedagriculture (Section 4.2). Theycontain mostly low salinitywater(<1000 mg/Ltotal dissolved solids,TDS) and can yield water at a sufficient rate to support irrigation development (>10 L/second). Theseaquifersalsocontain larger volumesof groundwater instorage (gigalitresto teralitres)than local-scale aquifers andtheir storage and dischargecharacteristicsareoften less affected by short-term (yearly) variations in recharge ratescaused byinter-annual variability in rainfall. Furthermore,their larger spatial extentprovides greater opportunitiesfor groundwater resourcedevelopment away fromexisting waterusers andGDEs atthe land surface such assprings, spring-fed vegetation and surface water,which can beecologically and culturally significant. In contrast,local-scale aquifers intheRopercatchment, suchas fracturedand weatheredrock and alluvial aquifers,hostlocal-scale groundwater systems thatare highly variable in composition, salinity and yield. They also have a small andvariablespatialextent and less storage compared tothelargeraquifers, limiting groundwater resourcedevelopment to localised opportunities such as stock and domesticuse,or as a conjunctive waterresource(i.e. combined use of surface water, groundwateror rainwater). The Assessment identified fivehydrogeological units hosting aquifersthat mayhave potential forfuture groundwater resource development in theRopercatchment(see Table5-2for details): •Cambrian limestone •Proterozoic dolostone and sandstone •Cretaceous sandstone, siltstone and claystone •Proterozoic sedimentaryand igneous rocks •Cambrian basalt. 280|Water resource assessment for the Roper catchment Table 5-2 Opportunity-level estimates of the potential scale of groundwater resource development opportunities in the Roper catchment For locations of the hydrogeological units see Figure 5-2. The indicative scale of the groundwater resource is based on the magnitude of the inputs and outputs of the groundwater balance. The actual scale will depend upon government and community acceptance of potential impacts of groundwater dependent ecosystems and existing groundwater users. HYDROGEOLOGICAL UNIT LOCATION LEVEL OF KNOWLEDGE INDICATIVE SCALE OF RESOURCE (GL/y) † COMMENT Cambrian limestone South to south- western part of the catchment Medium to high 35–105 Most promising regional-scale aquifer, the CLA, typically tens of metres thick, with high bore yields (5–50 L/s) and good water quality (<1000 mg/L TDS). Has potential to support multiple large-scale (5–15 GL/y) developments. Greatest opportunities exist in the Georgina Basin Water Management Zone of the Georgina Wiso water allocation plan area as well as the Larrimah Water Management Zone in the proposed Georgina Wiso and Mataranka water allocation plan areas, respectively. Opportunities are limited where water management zones have reached or are close to full allocation, or if the nature and cumulative scale of development will potentially affect water availability to existing licensed water users, the Mataranka Springs Complex and upper Roper River and its major tributaries. Proterozoic dolostone and sandstone North- eastern part of the catchment Low to medium <20 Promising intermediate-scale aquifer, the DCA, hosted in the Proterozoic dolostone (Dook Creek Formation). The aquifer is typically tens of metres thick, with high bore yields (5–50 L/s) and good water quality (<500 mg/L TDS). Has potential to support multiple small to intermediate-scale (1–3 GL/y) developments. Greatest opportunities exist where the aquifer is unconfined west of the Central Arnhem Road between Flying Fox Creek and the Wilton River. Opportunities are limited near community water supplies for Beswick, Bulman and Weemol as well as where the aquifer is connected to Flying Fox Creek and the Mainoru and Wilton rivers, and where springs occur e.g. Top Spring, Lindsay Spring and Weemol Spring. The Proterozoic dolostone and sandstone aquifers (Yalwarra Volcanics, Knuckey Formation and Mount Birch Sandstone) on the southern side of the Roper River away from Ngukurr may offer some potential opportunities for future groundwater development but this requires further investigation. Cretaceous sandstone, siltstone and claystone Southern part of the catchment Low <5 Local-scale sandstone aquifers occurring as localised basal quartzose sandstones. Variable bore yields, often <5 L/s, and variable water quality. Only likely to offer potential for small- scale (<0.5 GL/y) localised developments (i.e. mostly suited to stock and domestic water supplies) or as a conjunctive water resource where basal sandstone units are present. Proterozoic sedimentary and igneous rocks Central part of the catchment Low <5 Local-scale fractured and weathered rock aquifers composed mostly of sandstone and siltstone with some dolerite. Variable bore yields, often <2 L/s, and variable water quality. Only likely to offer potential for small-scale (<0.25 GL/y) localised developments (i.e. mostly suited to stock and domestic water supplies) or as a conjunctive water resource in the outcropping area where fracturing and weathering is high. Cambrian basalt Small patches in the south of the catchment Low <5 Local-scale fractured and weathered rock aquifers composed mostly of basalt and breccia. Variable bore yields, often <2 L/s, and variable water quality. Only likely to offer potential for small-scale (<0.25 GL/y) localised developments (i.e. mostly suited to stock and domestic water supplies) or as a conjunctive water resource in the outcropping/subcropping areas where fracturing and weathering are high. †Actual scale will depend upon government and community acceptance of impacts to GDEs and existing water users. Figure 5-2 Hydrogeological units with potential for future groundwater resource development To show the spatial extent of key regional hydrogeological units in the subsurface, the blanket of surficial Cretaceous to Quaternary rocks and sediments has been removed. The extent of the surficial Cretaceous to Quaternary rocks and sediments is shown in the lower right inset. The right inset also shows the entire spatial extent of the Cambrian limestone and Proterozoic dolostone outside the Roper catchment. Groundwater development costs The cost of groundwater development is the cost of the infrastructure plus the cost of the hydrogeological investigations required to understand the resource and risks associated with its development. This section presents information relevant to the cost of further developing the groundwater resources of the CLA and DCA, including but not limited to the depth to water-bearing formation (control over cost of drilling) and the depth to groundwater (control over cost of pumping). Information on the spatial extent of drawdown of groundwater levels is also presented. This is relevant to the potential hydraulic impact of future development on receptors such as existing licensed water users and culturally and ecologically important GDEs. Aquifer yield information is presented in Section 2.5.2. At a local development scale, individual proponents will need to undertake sufficient localised investigations to provide confidence around aquifer properties and bore performance. This information will also form part of an on-site hydrogeological assessment required by the regulator in order to grant an authorisation to extract groundwater. Key considerations for an individual proponent include: • determining the location to drill a production bore • testing the production bore • determining the location and number of monitoring bores required • conducting a hydrogeological assessment as part of applying for an authorisation to extract groundwater. Estimates of costs associated with these local-scale investigations are summarised in Table 5-3. Table 5-3 Summary of estimated costs for a 500-ha irrigation development using groundwater Assumes mean bore yield of 25 L/s and with 32 production bores required to meet peak evaporative demands of 500 ha area. Does not include operating and maintenance costs. DRILLING, CONSTRUCTION, INSTALLATION AND TESTING OF BORES ESTIMATED COST ($) Production bore 5,120,000† Monitoring bores 660,000‡ Submersible pumps 2,720,000§ Mobilisation/demobilisation 12,000§§ Aquifer testing 240,000 Hydrogeological assessment 100,000†† †Value assumes 32 production bores drilled and constructed at a mean depth of 60 m at a cost per bore of $750/m, constructed with 200m steel casing at a cost of $82/m and 18 m stainless steel wire-wound screen at a cost of $150/m. Assumes on average two holes need to be drilled for every cased production bore. ‡Value assumes twelve PVC monitoring bores drilled and constructed at a mean depth of 60 m at a cost of $500/m, constructed with 150 mm PVC and machine-slotted 5 m screen at a cost of $50/m §Value assumes a pump that is rated to draw water at a rate of up to 60 L/second, as well as rated to draw water from depths of up to 50 mBGL. Value based on 32 pumps. §§Value assumes a mobilisation/demobilisation rate of $10/km from Darwin to Daly Waters and return (approximately 1200 km round trip). *Value assumes six 72-hour aquifer tests (48 hours pumping, 24 hours recovery) at a cost of $500 per hour and $4000 mobilisation/demobilisation. †† Indicative cost to proponent. Value assumes a small-scale development away from existing users and GDEs. Assumes the regulator has already characterised the aquifers at the intermediate/regional scale to better understand the resource potential under cumulative extraction scenarios, as well as current and future constraints to development. While the CLA is well described at the regional scale relative to many other systems across northern Australia, the DCA has had little investigation prior to the Assessment. Pre-feasibility-level assessment of groundwater resource development opportunities and risks associated with the Cambrian Limestone Aquifer The Assessment identified the Cambrian Limestone Aquifer (CLA) and Dook Creek Aquifer (DCA) to be the most promising regional- and intermediate-scale aquifers with potential for future groundwater resource development in the Roper catchment. The CLA is almost exclusively unconfined. That is, it outcrops at the surface or is within tens of metres of it and is directly recharged via outcrop areas or via overlying variably-permeable rocks (sandstone, siltstone and claystone) across its extent in the Roper catchment. Only minor parts of the aquifer are confined by the overlying siltstone in the north-west and south-east of the aquifer (confining occurs where groundwater is pressurised due to an overlying veneer of very low permeability or impermeable rocks sealing off the aquifer from overlying rock layers). The thickness of the CLA varies spatially beneath this south to south-west part of the Roper catchment and is influenced by historical weathering of the limestone in places as well as changes in the topography of the underlying volcanic rocks (see Figure 5-3). The CLA is generally about 80 to 100 m thick but increases to over 300 m thick in the Georgina Basin south of Daly Waters beyond the catchment boundary. The saturated thickness (amount of saturated rock) also varies spatially and is an important characteristic along with aquifer hydraulic properties in relation to groundwater storage and flow. In some parts of the northern Wiso Basin beneath the Roper catchment, the saturated thickness can be thin (i.e. <20 m) as exhibited from historical drilling having mixed success (i.e. dry holes or bores with little water). In the southern Daly Basin and northern Georgina Basin beneath the Roper catchment, the saturated thickness is generally much thicker (i.e. >50 m) and the success rate for installing productive groundwater bores has been higher (Figure 5-3). See Figure 2-5 in Section 2.2.3 for an overview of the spatial extent of the different geological basins in the Roper catchment. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-3 Hydrogeological cross-section through the Cambrian Limestone Aquifer (CLA) in the south to south-west of the Roper catchment See Figure 5-2 for the spatial location of the cross-section. Figure source: adapted from Tickell (2016) The CLA beneath the Roper catchment is generally flat and only dips subtly towards the southern boundary of the surface water catchment. Depth to the top of the CLA, while spatially variable, occurs at depths of <200 metres below ground level (mBGL) across its extent in the south to south-west of the catchment.The top of theaquifer is generally shallow (<50 mBGL) alongthenorth and north-easternmarginsof the aquifer in the DalyB(seeSection2.2.3, Figure 2-5for information on the spatial occurrence andextent of the geological basins). Thedepth to thetop of the CLAthen generallyincreasessubtly inasoutherly andsouth-easterlydirectionaway from thenorthern aquifer boundarybeneath thecatchment.Depthsincreaseinitially to about 50to 100mBGLin the subsurfacearoundLarrimahbefore increasingto between 100and 200mBGL towardthe south-east, south and south-westerncatchment boundary. The most abrupt increase in depth to thetop of theaquifer awayfrom thenorthern aquifer boundaryisimmediatelywest of Mataranka where theaquifer is confined by a small portion of the overlying Cambrian siltstone(see Figure5-2).Here thedepth tothe top of the aquifer is between 200and 250 mBGL (Figure5-4). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure5-4Depth to the top of the Cambrian Limestone Aquifer (CLA) Only a partial spatial extent of the CLA is shown beyond theRoper catchment boundary. Depths are in metres belowthe groundsurface.Stratigraphic data, represents a bore with stratigraphic data to obtain information about changesin geology with depth. Aquifer extentdata source: Knapton (2020) Chapter5 Opportunitiesfor water resource development in the Roper catchment|285 Changes in the depth to groundwater across the CLA, also referred to as depth to standing water level (SWL), exhibit similar spatial patterns to the depth to the top of the aquifer. For example, groundwater is shallow (i.e., >10 mBGL) along the northern margin of the aquifer around Mataranka (Figure 5-5) where groundwater discharges via: (i) diffuse seepage and localised discharge via in-river springs to the upper Roper River and its major tributaries; (ii) via localised discharge at the Mataranka Spring Complex to the Mataranka Thermal Pools; and (iii) via evapotranspiration from groundwater dependant vegetation (GDV) in and nearby Elsey National Park. Depth to groundwater then increases subtly in a somewhat radial pattern south, east and west from the northern aquifer boundary beneath the catchment. Groundwater depth beneath Larrimah is approximately 50 mBGL increasing to >100 mBGL south of Daly Waters (Figure 5-5). For this reason, GDEs associated with the CLA in the Roper catchment are largely limited to the northern margin of the aquifer around Mataranka. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-5 Depth to standing water level (SWL) of the Cambrian Limestone Aquifer (CLA) Only a partial spatial extent of the CLA is shown beyond the Roper catchment boundary. Depths are in metres below the land surface. Aquifer extent data sources: Knapton (2020) The Assessment undertook a number of groundwater investigations to refine the conceptual model of the CLA, including updated geological and groundwater flow information. This refined conceptual model was used to test a range of climate and hypothetical groundwater extraction scenarios in order to evaluate the CLA response to different scales of groundwater extraction. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-6 Conceptual hydrogeological block model of the Cambrian Limestone Aquifer and aquifers hosted in adjacent hydrogeological units Textured and coloured geological units highlight the structural controls on vertical and horizontal groundwater flow in the Cambrian Limestone Aquifer in the regional groundwater discharge zone around the Mataranka Springs Complex and the upper Roper River and its tributaries. Blue arrows highlight the spatial variability in groundwater flow directions that converge in the discharge zone at different springs and different parts of the upper Roper River and its tributaries. Figure source: schematic adapted and updated from Department of Environment and Natural Resources (2017) Impacts of extracting groundwater from the Cambrian Limestone Aquifer to groundwater dependent ecosystems and existing groundwater users The impacts of incrementally larger groundwater extractions on groundwater discharge to the Roper River near Mataranka and existing groundwater users under historical and future climates are summarised in Table 5-4 and Table 5-5. Detailed in the companion technical report on groundwater modelling (Knapton et al., 2023). Scenarios are summarised in Section 1.4.3 and detailed in Knapton et al., 2023. The potential impacts, in terms of groundwater drawdown, of three different hypothetical groundwater extraction rates (5, 10 and 15 GL/year) at seven hypothetical locations within the CLA are reported at six bores (each with a registered number – RN) installed in a range of different hydrogeological settings and proximity to existing users. The seven hypothetical locations were selected considering the location of existing groundwater licences, suitability of soil for irrigated agriculture, suitable hydrogeological properties for groundwater extraction and distance from existing infrastructure (see Knapton et al., 2023 for more detail). The location of the hypothetical groundwater extraction locations and the reporting locations is shown in Figure 5-8. The CLA being a regional-scale groundwater system, changes in climate and increases in groundwater extraction can take many hundreds of years to fully propagate through the system. Consequently, the time period over which results are reported becomes relevant. The results reported in this section involve running the model to 2070 (~50 years). This is considered a pragmatic time period over which to consider the impacts of changes in climate and groundwater extraction because: (i) it is equivalent to more than twice the length of the investment period of a typical agricultural enterprise; (ii) it is roughly equivalent to the service life of a commissioned groundwater borefield; and (iii) it is consistent with the time period over which future climate projections have been evaluated. It should also be noted that this time period is about five times the length of the current period over which NT water licences are assigned. Importantly, in reporting the results of the hypothetical groundwater development scenarios no judgement is made as to whether the impact of the modelled groundwater-level drawdown to receptors such as groundwater-dependant environmental assets or existing users are acceptable. Drawdown in groundwater levels in the CLA under scenarios A, B35 (Figure 5-8), B70 and B105 is concentric around the seven hypothetical development sites between Larrimah and Daly Waters. At the smallest cumulative hypothetical extraction rate (35 GL/year, Scenario B35) the maximum mean modelled drawdown in groundwater level after the 50-year period (~2070) is about 5 m occurring south of Larrimah in the centre of the hypothetical extraction sites. At the largest cumulative extraction rate (105 GL/year, Scenario B105), the maximum mean modelled drawdown in groundwater level after the 50-year period (~2070) is about 12 m also occurring in the centre of the hypothetical extraction sites (RN029013 – see Table 5-4). Drawdown of about 1 m in groundwater level – a value that can be considered measurable – is modelled to extend >100 km north of the centre of the hypothetical development sites to the groundwater discharge zone (RN035796 in Table 5-4), as well as south and outside of the catchment south of Daly Waters (RN005621 in Table 5-4 and Figure 5-8. The widespread propagation of drawdown arising from modest levels of groundwater extraction is due to the low storage properties of the limestone aquifer. At the centre of the hypothetical extraction sites (RN029013) the modelled groundwater drawdown under scenarios B35, B70 and B105 exceeds the groundwater drawdown under Scenario Cdry, however, at Mataranka the modelled groundwater drawdown under Scenario Cdry (117.6m) exceeds the drawdown under scenarios B35, B70 and B105 (118.4m). A dry future climate and hypothetical groundwater development (i.e. Scenario D) exacerbates the groundwater drawdown modelled under Scenario B. Table 5-4 Mean modelled groundwater levels at six locations within the Cambrian Limestone Aquifer (CLA) under scenarios A and B Locations shown on Figure 5-8. Maps of groundwater drawdown are provided in the companion technical report on groundwater modelling, Knapton et al. (2023). See Knapton et al. (2023) for more information. SCENARIO RN005621 – NEAR NEWCASTLE WATERS (mAHD) DIFF TO AN (m) RN024536 – NEAR DALY WATERS (mAHD) DIFF TO AN (m) RN028082 – NEAR LARRIMAH (mAHD) DIFF TO AN (m) RN029012 – SOUTH OF MATARANKA (mAHD) DIFF TO AN (m) RN029013 – SOUTH OF LARRIMAH (mAHD) DIFF TO AN (m) RN035796 – AT MATARANKA (mAHD) DIFF TO AN (m) AN 160.5 – 154.2 – 142.7 – 134.1 – 148.6 – 119 – A 160.3 -0.2 153.5 -0.7 140.4 -2.3 132.5 -1.6 147.0 -1.6 118.5 -0.5 B35 160.1 -0.4 151.1 -3.1 137.6 -5.1 131.6 -2.5 143.6 -5.0 118.4 -0.6 B70 159.9 -0.6 148.7 -5.5 134.8 -7.9 130.7 -3.4 140.2 -8.4 118.4 -0.6 B105 159.7 -0.8 146.4 -7.8 132.0 -10.7 129.7 -4.4 136.8 -11.8 118.4 -0.6 Cdry 158.8 -1.7 151.9 -2.3 137.5 -5.2 127.9 -6.2 144.7 -3.9 117.6 -1.4 Cmid 159.6 -0.9 152.7 -1.5 138.3 -4.4 128.9 -5.2 145.6 -3 118.1 -0.9 SCENARIO RN005621 – NEAR NEWCASTLE WATERS (mAHD) DIFF TO AN (m) RN024536 – NEAR DALY WATERS (mAHD) DIFF TO AN (m) RN028082 – NEAR LARRIMAH (mAHD) DIFF TO AN (m) RN029012 – SOUTH OF MATARANKA (mAHD) DIFF TO AN (m) RN029013 – SOUTH OF LARRIMAH (mAHD) DIFF TO AN (m) RN035796 – AT MATARANKA (mAHD) DIFF TO AN (m) Cwet 162.8 +2.3 155.4 +1.2 140.5 -2.2 131.2 -2.9 148 -0.6 119.3 +0.3 Ddry35 158.6 -1.9 149.5 -4.7 134.7 -8 126.9 -7.2 141.4 -7.2 117.5 -1.5 Ddry70 158.4 -2.1 147.1 -7.1 131.8 -10.9 125.9 -8.2 138 -10.6 117.5 -1.5 Ddry105 158.2 -2.3 144.7 -9.5 128.9 -13.8 124.9 -9.2 134.6 -14 117.4 -1.6 Dmid35 159.4 -1.1 150.3 -3.9 135.5 -7.2 127.9 -6.2 142.2 -6.4 118.1 -0.9 Dmid70 159.3 -1.2 147.9 -6.3 132.7 -10 126.9 -7.2 138.8 -9.8 118 -1 Dmid105 159.1 -1.4 145.5 -8.7 129.8 -12.9 126 -8.1 135.4 -13.2 118 -1 Dwet35 162.6 +2.1 153.1 -1.1 137.8 -4.9 130.3 -3.8 144.6 -4 119.3 +0.3 Dwet70 162.4 +1.9 150.7 -3.5 134.9 -7.8 129.4 -4.7 141.3 -7.3 119.3 +0.3 Dwet105 162.2 +1.7 148.3 -5.9 132.1 -10.6 128.4 -5.7 137.9 -10.7 119.2 +0.2 Note: (-) value represents a decrease in groundwater level relative to Scenario AN; (+) represents an increase in groundwater discharge relative to Scenario AN. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-7 Lower reach of Elsey Creek that is groundwater-fed near the junction with the Roper River Photo: CSIRO For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-8 Modelled drawdown in groundwater level in the Cambrian Limestone Aquifer (CLA) under (a) Scenario A current licensed entitlements and (b) Scenario B35 at ~2070 Drawdown contours shown as the 5th, 50th and 95th percentiles relate to drawdown in groundwater level. See companion technical report on groundwater modelling (Knapton et al., 2023) for more information. Under Scenario B35 the modelled mean groundwater discharge from the CLA to the Roper River is 3.0 m3/second, the same as the discharge under Scenario A (i.e. assuming full use of existing entitlements). Under Scenario B135, the modelled mean groundwater discharge from the CLA to the Roper River is 2.9 m3/second. This is 0.1 m3/second less (3% reduction) than the mean groundwater discharge under Scenario A and 0.4 m3/second less (11% reduction) than the mean groundwater discharge under Scenario AN (no groundwater extraction) (Table 5-5). The small (i.e. smaller than what is considered measurable) modelled drawdown at Mataranka (Table 5-4) and reduction in groundwater discharge under groundwater extraction scenarios B35, B70 and B105 is due to the long distance (and hence time lags) between the groundwater extraction locations and Mataranka. The hypothetical extraction occurs between about 60 and 160 km from the discharge areas of the aquifer, whereas existing developments are closer (between about 5 and 90 km). The timescales for groundwater flow in the aquifer can range from a few years near the groundwater discharge area around Mataranka to several hundred years in the southern part of the aquifer around Daly Waters. Table 5-5 presents the mean modelled groundwater discharge from the CLA at streamflow gauging station G9030013 at ~2070. The results illustrate that changes in climate have a considerably larger impact on groundwater discharge to the Roper River than groundwater extractions 60 to 160 km distant. This is because the CLA outcrops near Mataranka, and which receives localised recharge, have relatively short groundwater flow paths to the Roper River and consequently interannual variations in climate are evident in interannual variations in discharge. Table 5-5 Mean modelled groundwater discharge from the CLA at streamflow gauging station (G9030013) Scenario G9030013 m3/second % change AN 3.3 - A 3.0 −9 B35 3.0 −10 B70 2.9 −10 B105 2.9 −11 Cdry 2.3 -30 Cmid 2.6 -19 Cwet 3.4 +4 Ddry35 2.3 -31 Ddry70 2.2 -32 Ddry105 2.2 -33 Dmid35 2.6 -20 Dmid70 2.6 -21 Dmid105 2.6 -22 Dwet35 3.4 +3 Dwet70 3.4 +3 Dwet105 3.3 +2 Note: (-) value represents a decrease in groundwater level relative to Scenario AN; (+) represents an increase in groundwater discharge relative to Scenario AN. Pre-feasibility-level assessment of groundwater resource development opportunities and risks associated with the Dook Creek Aquifer The entire western portion of the DCA, west of the Central Arnhem Road is unconfined (Figure 5-9). That is, it outcrops at the surface or is within tens of metres of it and is directly recharged via outcrop areas or via a thin (i.e. <20 m) and patchy veneer of overlying variably permeable rocks (sandstone, siltstone and claystone). To the east of the Central Arnhem Road, the DCA is confined and dips steeply in the subsurface. Initially, depths increase to a few hundred metres before the aquifer reaches depths of >1000 mBGL. In the deeper confined parts of the aquifer (i.e. depth >500 mBGL) drilling is sparse with the exception of a few mineral exploration holes as it is prohibitively expensive to instal bores for extracting groundwater at these depths. Drilling in the unconfined parts of the aquifer while sparse, indicates the DCA has a mean saturated thickness of about 100 m. However, most groundwater bores have only been drilled for stock and domestic purposes and have been installed only tens of metres below the watertable. Similar to the CLA, the saturated thickness is an important characteristic along with aquifer hydraulic properties in relation to groundwater storage and flow. Though drilling maybe sparse across unconfined parts of the DCA, most bores have been installed in areas where information indicates the saturated thickness is >20 m, and where few appropriately constructed production bores have been installed and tested, they have yielded >10 L/second. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-9 North-west to south-east cross section traversing the Dook Creek Formation See Figure 5-2 for the spatial location of the cross-section. Depth to the top of the DCA, while spatially variable, occurs generally at depths of <100 metres below ground level (mBGL) across the western unconfined portion of the aquifer, west of the Central Arnhem Road (Figure 5-10). However, information is sparse. Depth to the top of the DCA east of the Central Arnhem Road increases from about 100 m BGL to 500 mBGL where the confluence between the Mainoru River and Wilton River occurs. Below the lower reaches of the Wilton River, the depth to the top of the DCA is >1000 mBGL (Figure 5-10). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-10 Depth to the top of the Dook Creek Aquifer (DCA) Mapped spatial extent of the DCA both within and beyond the Roper catchment boundary. Depths are in metres below the land surface. Stratigraphic data, represents a bore with stratigraphic data to obtain information about changes in geology with depth. Aquifer extent data source: Knapton (2009) Changes in the depth to groundwater across the DCA (derived from limited SWL data), indicate that groundwater occurs at depths ranging between 10 and 50 mBGL across the western unconfined part of the aquifer (Figure 5-12). Groundwater is shallowest (i.e., >10 mBGL) in the vicinity of groundwater discharge zones where discharge occurs via: (i) diffuse seepage and localised discharge to lower reaches of Flying Fox Creek and the Mainoru and Wilton Rivers; and (ii) via localised discharge at discrete springs such Weemol Spring near Bulman (Figure 5-9 and Figure 5-11). East of the Central Arnhem Road where the aquifer is deep and confined, groundwater is modelled to be under natural pressure and in parts is highly artesian (Figure 5-12). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-11 Dolostone outcrop in the bed of Weemol Spring Photo: CSIRO The Assessment undertook a number of groundwater investigations to confirm the current conceptual model of the DCA. The conceptual model was used to test a range of climate and hypothetical groundwater extraction scenarios in order to evaluate the DCA response to different scales of groundwater extraction. Importantly, in reporting the results of the hypothetical groundwater development scenarios no judgement is made as to whether the impact of the modelled groundwater-level drawdown to receptors such as groundwater-dependant environmental assets (see Figure 5-11) or existing users are acceptable. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-12 Depth to standing water level (SWL) of the Dook Creek Aquifer (DCA) Mapped spatial extent of the DCA both within and beyond the Roper catchment boundary. Positive values are depth below the land surface representing sub-artesian conditions, negative values are depths above the land surface representing artesian conditions. Aquifer extent data source: Knapton (2009) Impacts of extracting groundwater from the Dook Creek Aquifer to groundwater-dependent ecosystems and existing groundwater users Scenario-based modelling was conducted to assess the impacts of long-term changes in rainfall and potential evaporation and/or potential increased groundwater resource development on the streamflow of some of the Roper River’s northern tributaries (Flying Fox Creek, Mainoru and Wilton Rivers), existing groundwater users and environmental receptors such as the groundwater- fed creeks and rivers. Detailed in the companion technical report on groundwater modelling (Knapton et al., 2023). Scenarios are summarised in Section 1.4.3 and detailed in Knapton et al., 2023. The potential impacts of three different hypothetical groundwater extraction rates (1, 2 and 3 GL/year) at each of the six locations within the DCA have been reported at specified locations, including six bores (eachwith a registered numbers–RN)that are installed in a rangeof different settings acrossthe shallow unconfined parts of the aquifer, such as in close vicinitytogroundwater discharge zones at Flying Fox Creek and the Wilton River and existinggroundwater users such asthe communities at and near Bulman(Table5-6). Potential impactsto groundwaterdischarge werealsomodelled attwo streamflowgauging stations (G9030108 on Flying Fox Creekand G903003 on the Wilton River),which are considered bestrepresentative of groundwaterdischargefrom the aquifer(Table5-7). The location of the size hypothetical groundwaterextraction locations and the reporting locations are shown inFigure5-13. The sixhypothetical locations were selected considering the location of existing groundwaterlicences, suitability of soil for irrigated agriculture, suitable hydrogeological propertiesfor groundwaterextraction and distance fromexistinginfrastructure(see Knapton et al., 2023formoredetail). The location of the hypothetical groundwater extraction locations and the reportinglocations is shown inFigure5-8. TheDCAis an intermediate-scale groundwater system and consequently changesin climateand increases in groundwater extraction can take many hundreds of yearstofully propagatethroughthe system.Similar to theCLA thetime period over which the resultswere reported involved runningthe model to 2070 (~50 years). This wasconsidered a pragmatictime period over whichtoconsider the impactsof changesin climateand groundwater extraction because: (i) it is equivalentto morethan twice the length ofthe investmentperiod of a typical agricultural enterprise;(ii) it isroughly equivalent totheservicelife of a commissionedgroundwater borefield; (iii) it is consistentwith thetime period over whichfuture climate projections have been evaluated. Drawdown in groundwater levels in the DCA for each of thethree scenarios (B6, B12andB18) isconcentric around the six hypotheticalextraction sites (Figure5-13). Atthe smallest cumulativehypothetical extraction rate (6 GL/year,ScenarioB6) the maximum meanmodelleddrawdown ingroundwater level afterthe 50-year period (~2070) is <1 m occurring in the centreof thehypothetical extraction sites (Table5-6andFigure5-13). At the largest cumulative extraction rate(18 GL/year, Scenario B18), the maximum meanmodelleddrawdown in groundwater level after the 50-year period(~2070) is <2 m also occurringin the centre ofthe hypothetical extraction sites. Drawdown of about1m in groundwater level–a value thatcanbeconsidered measurable–is modelled to extend >50kmwest of the centre of thehypotheticalgroundwater extractionsites toFlying Fox Creek (RN031983 inTable5-6andFigure5-13), as well as > 50 km east and outsideofthe catchment east of the Wilton River (RN028226 inTable5-6andFigure5-13).Similar to theCLA, the widespread propagation of small drawdown impacts isdue to the low storagepropertiesof thedolostoneaquifer. Table5-6Mean modelled groundwater levelsin different parts of the DCAfor scenarios A and B Locations shown onFigure5-8. Maps of groundwater drawdown are provided in the companion technical report on groundwater modelling, Knapton et al. (2023). See Knapton et al. (2023)for more information. SCENARIORN006546 EAST OF FLYING FOXCK(mAHD) DIFF TOA'N (m) RN027811 WEST OF WILTON R(mAHD) DIFF TO A'N(m) RN028226 EAST OF WILTON R(mAHD) DIFF TO A'N(m) RN031983 WEST OF FLYING FOXCK(mAHD) DIFFTO AN (m) RN036302WEST OFMAINORU R(mAHD) DIFFTO AN (m) AN139.8–97.3–93.3–146.1–134– B6139.4-0.497.0-0.393.1-0.2145.8-0.3133.5-0.5B12138.9-0.996.7-0.692.9-0.4145.4-0.7132.9-1.1 296|Water resource assessment for the Roper catchment SCENARIO RN006546 – EAST OF FLYING FOX CK (mAHD) DIFF TO A'N (m) RN027811 – WEST OF WILTON R (mAHD) DIFF TO A'N (m) RN028226 – EAST OF WILTON R (mAHD) DIFF TO A'N (m) RN031983 – WEST OF FLYING FOX CK (mAHD) DIFF TO A'N (m) RN036302 – WEST OF MAINORU R (mAHD) DIFF TO A'N (m) B18 138.4 -1.4 96.4 -0.9 92.7 -0.6 145.1 -1.0 132.3 -1.7 Cdry 135.9 -3.9 95.6 -1.7 92.1 -1.2 142.1 -4 130.5 -3.5 Cmid 138.9 -0.9 96.9 -0.4 93 -0.3 145.1 -1 133.3 -0.7 Cwet 143.5 +3.7 98.7 +1.4 94.1 +0.8 149.9 +3.8 136.8 +2.8 Ddry6 135.3 -4.5 95.2 -2.1 91.8 -1.5 141.6 -4.5 129.7 -4.3 Ddry12 134.6 -5.2 94.8 -2.5 91.5 -1.8 141.1 -5 128.9 -5.1 Ddry18 133.9 -5.9 94.4 -2.9 91.2 -2.1 140.6 -5.5 128 -6 Dmid6 138.4 -1.4 96.6 -0.7 92.8 -0.5 144.8 -1.3 132.7 -1.3 Dmid12 137.9 -1.9 96.3 -1 92.6 -0.7 144.4 -1.7 132 -2 Dmid18 137.3 -2.5 95.9 -1.4 92.4 -0.9 144 -2.1 131.3 -2.7 Dwet6 143.2 +3.4 98.5 +1.2 94 +0.7 149.7 +3.6 136.4 +2.4 Dwet12 142.8 +3 98.2 +0.9 93.9 +0.6 149.4 +3.3 136.1 +2.1 Dwet18 142.5 +2.7 98 +0.7 93.8 +0.5 149.2 +3.1 135.7 +1.7 Note: (-) value represents a decrease in groundwater level relative to Scenario AN; (+) represents an increase in groundwater discharge relative to Scenario AN. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-13 Modelled drawdown in groundwater level in the Dook Creek Aquifer (DLA) for (a) Scenario B6, 6 GL/year hypothetical groundwater development (1 GL/year at six locations for the period 2059 to 2069); (b) Scenario B12, 12 GL/year hypothetical groundwater development (2 GL/year at six locations for the period 2059 top 2069); and (c) Scenario B18, 18 GL/year hypothetical groundwater development (3 GL/year at six locations for the period 2059 to 2069) Drawdown contours shown as the 5th, 50th and 95th percentiles relate to drawdown in groundwater level at depth in the aquifers beneath the land surface. Under Scenario B6 the modelled mean groundwater discharge from the DCA to the Wilton River and Flying Fox Creek is 0.10 m3/second and 0.61 m3/second respectively, representing reductions in groundwater discharge of between 3% and 5% (Table 5-7). Under Scenario B18, the modelled mean groundwater discharge from the DCA to the Wilton River and Flying Fox Creek is 0.10 m3/second and 0.57 m3/second respectively, representing reductions in groundwater discharge of between 10% and 12% (Table 5-7). Similar to the CLA, there are time lags ranging from tens to hundreds of years for small changes in groundwater level drawdown and groundwater discharge to occur at different spatial scales across the DCA. Table 5-7 also presents the mean modelled groundwater discharge from the DCA at streamflow gauging stations G9030003 (Wilton River) and G9030108 (Flying Fox Creek) at ~2070 for the future climate scenarios. The results illustrate that changes in climate have a considerably larger impact on groundwater discharge to the Wilton River and Flying Fox Creek than groundwater extractions at distances varying between 10 to 30 km from groundwater-fed streams. This is because, the DCA has a large outcropping area where the aquifer is recharged. Therefore, modelled hypothetical changes in climate have a larger impact on the water balance (i.e. recharge and discharge) across the unconfined extent of the aquifer than the modelled volumes of hypothetical groundwater extraction at specified locations. Table 5-7 Mean modelled groundwater discharge at streamflow gauging station (G9030003) and (G9030108) representative of groundwater discharge from the DCA to the Wilton River and Flying Fox Creek respectively for the period 2059 to 2069 SCENIARIO G9030003 G9030108 M3/SECOND % CHANGE M3/SECOND % CHANGE AN 0.11 - 0.63 - A 0.11 0.0 0.63 0.0 B6 0.10 -5.0 0.61 -3.2 B12 0.10 -9.0 0.59 -6.3 B18 0.10 -12.0 0.57 -10.0 Cdry 0.08 -22 0.35 -45 Cmid 0.10 -6 0.56 -12 Cwet 0.13 +22 0.98 +54 Ddry6 0.08 -26 0.33 -48 Ddry12 0.08 -30 0.31 -51 Ddry18 0.07 -34 0.29 -54 Dmid6 0.10 -9 0.53 -16 Dmid12 0.09 -13 0.51 -19 Dmid18 0.09 -17 0.49 -23 Dwet6 0.13 +19 0.96 +51 Dwet12 0.13 +16 0.94 +48 Dwet18 0.12 +13 0.92 +45 Note: (-) value represents a decrease in groundwater level relative to Scenario AN; (+) represents an increase in groundwater discharge relative to Scenario AN. 5.3.3 Managed aquifer recharge Introduction MAR is the intentional recharge of water to aquifers for subsequent recovery or environmental benefit (NRMMC-EPHC-NHMRC, 2009). Importantly for northern Australia, which has high intra- annual variability in rainfall, MAR can contribute to planned conjunctive use, whereby excess surface water can be stored in an aquifer in the wet season for subsequent reuse in the dry season (Evans et al., 2013; Lennon et al., 2014). Individual MAR schemes are typically small- to-intermediate-scale storages with annual extractable volumes of up to 20 GL/year. In Australia, they currently operate predominantly within the urban and industrial sectors but also in the agricultural sector. This scale of operation can sustain rural urban centres, contribute to diversified supply options in large urban centres and provide localised water management options, and it is suited to mosaic-type irrigation developments. The basic requirements for a MAR scheme are the presence of a suitable aquifer for storage, availability of an excess water source for recharge and a demand for water. The presence of suitable aquifers is determined from previous regional-scale hydrogeological and surface geological mapping (see companion technical report on hydrogeological assessment (Taylor et al., 2023)). Source water availability is considered in terms of presence/absence rather than volumes with respect to any existing water management plans. Pre-feasibility assessment was based on MAR scheme entry-level assessment in the Australian guidelines for water recycling: managed aquifer recharge (NRMMC-EPCH-NHMRC, 2009 – referred to as the ‘MAR guidelines’). The MAR guidelines provide a framework to assess feasibility of MAR; they incorporate four stages of assessment and scheme development. Stage one is entry-level assessment (pre-feasibility), stage two involves investigations and risk assessment, stage three is MAR scheme construction and commissioning, and stage four is operation of the scheme. There are numerous types of MAR (Figure 5-14) and the selection of MAR type is influenced by the characteristics of the aquifer, the thickness and depth of low-permeability layers, land availability and proximity to the recharge source. Infiltration techniques can be used to recharge unconfined aquifers, with water infiltrating through permeable sediments beneath a dam, river or basin. If infiltration is restricted by superficial clay, the recharge method may involve a pond or sump that penetrates the low-permeability layer. Bores are used to divert water into deep or confined aquifers. Infiltration techniques are typically lower cost than bore injection (Dillon et al., 2009; Ross and Hasnain, 2018) and are generally favoured in this Assessment. The challenge in northern Australia is to identify a suitable unconfined aquifer with capacity to store more water when water is available for recharge. In the Roper catchment, suitable unconfined aquifers are typically thought to rapidly recharge to full capacity during the wet season. Unless stated otherwise, the material presented in Section 5.3.3 has been summarised from the Northern Australia Water Resource Assessment technical report on managed aquifer recharge (Vanderzalm et al. 2018). Figure 5-14 Types of managed aquifer recharge (MAR) ASR = aquifer storage and recovery; ASTR = aquifer storage, transfer and recovery. Groundwater level indicated by triangle. Arrows indicate nominal movement of water. Source: Adapted from NRMMC-EPHC-NHMRC (2009) Opportunity-level assessment of infiltration based MAR in the Roper catchment The most promising aquifers for infiltration based MAR in the Roper catchment are within limestones and dolostones because these formations host the major aquifer systems in the Roper catchment: the sedimentary limestone aquifers of the Tindall Limestone and the sedimentary dolostone aquifers of the Dook Creek Formation (Figure 5-15). The sedimentary dolostone and sandstone aquifers of the Nathan Group and the sedimentary sandstone aquifers of the Bukalara Sandstone and Roper Group are also classified as having potentially being suitable. MAR potential was also assessed in the fractured rock aquifers of the Derim Derim Dolerite; however, these systems are low yielding and poorly characterised. Groundwater use lowers groundwater levels and therefore creates storage capacity in the aquifer, which is required for MAR. However, the challenge remains to target aquifers with storage capacity at the end of the wet season, or to identify an available recharge source when there is sufficient storage capacity (i.e. early in the dry season). Infiltration techniques recharging unconfined aquifers are generally favoured for producing cost-effective water supplies, hence the initial focus on recharge techniques and limitations for unconfined aquifers. MAR opportunity maps were developed from the best available data at the catchment scale using the method outlined in the Northern Australia Water Resource Assessment technical report on managed aquifer recharge (Vanderzalm et al., 2018). This method categorised the suitability of the more promising aquifers for MAR into four suitability classes: • Class 1 – highly permeable and low slope (<5%) • Class 2 – highly permeable and moderate slope (5% to 10%) • Class 3 – moderately permeable and low slope (<5%) • Class 4 – moderately permeable and moderate slope (5% to 10%). Class 1 is considered most suitable for MAR and Class 4 least suitable. Figure 5-15 shows the suitability map for MAR in the Roper catchment, with classes 1 and 2 considered potentially suitable for MAR. The opportunity assessment (Figure 5-16) indicates approximately 480 km2 (0.5%) of the Roper catchment may have aquifers with potential for MAR within 5 km of a major drainage line (excluding the highly intermittent drainage lines on the Sturt Plateau). Approximately 75 km2 (~0.1%) of the catchment is considered class 1 or 2 and is within 1 km2 of a major drainage line. Water-level data for monitoring bores across the Roper catchment provide some insight into the potential for aquifers to store additional water. A watertable level deeper than 4 m is recommended to in order to have sufficient storage capacity for MAR. Sufficient aquifer storage space is indicated where depth to water is either greater than 4 m at the end of the wet season (i.e. available for recharge year round) or greater than 4 m at the end of the dry season (i.e. available for seasonal recharge). Bores recording depth to water of less than 4 m at the end of the dry season could be considered indicative that no storage space exists at any time of year. These bores are not identified as targeting specific aquifers, but they plot approximately over corresponding regional watertable aquifers as shown by aquifer type. This is not precise, as there are doubts about the integrity of some bores as well as the potential for interaction between readings from underlying aquifers where there is layered stratigraphy, but it is considered adequate for interpretation at a regional scale. Figure 5-15 Managed aquifer recharge (MAR) opportunities for the Roper catchment independent of distance from a water source for recharge Analysis based on the permeability (Thomas et al., 2022) and terrain slope (Gallant et al., 2011) datasets and limited to the following aquifer formations: Cambrian Limestone, DCA hosted in the Dook Creek Formation, and aquifers hosted in the Proterozoic dolostone and sandstones (Figure 2-23). Water persistence in dry seasons is shown for context; presence of water in 90% or more of dry seasons is considered indicative of perennial flow. Gr-R-518_MAR_Suitability_CLA_DCA_aquifer_5kmMRivers_CR_v04.png For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-16 Managed aquifer recharge (MAR) opportunities in the Roper catchment within 5 km of major rivers Analysis based on the permeability (Thomas et al., 2022) and terrain slope (Gallant et al., 2011) datasets and limited to the following aquifer formations: Cambrian Limestone, DCA hosted in the Dook Creek Formation, and aquifers hosted in the Proterozoic dolostone and sandstones (Figure 2-23). The available depth-to-water data suggest that, in general, there is sufficient storage capacity in areas identified as having regional MAR opportunities on the Sturt Plateau. However, no existing surface water storages in the Roper catchment could provide a source of water for MAR. Furthermore, the areas with the greatest potential for new surface water storages to provide source water have limited opportunities for MAR and vice versa. For example, there is moderate potential for MAR on the Sturt Plateau but there is no reliable surface water and the soils are unlikely to be suitable for the construction of offstream storages. Where there are opportunities, some of these options may complement MAR operations where surface water capture, storage and detention offer a degree of treatment through sedimentation. Particulates in the recharge source may lead to clogging and reduce the recharge (infiltration or injection) rate. Pre-treatment before recharge can be used to manage clogging and reduce the need for ongoing maintenance. Also, intentional release of water from surface storage to provide groundwater recharge is an example of MAR. See the Northern Australia Water Resource Assessment technical report on MAR schemes in northern Australia In this report costs were estimated for ten hypothetical MAR schemes in northern Australia. 5.4 Surface water storage opportunities 5.4.1 Introduction In a highly seasonal climate, such as that of the Roper catchment, and in the absence of suitable groundwater, water storages are essential to enable irrigation during the dry season and other periods when soil water is insufficient for crop growth. The Assessment undertook a pre-feasibility-level assessment of three types of surface water storage options. These were: • major dams that could potentially supply water to multiple properties (Section 5.4.2) • re-regulating structures such as weirs (Section 5.4.3) • large farm-scale or on-farm dams, which typically supply water to a single property (Section 5.4.4 and Section 5.4.5). Both major dams and large farm-scale dams can be further classified as instream or offstream water storages. In this Assessment, instream water storages are defined as structures that intercept a drainage line (creek or river) and are not supplemented with water from another drainage line. Offstream water storages are defined as structures that: (i) do not intercept a drainage line, or (ii) intercept a small drainage line and are largely supplemented with water extracted from another larger drainage line. Ringtanks and turkey nest tanks are examples of offstream storages with a continuous embankment; the former is the focus in the Assessment due to their higher storage-to-excavation ratios, relative to the latter. The performance of a dam is often assessed in terms of water yield or demand. This is the amount of water that can be supplied for consumptive use at a given reliability. For a given dam, an increase in water yield results in a decrease in reliability. Importantly, the Assessment does not seek to provide instruction on the design and construction of farm-scale water storages. Numerous books and online tools provide detailed information on nearly all facets of farm-scale water storage (e.g. QWRC, 1984; Lewis, 2002; IAA, 2007). Siting, design and construction of weirs, large farm-scale ringtanks and gully dams is heavily regulated in most jurisdictions across Australia and should always be undertaken in conjunction with a suitably qualified professional and tailored to the nuances that occur at every site. Major dams are complicated structures and usually involve a consortium of organisations and individuals. Unless otherwise stated, the material in Section 5.4 originates from the companion technical report on surface water storage (Petheram et al., 2022). 5.4.2 Major dams Introduction Major dams are usually constructed from earth, rock and/or concrete materials, and typically act as a barrier wall across a river to store water in the reservoir created. They need to be able to safely discharge the largest flood flows likely to enter the reservoir and the structure has to be designed so that the dam meets its purpose, generally for at least 100 years. Some dams, such as the Kofini Dam in Greece and the Anfengtang Dam in China, have been in continuous operation for over 2000 years, with Schnitter (1994) consequently coining dams as ‘the useful pyramids’. An attraction of major dams over farm-scale dams is that if the reservoir is large enough relative to the demands on the dam (i.e. water supplied for consumptive use and ‘lost’ through evaporation and seepage), when the reservoir is full, water can last 2 or more years. This has the advantage of mitigating against years with low inflows to the reservoir. For this reason, major dams are sometimes referred to as ‘carry-over storages’. Major instream versus offstream dams Offstream water storages were among the first man-made water storages (Nace, 1972; Scarborough and Gallopin, 1991) because people initially lacked the capacity to build structures that could block rivers and withstand large flood events. One of the advantages of offstream storages is that, if properly designed, they can cause less disruption of the natural flow regime than large instream dams. Less disruption occurs if water is extracted from the river using pumps, or if there is a diversion structure with raiseable gates, which allow water and aquatic species to pass through when not in use. In the remote environments of northern Australia, the period in which these gates need to be operated is also the period in which it is difficult to move around wet roads and flooded waterways. The primary advantage of large instream dams is that they provide a very efficient way of intercepting the flow in a river, effectively trapping all flow until the full supply level (FSL) is reached. For this reason, however, they also provide a very effective barrier to the movement of fish and other species within a river system, alter downstream flow patterns and can inundate large areas of land upstream of the dam. Types of major dams Two types of major dams are particularly suited to northern Australia: embankment dams and concrete gravity dams. Embankment dams (EB) are usually the most economic, provided suitable construction materials can be found locally, and are best suited to smaller catchment areas where the spillway capacity requirement is small. Concrete gravity dams with a central overflow spillway are generally more suitable where a large capacity spillway is needed to discharge flood inflows, as is the case in most large catchments in northern Australia. Traditionally, concrete gravity dams were constructed by placing conventional concrete in formed ‘lifts’. Since 1984 in Australia, however, roller compacted concrete (RCC) has been used, where low-cement concrete is placed in continuous thin layers from bank to bank and compacted with vibrating rollers. This approach allows large dams to be constructed in a far shorter time frame than required for conventional concrete construction, often with large savings in cost (Doherty, 1999). RCC is best used for high dams where a larger scale plant can provide significant economies of scale. This is now the favoured type of construction in Australia whenever foundation rock is available within reasonable depth, and where a larger capacity spillway is required. In those parts of the Roper catchment with topography and hydrology most suited to large instream dams, RCC was deemed to be the most appropriate type of dam. Opportunity-level assessment of potential major dams in the Roper catchment A promising dam site requires inflows of sufficient volume and frequency, topography that provides a constriction of the river channel, and critically, favourable foundation geology. With no studies of large dams in the Roper catchment identified, the opportunity-level assessment of potential major dams in the Roper catchment was undertaken using a spatial analysis approach – to ensure no potential dam site had been overlooked, the Assessment used a bespoke computer model, the DamSite model (Petheram et al., 2017), to assess over 50 million sites in the Roper catchment for their potential as major offstream or instream dams. Broad-scale geological considerations Favourable foundation conditions include a relatively shallow layer of unconsolidated materials, such as alluvium, and rock that is relatively strong, resistant to erosion, non-permeable or capable of being grouted. Geological features that make dam construction challenging include the presence of faults, weak geological units, landslides and deeply weathered zones. Potentially feasible large dam sites in the Roper catchment occur where resistant ridges of Proterozoic sandstone beds that have been incised by the river systems outcrop on both sides of river valleys. The sandstones are generally weathered to varying degrees and the depth of weathering and the amount of sandstone outcrop on the valley slopes is a fundamental control on the suitability of the potential dam sites. Where the sandstones are relatively unweathered and outcrop on the abutments of the potential dam site, less stripping will be required to achieve a satisfactory founding level for the dam. The other fundamental control on the suitability of the dam is the extent and depth of the Quaternary alluvial sands and gravels in the floor of the valley, as these materials will have to be removed to achieve a satisfactory founding level for the dam. In general, where stripping removes the more weathered rock, it is anticipated that the Proterozoic sandstones will form a reasonably watertight dam foundation requiring conventional grout curtains and foundation preparation. Where potentially soluble dolomites occur within the Proterozoic sequences (soluble over a geological timescale) then it is possible that potentially leaky dam abutments and reservoir rims may be present, requiring specialised and costly foundation treatment such as extensive grouting. Where the rivers are tidal (i.e. lower Roper River), the presence of soft estuarine sediments has the potential to make dam design more challenging and construction more expensive, which may compromise the feasibility of a dam. Major offstream storages for water and irrigation supply Figure 5-17 displays the most promising sites across the Roper catchment in terms of topography, assessed in terms of approximate cost of construction per storage volume (ML). Favourable locations with a small catchment area and adjacent to a large river may be suitable as major offstream storages. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-17 Potential storage sites in the Roper catchment based on minimum cost per ML storage capacity This figure can be used to identify locations where topography is suitable for large offstream storages. At each location the minimum cost per ML storage capacity is displayed. The smaller the minimum cost per ML storage capacity ($/ML) the more suitable the site for a large offstream storage. Analysis does not take into consideration geological considerations, hydrology or proximity to water. Only sites with a minimum cost to storage volume ratio of less than $5000/ML are shown. $1000/ML is equivalent to 1 GL per million dollars. Costs are based on unit rates and quantity of material and site establishment for a roller compacted concrete (RCC) dam. Data are underlain by a shaded relief map. Inset displays height of full supply level (FSL) at the minimum cost per ML storage capacity. For more details see companion technical report on surface water storage (Petheram et al., 2022). In Figure 5-17 only those locations with a ratio of cost to storage less than $5000/ML are shown. This provides a simple way of displaying those locations in the Roper catchment with the most favourable topography for a large reservoir relative to the size (i.e. cost) of the dam wall necessary to create the reservoir. This figure can be used to identify more promising sites for offstream storage (i.e. where some or all of the water is pumped into the reservoir from an adjacent drainage line). The threshold value of $5000/ML is nominal and was used to minimise the amount of data displayed. This analysis does not consider evaporation or hydrology or geological suitability for dam construction. Figure 5-17 shows that those parts of the Roper catchment with the most favourable topography for storing water are on the Wilton and upper Waterhouse rivers and mid-reaches of a number of tributaries of the Roper River including Flying Fox Creek and Hodgson River. There is little favourable topography for large instream dams on the Sturt Plateau, where the largest contiguous areas of soils suitable for irrigated agriculture occur in the Roper catchment. Major instream dams for water and irrigation supply In addition to suitable topography (and geology), instream dams require sufficient inflows to meet a potential demand. Potential dams that command smaller catchments with lower runoff have smaller yields. Results concerning this criterion are presented in terms of minimum cost per unit yield (ML). The potential for major instream dams to cost-effectively supply water is presented in Figure 5-18. No values greater than $10,000/ML are shown. The highest yielding sites per unit cost are along the Wilton River, the lower Hodgson River and along the lower reaches of the Roper River. The results presented in Figure 5-18 do not take into consideration the geological suitability of a site for dam construction. Based on this analysis and a broad-scale desktop geological evaluation, four of the more costing effective larger yielding sites in proximity to soils suitable for irrigated agriculture were selected for pre-feasibility analysis (see the companion technical report on surface water storage (Petheram et al., 2022)) to explore the potential opportunities and risks of dams in the Roper catchment. The locations of the pre-feasibility potential dam sites are denoted in Figure 5-18 by black circles. It should be noted a fifth potential site, Site E on the Wilton River, was also selected for pre- feasibility analysis based on its potential for hydropower generation. Key parameters and performance metrics are summarised in Table 5-8 and an overall summary comment is recorded in Table 5-9. More detailed analysis of the five pre-feasibility sites is provided in the companion technical report on surface water storage (Petheram et al., 2022). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-18 Potential storage sites in the Roper catchment based on minimum cost per ML yield at the dam wall This figure indicates those sites more suitable for major dams in terms of cost per ML yield at the dam wall in 85% of years overlain on versatile land surface (see companion technical report on land suitability, Thomas et al., 2022). At each location the minimum cost per ML storage capacity is displayed. The smaller the cost per ML yield ($/ML) the more favourable the site for a large instream dam. Only sites with a minimum cost to yield ratio less than $10,000/ML are shown. Costs are based on unit rates and quantity of material required for a roller compacted concrete (RCC) dam with a flood design of 1 in 10,000. Right inset displays height of full supply level (FSL) at the minimum cost per ML yield and left inset displays width of FSL at the minimum cost per ML yield. Letters indicate potential dams listed in Table 5-8 and Table 5-9; A – Waterhouse River west branch; B – Waterhouse River; C – upper Flying Fox Creek; D – Jalboi River; E – Wilton River. See companion technical report on surface water storage (Petheram et al., 2022) for more information. Hydro-electric power generation potential in the Roper catchment The potential for major instream dams to generate hydro-electric power is presented in Figure 5-19, following an assessment of more than 50 million potential dam sites in the Roper catchment (Petheram et al., 2022). This figure provides indicative estimates of hydro-electric power generation potential but does not consider the existence of supporting infrastructure or geological suitability for dam construction. No values greater than $20,000/ML are shown For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-19 Roper catchment hydro-electric power generation opportunity map Costs are based on unit rates and quantity of material required for a roller compacted concrete (RCC) dam with a flood design of 1 in 10,000. Data are underlain by a shaded relief map. Letter E indicates location of Wilton River. For more details see companion technical report on surface water storage (Petheram et al., 2022). Pre-feasibility-level assessment of potential major dams in the Roper catchment Five potential dam sites in the Roper catchment were examined as part of this pre-feasibility assessment. They are summarised in Table 5-8 and Table 5-9. Table 5-8 Potential dam sites in the Roper catchment examined as part of the Assessment All numbers have been rounded. Locations of potential dams are shown in Figure 5-18. FSL = full supply level. NAME MAP ID DAM TYPE† SPILLWAY HEIGHT ABOVE BED‡ (m) CAPACITY AT FSL (GL) CATCHMENT AREA (km2) ANNUAL WATER YIELD§ (GL) CAPITAL COST* ($ MILLION) UNIT COST†† ($/ML) ANNUAL EQUIVALENT UNIT COST ‡‡ ($/y PER ML/y) Waterhouse River west branch ATMD 70.5 km A RCC 23 128 795 48 415 8,646 640 Waterhouse River ATMD 70.5 km B RCC 21 219 1,477 89 253 2,843 211 Upper Flying Fox Creek ATMD 105 km C RCC 22 133 1,173 68 318 4,676 346 Jalboi River ATMD 53 km D RCC 25 174 828 40 463 11,575 857 Wilton River ATMD 33 km§§ E RCC 39 4541 12,073 926 849& 916 68 †Roller compacted concrete (RCC) dam. ‡The height of the dam abutments and saddle dams will be higher than the spillway height. §Water yield is based on 85% annual time-based reliability using a perennial demand pattern for the baseline river model under historical climate and current development. This is yield at the dam wall (i.e. does not take into account distribution losses or downstream transmission losses). These yield values do not take into account downstream existing entitlement holders or environmental considerations. * indicates manually derived preliminary cost estimate, which is likely to be –10% to +50% of ‘true cost’.  indicates modelled preliminary cost estimate, which is likely to be –25% to +75% of ‘true’ cost. Should site geotechnical investigations reveal unknown unfavourable geological conditions, costs could be substantially higher. ††This is the unit cost of annual water supply and is calculated as the capital cost of the dam divided by the water yield at 85% annual time reliability. ‡‡Assumes a 7% real discount rate and a dam service life of 100 years. Includes operation and maintenance costs, assuming operation and maintenance costs are 0.4% of the total capital cost. §§Note there is limited soil suitable for irrigated agriculture downstream of this potential dam site. &Includes cost of power station. It does not include cost transmission lines. Table 5-9 Summary comments for potential dams in the Roper catchment Locations of potential dams are shown in Figure 5-18. NAME MAP ID SUMMARY COMMENT Waterhouse River west branch ATMD 70.5 km A A low-yielding site on the Waterhouse River west branch. Upstream of large areas of sandy loam soils moderately suitable for irrigated agriculture. The site is situated on Aboriginal land scheduled under the Aboriginal Land Rights (Northern Territory) Act 1976 (ALRA) and is near the community of Beswick. The site would not be able to be developed without the agreement of the Traditional Owners. Although data from the NT cultural heritage sites register were not made available to the Assessment, it is likely that the site and parts of the potential inundation area would contain cultural heritage sites of significance. Waterhouse River ATMD 70.5 km B The Waterhouse River potential dam site has one of the highest yield-to-cost ratios in the upper parts of the Roper catchment. The site appears to be suitable for a roller compacted concrete (RCC) type dam with a central uncontrolled spillway. The potential dam site is upstream of large areas of sandy loam soils moderately suitable for irrigated agriculture. The site is situated on Aboriginal land scheduled under the ALRA and is near the community of Beswick. The site would not be able to be developed without the agreement of the Traditional Owners. Although data from the NT cultural heritage sites register were not made available to the Assessment, it is likely that the site and parts of the potential inundation area would contain cultural heritage sites of significance. Upper Flying Fox Creek ATMD 105 km C A moderately high-yielding site relative to other sites in the Roper catchment that appears to be suitable for an RCC-type dam with a central uncontrolled spillway 150 m wide. It would be on land held under pastoral tenure. There is potential for a regulating weir downstream from the storage that would enable additional inflows to be captured and allow for the more efficient use of water released from the storage. Conceptual arrangement releases would be made from the storage downstream to a regulating weir for diversion by irrigators. There is a high likelihood of unrecorded cultural heritage sites of significance in the inundation area. NAME MAP ID SUMMARY COMMENT Jalboi River ATMD 53 km D A low-yielding site on the Jalboi River that appears to be suitable for an RCC-type dam with a central uncontrolled spillway. The potential dam site has a lower yield-to-cost ratio than other sites selected for pre-feasibility analysis; however, it is relatively close to large areas of alluvial soils moderately suitable for irrigated agriculture. Being north of the Roper River, accessing the dam site and potential irrigation areas during the wet season would be challenging without major road and bridge infrastructure. The affected area would primarily have an impact on the Lonesome Dove pastoral lease area. There is a high likelihood of unrecorded cultural heritage sites of significance in the inundation area. Wilton River ATMD 33 km E A very high-yield potential dam site. However, there are limited areas of land that would be suitable for irrigated agriculture downstream of the site. The site has the most suitable characteristics for a dam supplying water for hydro-electric power in the Roper catchment; however, the site is remote and there is no electricity transmission infrastructure. The site is situated on Aboriginal land scheduled under the ALRA and would not be able to be developed without the agreement of the Traditional Owners. Although data from the NT cultural heritage sites register were not made available to the Assessment, it is highly likely that the site and parts of the potential inundation area would contain cultural heritage sites of significance. The investigation of a potential large dam site generally involves an iterative process of increasingly detailed studies over a period of years, occasionally over as few as 2 or 3 years but often over 10 or more years. It is not unusual for the cost of the geotechnical investigations for a potential dam site alone to exceed several million dollars. For any of the options listed in this report to advance to construction, far more comprehensive studies would be needed, including not just bio-physical studies such as geotechnical investigations, field measurements of sediment yield, archaeological surveys and ground-based vegetation and fauna surveys, but also extensive consultations with Traditional Owners (e.g. see companion technical report on Indigenous aspirations, interests and water values (Lyons et al., 2023) and other stakeholders. Studies at that detail are beyond the scope of this regional-scale resource assessment. The companion technical report on surface water storage (Petheram et al., 2022) outlines the key stages in investigation of design, costing and construction of large dams. More comprehensive descriptions are provided by Fell et al. (2005), while Indigenous peoples’ views on large-scale water development in the catchment can be found in the companion technical report on Indigenous aspirations, interests and water values (Lyons et al., 2023). Other important considerations Cultural heritage considerations Indigenous people traditionally situated their campsites and hunting and foraging activities along major watercourses and drainage lines. Consequently, dams are more likely to affect areas of high cultural significance than most other infrastructure developments (e.g. irrigation schemes, roads). No field-based cultural heritage investigations of potential dam and reservoir locations were undertaken in the Roper catchment as part of the Assessment. However, based on existing records and statements from Indigenous participants in the Assessment, it is highly likely such locations will contain heritage sites of cultural, historical and wider scientific significance. There is insufficient information relating to the cultural heritage values of the potential major dam sites to allow full understanding or quantification of the likely impacts of water storages on Indigenous cultural heritage. The cost of cultural heritage investigations associated with large instream dams that could potentially impound large areas is high relative to other development activities. Ecological considerations of the dam wall and reservoir The water impounded by a major dam inundates an area of land, drowning not only instream habitat but surrounding flora and fauna communities. Complex changes in habitat resulting from inundation could create new habitat to benefit some of these species, while other species would be affected by loss of habitat. For instream ecology, the dam wall acts as a barrier to the movements of plants, animals and nutrients, potentially disrupting connectivity of populations and ecological processes. There are many studies linking water flow with nearly all the elements of instream ecology in freshwater systems (e.g. Robins et al., 2005). The impact of major dams on the movement and migration of aquatic species will depend upon the relative location of the dam walls in a catchment (Stratford al., 2022). For example, generally a dam wall in a small headwater catchment will have less of an impact on the movement and migration of species than a dam lower in the catchment. A dam also creates a large, deep lake, a habitat that is in stark contrast to the usually shallow and often flowing, or ephemeral, habitats it replaces. This lake-like environment favours some species over others and will function completely differently to natural rivers and streams. The lake-like environment of an impoundment is often used by sports anglers to augment natural fish populations by artificial stocking. Whether fish stocking is a benefit of dam construction is a matter of debate and point of view. Stocked fisheries provide a welcome source of recreation and food for fishers, and no doubt an economic benefit to local businesses, but they have also created a variety of ecological challenges. Numerous reports of disruption of river ecosystems (e.g. Drinkwater and Frank, 1994; Gillanders and Kingsford, 2002) highlight the need for careful study and regulatory management. Impounded waters may be subject to unauthorised stocking of native fish and releases of exotic flora and fauna. Further investigation of any of these potential dam sites would typically involve a thorough field investigation of vegetation and fauna communities. Potential changes to instream, riparian and near-shore marine species arising from changes in flow are discussed in Section 7.2. Sedimentation Rivers carry fine and coarse sediment eroded from hill slopes, gullies and banks and sediment stored within the channel. The delivery of this sediment into a reservoir can potentially be a problem because it can progressively reduce the volume available for active water storage. The deposition of coarser grained sediments in backwater (upstream) areas of reservoirs can also cause back-flooding beyond the flood limit originally determined for the reservoir. Although infilling of the storage capacity of smaller dams has occurred in Australia (Chanson, 1998), these dams had small storage capacities, and infilling of a reservoir is generally only a potential problem where the volume of the reservoir is small relative to the catchment area. Sediment yield is strongly correlated to catchment area (Wasson, 1994; Tomkins, 2013). Sediment yield to catchment area relationships developed for northern Australia (Tomkins, 2013) were found to predict lower sediment yield values than global relationships. This is not unexpected given the antiquity of the Australian landscape (i.e. it is flat and slowly eroding under ‘natural’ conditions). Using the relationships developed by Tomkins (2013), potential major dams in the Roper catchment were estimated to have about 2% or less sediment infilling after 30 years and less than 5% sediment infilling after 100 years. Cumulative yield of multiple dams in the Roper catchment This analysis explores the cumulative divertible yield and marginal returns of additional dam development of five of the more cost effective potential dam sites in the Roper catchment in terms of yield per unit cost in close proximity to soil suitable for irrigated agriculture and geographically distinct areas (but blind to other important considerations such as community and cultural values, land tenure and ecological impacts). The results in this section are used to report the cumulative ecological impacts of additional dam development. Figure 5-20a shows that the total divertible yield, before losses, from the five potential dams was about 348 GL in 85% of years at the dam wall and would cost approximately $2.16 billion. The construction cost per ML of yield increased from about $2840/ML with the first potential dam site (i.e. Waterhouse River) to $6220/ML for all five dams. Figure 5-20b is indicative of the amount of water available to go through the crop/plant after losses (i.e. assuming 75% efficiency to the farm gate, 95% efficiency of on-farm storage and delivery and 85% field application efficiency). The results from this analysis were used to investigate the cumulative impacts of multiple dams in the Roper catchment (see Section 7.2). a) b) For more information on this figure please contact CSIRO on enquiries@csiro.au 0% 25% 50% 75% 100% 09018027036002000400060008000Change in median annual flowCumulative yield at the dam wall (GL) $/ML supplied at the dam wallYieldChange in median annual flowWaterhouse RiverUpper Flying Fox CreekHodgson RiverWaterhouse River West BranchJalboi River For more information on this figure please contact CSIRO on enquiries@csiro.au 0% 25% 50% 75% 100% 060120180240030006000900012000Change in median annual flowCumulative yield after losses (GL) $/ML supplied through the cropYieldChange in mean annual flowWaterhouse RiverUpper Flying Fox CreekHodgson RiverWaterhouse River West BranchJalboi River Figure 5-20 Cost of water in $/ML versus cumulative divertible yield at 85% annual time reliability (a) Yield at the dam wall versus cost of water at the dam wall under historical climate and future development and (b) yield after river, channel (10%), on-farm (10%) and field application (15%) losses (i.e. equivalent to the amount of water available to go through the plant) versus cost of water after losses under historical climate and future development. Exploration of two potential dam sites in the Roper catchment Two potential dam sites on different rivers are summarised here. These sites are described because they are among the most cost-effective sites in close proximity to relatively large continuous areas of land suitable for irrigated agriculture in the Roper catchment. More detailed descriptions of the five sites selected for pre-feasibility assessment are provided in the companion technical report on surface water storage (Petheram et al., 2022). Potential dam on Waterhouse River ATMD 70.5 km This potential dam site is situated on the Beswick Aboriginal Land Trust area. This is Aboriginal land scheduled under the Commonwealth Aboriginal Land Rights (Northern Territory) Act 1976. It is near the community of Beswick and is classified under ‘inalienable freehold title’, which means that it cannot be bought, acquired or mortgaged. The site is situated on Proterozoic rocks of the Katherine River Group (Phs), which consist of medium to coarse and pebbly, trough cross-bedded quartz sandstone. The deeply weathered erosion surface characteristic of the region appears to have been locally removed by erosion at the site. On the dam abutments, blocky weathered sandstone bedrock appear to be exposed with open joints, suggesting erosion of weathered material from between the blocks. The site appears to be suitable for an RCC-type dam with a central uncontrolled spillway with crest length of approximately 75 m. Outlet works and a fish lift facility would be located on the left bank of the dam and access to the left bank of the site could be via 15 km of new road, branching from the Central Arnhem Highway a short distance east of the Beswick settlement. The total distance from Katherine via the Stuart and Central Arnhem highways would be some 127 km. Although data from the NT cultural heritage sites register were not made available to the Assessment, it is likely that the site and parts of the potential inundation area would contain cultural heritage sites of significance. Although there were no actual records of fish at this site, fish whose movement may be impeded by a dam include the mouth almighty (Glossamia aprion), giant gudgeon (Oxyeleotris selheimi), spangled grunter (Leiopotherapon unicolor), barramundi (Lates calcarifer), Hyrtl’s catfish (Neosilurus hyrtlii), and the northern snapping turtle (Elseya dentata) as they all occur in the neighbouring streams (Figure 5-21). The vegetation at this potential dam site is ‘Arnhem Plateau Sandstone Shrubland Complex’, listed as an Endangered Ecological Community (under the Commonwealth’s Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act)). The potential for ecological change as a result of changes to the downstream flow regime is examined in the companion technical report on ecological assets (Stratford et al., 2022). Modelled yield and cost versus dam FSL are shown in Figure 5-22. At a nominal FSL 200 mEGM96 (23 m above river bed), the reservoir of the dam would inundate 3540 ha at full supply and have a capacity of 219 GL (Figure 5-22). It would have the capacity to yield 89 GL of water in 85% of years. A manual cost estimate undertaken as part of the Assessment for an RCC dam on the Waterhouse River potential dam site at FSL 200 mEGM96 found the dam would cost approximately $253 million. A potential dam on the Waterhouse River west branch could supply water for irrigation to moderately suitable alluvial soils and sandplains downstream of the junction of the upper Waterhouse River and the Waterhouse River west branch. Under this hypothetical conceptual arrangement, water releases could be made from the storage to the stream for use by irrigators downstream (see Section 5.6). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-21 Migratory fish and water-dependent birds in the vicinity of the potential Waterhouse River dam site FSL = full supply level. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-22 Potential Waterhouse River dam site on the Waterhouse River: cost and yield at the dam wall (a) Dam length and dam cost versus full supply level (FSL), and (b) dam yield at 75% and 85% annual time reliability and yield/$ million at 75% and 85% annual time reliability. Potential dam on upper Flying Fox Creek ATMD 105 km The upper Flying Fox Creek potential dam site is one of the sites with the highest yield-to-cost ratio on non-Indigenous land and with soils that are moderately suited to irrigated agriculture downstream. The site is located on Proterozoic rocks of the Mount Rigg Group (Pooj), which consist of medium- to very thick-bedded quartz sandstone; chert and sandstone clast pebble to cobble conglomerate; and minor dolomitic siltstone with a sub-horizontal dip. The deeply weathered erosion surface characteristic of the region appears to have been partially removed by erosion at the site. On the dam abutments, weathered sandstone bedrock is partially exposed with some talus (blocky slope deposits) on the surface. The site appears to be suitable for an RCC-type dam with a central uncontrolled spillway 150 m wide. A potential dam with FSL 173 mEGM96 (22 m above (ALOS) bed level) could have a capacity of 133 GL and would inundate approximately 1924 ha at full supply. At this FSL a reservoir at this site could release 99 GL of water in 85% of years at the dam wall. Under this hypothetical conceptual arrangement, releases would be made from the storage downstream to a regulating weir for diversion by irrigators (see Section 5.6.3). Outlet works and a fish lift facility would be located on the right bank. No saddle dams are required at this level of development. A manual cost estimate undertaken as part of the Assessment for an RCC dam on the upper Flying Fox Creek potential dam site at FSL 173 mEGM96 found the dam would cost approximately $318 million. Access to the right bank of the site would be via a 10 km new road from the Central Arnhem Highway branching before the creek crossing. The total distance via the Stuart and Central Arnhem highways from Katherine would be some 207 km. Although data from the NT cultural heritage sites register were not made available to the Assessment, it is likely that the site and parts of the potential inundation area would contain cultural heritage sites of significance. The area below the potential dam site on the upper Flying Fox Creek is dominated by hills and undulating rises dissected by a narrow alluvial plain along the creek, then a relatively large area of alluvial plains of Flying Fox Creek around the Central Arnhem Road, and broad alluvial plains associated with the lower Flying Fox Creek approximately 45 km downstream of the Central Arnhem Road. The alluvial plains in the upper catchment are dominated by very deep sandy- surfaced brown Dermosols (soil generic group (SGG) 2) with moderately permeable, moderately well-drained to imperfectly drained, mottled structured clay subsoils. The broad alluvial plains approximately 45 km downstream of the Central Arnhem Road are dominated by very deep, moderately well-drained to imperfectly drained, slowly permeable brown to grey cracking clay soils (SGG 9) with strongly sodic subsoils, and soft, self-mulching or hard-setting structured clay surfaces. At this site there were no registered records of any species. The ‘Arnhem Plateau Sandstone Shrubland Complex’ listed as an Endangered Ecological Community (EPBC Act) surrounds the potential inundated area at FSL for this site (172 mEGM96) (Figure 5-23). The potential for ecological change as a result of changes to the downstream flow regime is examined in the companion technical report on ecological assets (Stratford et al., 2022). Modelled dam yield and dam cost versus FSL is shown in Figure 5-24. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-23 Migratory fish and water-dependent birds in the vicinity of the potential upper Flying Fox Creek dam site FSL = full supply level. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-24 Upper Flying Fox Creek dam site on the Flying Fox Creek: cost and yield at the dam wall (a) Dam length and dam cost versus full supply level (FSL) and (b) dam yield at 75% and 85% annual time reliability and yield/$ million at 75% and 85% annual time reliability. 5.4.3 Weirs and re-regulating structures Re-regulating structures, such as weirs, are typically located downstream of large dams. They allow for more efficient releases from the storages and for some additional yield from the weir storage itself, thereby reducing the transmission losses normally involved in supplemented river systems. As a rule of thumb, however, weirs are constructed to one-half to two-thirds of the river bank height. This height allows the weirs to achieve maximum capacity, while ensuring the change in downstream hydraulic conditions does not result in excessive erosion of the toe of the structure. It also ensures that large flow events can still be passed without causing excessive flooding upstream. Broadly speaking, there are two types of weir structure: concrete gravity type weirs and sheet piling weirs. These are discussed below. For each type of weir, rock-filled mattresses are often used on the stream banks, extending downstream of the weir to protect erodible areas from flood erosion. A brief discussion on sand dams is also provided. Weirs, sand dams and diversion structures obstruct the movement of fish in a similar way to dams during the dry season. Concrete gravity type weirs Where rock bars are exposed at bed level across the stream, concrete gravity type weirs have been built on the rock at numerous locations across north Queensland. This type of construction is less vulnerable to flood erosion damage both during construction and in service. Assuming favourable foundation conditions, the cost of a 6-m high and 400-m wide concrete gravity weir is estimated to be approximately $25 million (see companion technical report on surface water storage (Petheram et al., 2022)). This includes but is not limited to a fish lock ($1.1 million), bank protection ($900,000) and outlet works ($550,000), investigation and design ($700,000), on-site overheads ($2.15 million) and risk adjustment ($6 million). It does not include acquisition and approval costs. Sheet piling weirs Where rock foundations are not available, stepped steel sheet piling weirs have been successfully used in many locations across Queensland. These weirs consist of parallel rows of steel sheet piling, generally about 6 m apart with a step of about 1.5 to 1.8 m high between each row. Reinforced concrete slabs placed between each row of piling absorb much of the energy as flood flows cascade over each step. The upstream row of piling is the longest, driven to a sufficient depth to cut off the flow of water through the most permeable material (Figure 5-25). Indicative costs are provided in Table 5-10. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-25 Schematic cross-section diagram of sheet piling weir FSL = full supply level. Source: Petheram et al. (2013) Table 5-10 Estimated construction cost of 3-m high sheet piling weir Cost indexed to 2021 WEIR CREST LENGTH (m) ESTIMATED CAPITAL COST ($ million) 100 28 150 36 200 43 Sand dams Because many of the large rivers in northern Australia are very wide (e.g. >300 m), weirs are likely to be impractical and expensive at many locations. Alternative structures are sand dams, which are low embankments built of sand on the river bed. They are constructed at the start of each dry season during periods of low or no flow when heavy earth-moving machinery can access the bed of the river. They are constructed to form a pool of depth sufficient to enable pumping (i.e. typically greater than 4 m depth) and are widely used in the Burdekin River near Ayr in Queensland, where the river is too wide to construct a weir. Typically, sand dams take three to four large excavators about 2 to 3 weeks to construct, and no further maintenance is required until they need to be reconstructed again after the wet season. Bulldozers can construct a sand dam quicker than a team of excavators but have greater access difficulties. Because sand dams only need to form a pool of sufficient size and depth from which to pump water, they usually only partially span a river and are typically constructed immediately downstream of large, naturally formed waterholes. The cost of 12 weeks of hire for a 20-tonne excavator and float (i.e. transportation) is approximately $85,000. Although sand dams are cheap to construct relative to a weir, they require annual rebuilding and have very high seepage losses beneath and through the dam wall. No studies are known to have quantified losses from sand dams. The application of sand dams in the Roper catchment is likely to be limited. 5.4.4 Large farm-scale ringtanks Large farm-scale ringtanks are usually fully enclosed circular earthfill embankment structures constructed close to major watercourses/rivers so as to minimise the cost of pumping infrastructure by ensuring long ‘water harvesting’ windows. For this reason, they are often subject to reasonably frequent inundation, usually by slow-moving flood waters. In some exceptions embankments may not be circular; rather, they may be used to enhance the storage potential of natural features in the landscape such as horseshoe lagoons or cut-off meanders adjacent to a river (see Section 5.4.6 for discussion on extracting water from persistent waterholes). An advantage of ringtanks over gully dams is that the catchment area of the former is usually limited to the land that it impounds, so costs associated with spillways, failure impact assessments and constructing embankments to withstand flood surges are considerably less than large farm- scale gully dams. Another advantage of ringtanks is that unless a diversion structure is utilised in a watercourse to help ‘harvest’ water from a river, a ringtank and its pumping station do not impede the movement of aquatic species or transport of sediment in the river. Ringtanks also have to be sited adjacent to major watercourses to ensure there are sufficient days available for pumping. While this limits where they can be sited, it means that because they can be sited adjacent to major watercourses (on which gully dams would be damaged during flooding – large farm-scale gully dams are typically sited in catchments less than 30 km2), they often have a higher reliability of being filled each year than gully dams. However, operational costs of ringtanks are usually higher than gully dams because water must be pumped into the structure each year from an adjacent watercourse, typically using diesel-powered pumps (solar and wind energy do not generate sufficient power to operate high-volume axial flow or ‘china’ pumps). Even where diversion structures are utilised to minimise pumping costs, the annual cost of excavating sediment and debris accumulated in the diversion channel can be in the order of tens of thousands of dollars. For more information on ringtanks in the Roper catchment, refer to the companion technical reports on surface water storage (Petheram et al., 2022) and river model simulation (Hughes et al., 2023). Also of relevance is the Northern Australia Water Resource Assessment technical report on large farm-scale dams (Benjamin, 2018). A rectangular ringtank in the Flinders catchment (Queensland) is pictured in Figure 5-26. In this section, the following assessments of ringtanks in the Roper catchment are reported: • suitability of the land for large farm-scale ringtanks • reliability with which water can be extracted from different reaches • indicative evaporative and seepage losses from large farm-scale ringtanks • indicative capital, operation and maintenance costs of large farm-scale ringtanks. Figure 5-26 Rectangular ringtank and 500 ha of cotton in the Flinders catchment (Queensland) Channel along which water is diverted from the Flinders River to the ringtank can be seen in foreground. Photo: CSIRO Suitability of land for ringtanks in the Roper catchment Figure 5-27 displays the broad-scale suitability of land for large farm-scale ringtanks in the Roper catchment. Approximately 7% of the Roper catchment is classed as being suitable. Several land types are likely to be suitable for ringtanks. These include the poorly drained coastal marine clay plains; the cracking clay soils on the alluvial plains of the Roper River and major tributaries; and the Cenozoic clay plains on the Sturt Plateau. The low-lying, very deep (>1.5 m), very poorly drained, strongly mottled grey saline clay soils with potential acid sulfate deposits in the profile on the coastal marine plains are likely to be suitable for ringtanks but are subject to tidal inundation and storm surge from cyclones. Other areas likely to be suitable are the slowly permeable cracking clay soils on the alluvial plains of the Roper River and major rivers, which have very deep (>1.5 m), moderately well to imperfectly drained, slowly permeable brown to grey cracking clays that are usually strongly sodic at depth. The clay plains of the Roper River are subject to regular flooding and frequently have small (<0.3 m) gilgai depressions and numerous flood channels. These soils on the alluvial plains grade to seasonally wet soils lower in the catchment below Ngukurr. The Cenozoic clay plains of the Sturt Plateau with very deep (>1.5 m), impermeable, imperfectly drained grey cracking clay soils often have large deep gilgai (>0.3–0.8 m). This relict alluvium occurs in drainage depressions enabling collection and storage of overland flows. The non-cracking clay soils associated with the Cenozoic clay plains on the Sturt Plateau are possibly suitable for ringtanks. These very deep (>1.5 m), gilgaied non-cracking soils with moderately permeable clay loam to clay surfaces up to 1 m deep overlie impermeable, mottled, structured, brown vertic (shrink–swell properties) clay subsoils. However, the streams and rivers on the Sturt Plateau are highly intermittent and hence do not provide a reliable source of water for ringtanks (Figure 2-41b). Other soils that are possibly suitable are the gently undulating plains with deep (1–1.5 m), non-rocky, non-cracking clay soils developed on mudstones in the upper Hodgson River catchment. This latter group frequently occurs in association with shallow or rocky soils on slopes that are dissected by numerous creeks. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-27 Suitability of land for large farm-scale ringtanks in the Roper catchment Soil and subsurface data were only available to a depth of 1.5 m; hence the Assessment does not consider the suitability of subsurface material below this depth. This figure does not take into consideration the availability of water. Data are overlaid on a shaded relief map. The results presented in this figure are only indicative of where suitable locations for siting a ringtank may occur and site-specific investigations by a suitably qualified professional should always be undertaken prior to their construction. Reliability of water extraction The reliability at which an allocation or volume of water can be extracted from a river depends upon a range of factors including: • the quantity of discharge and the natural inter- and intra-variability within a river system (Section 2.5.5) • the capacity of the pumps or diversion structure (expressed here as the number of days taken to pump an allocation) • the quantity of water being extracted by other users and their location • conditions associated with a licence to extract water, such as: – a minimum threshold (i.e. water height level/discharge) at which pumping can commence (pump start threshold) – an end-of-system flow requirement, the minimum flow that must pass the lowest gauge in the system before pumping can commence. In this case the end-of-system node is the river model node at Ngukurr. Licence conditions can be imposed on a potential water user to ensure downstream entitlement holders are not affected by new water extractions and to minimise environmental change that may arise from perturbations to streamflow. In some cases a pump start threshold may be a physical threshold below which it is difficult to pump water from a natural pumping pool, but it can also be a regulatory requirement imposed to minimise impacts to existing downstream users and mitigate changes to existing water dependent ecosystems. The reliability at which water can be extracted under different conditions and different locations in the Roper catchment is detailed in the companion technical report for river modelling, Hughes et al., 2023. A selection of plots from this report are provided below to illustrate key concepts. Figure 5-28 can be used to explore the reliability at which increasing volumes of water can be extracted (‘harvested’) or diverted at five locations in the Roper catchment under varying pump start thresholds. The left vertical axis (y1-axis) indicates the system target volume, which is the maximum volume of water extracted across the whole catchment each season (nominal catchment-wide entitlement volume). The right vertical axis (y2-axis) is the maximum volume of water extracted in that reach each season (nominal reach entitlement volume). This example assumes a 20-day pump capacity, that is, the system and reach target volumes (i.e. nominal entitlement volume) that can be pumped in 20 days (not necessarily consecutive). This means an irrigator with a 4 GL ringtank would need a pump capacity of 200 ML/day to fill their ringtank in 20 days. In this example there is no end-of-system flow requirement. The impact of pump start thresholds and end-of-system flow requirements on extraction reliability are explored because they are the least complex environmental flow provision to regulate and ensure compliance in remote areas. Although more targeted environmental flow provisions may be possible, these are inevitably more complicated for irrigators to adhere to (usually requiring many dozens of pump operations during the course of a single season) and more difficult for regulators to ensure compliance. Within each river reach, water could be harvested by one or more hypothetical water harvesters and the water nominally stored in ringtanks adjacent to the river reach. The locations of the hypothetical extractions are illustrated in the map in the bottom right corner of Figure 5-28 to Figure 5-33 and their relative proportion of the total system allocation (left vertical axis) were assigned based on joint consideration of crop versatility, broad-scale flooding, ringtank suitability and river discharge (see companion technical report on river modelling (Hughes et al., 2023)). Plots - pump rate = 20 days "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\7_RiverModel\14_river_9\8_waterHarvest\2_output\6_catchReportChap5_plots\plot1_thresh_v_alloc_0eos_rate20days.png" For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-28 Annual reliability of diverting annual system and reach target for varying pump start thresholds No end-of-system flow requirement before pumping can commence. Cross-shading indicates volumes of water for which there is insufficient soil suitable for irrigated agriculture in close proximity to the river. Eight digit numbers refer to model node location. See companion technical report on river modelling (Hughes et al., 2023), for more detail. At the smallest pump start threshold examined, 200 ML/day (nominally representative of a lower physical pumping limit), approximately 1500 GL of water can be extracted in the Roper catchment in 75% of years, however, there is insufficient soil suitable for irrigated agriculture in close proximity (~5 km) to the rivers to fully utilise this volume of water for irrigated agriculture. The hashed shading in Figure 5-28 indicates where the system target volumes are in excess of that required to irrigate the area of land suitable for irrigated agriculture (assuming 10 ML is required to be extracted per hectare). This figure shows that as the total system and reach targets increase, the reliability at which the full system and reach targets can be extracted decreases. Similarly, as the pump start threshold increases the reliability at which the full system and reach targets can be extracted decreases. The data presented in Figure 5-29 and Figure 5-30 are similar to those presented in Figure 5-28 except in Figure 5-29 and Figure 5-30 an additional extraction condition is imposed where 400 GL (Figure 5-29) and 1000 GL (Figure 5-30), respectively, has to flow past Ngukurr each wet season before any water can be extracted. These figures show that increasing the end-of-system flow requirement reduces the reliability at which the system and reach targets can be extracted. As shown in Figure 5-31 and Figure 5-32, which indicate the post-extraction 50% and 80% annual flow exceedance at the last streamflow gauge relative to Scenario A, the end-of-system flow requirement has the effect of ‘protecting’ streamflow during drier years. Figure 5-33 shows the relationship between the reliability of achieving system and reach target volumes and pump capacity, expressed in days to pump target. As shown in this figure, with a pump start threshold of 1000 ML/day and an annual end-of-system flow requirement of 400 GL, large pump capacities (i.e. 10 days or less) are required to extract the system and reach targets in 75% of years or greater. "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\7_RiverModel\14_river_9\8_waterHarvest\2_output\6_catchReportChap5_plots\plot3_thresh_v_alloc_400eos_rate20days.png For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-29 Annual reliability of diverting annual system and reach target for varying pump start thresholds assuming end-of-system flow requirement before pumping can commence is 400 GL Assumes pumping capacity of 20 days (i.e. system and reach targets can be pumped in 20 days). Cross-shading indicates volumes of water for which there is insufficient soil suitable for irrigated agriculture in close proximity to the river. Eight digit number refers to model node location. See companion technical report on river modelling (Hughes et al., 2023, for more detail. "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\7_RiverModel\14_river_9\8_waterHarvest\2_output\6_catchReportChap5_plots\plot4_thresh_v_alloc_1000eos_rate20days.png For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-30 Annual reliability of diverting annual system and reach target for varying pump start thresholds assuming end-of-system flow requirement before pumping can commence is 1000 GL Assumes pumping capacity of 20 days (i.e. system and reach targets can be pumped in 20 days). Cross-shading indicates volumes of water for which there is insufficient soil suitable for irrigated agriculture in close proximity to the river. Eight digit number refers to model node location. See companion technical report on river modelling (Hughes et al., 2023, for more detail. "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\7_RiverModel\14_river_9\8_waterHarvest\2_output\6_catchReportChap5_plots\catchRep_50annual_residiual_flow.png" For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-31 50% annual exceedance (median) streamflow relative to Scenario A in the Roper catchment for a pump start threshold of 1000 ML/day and a pump capacity of 20 days Cross-shading indicates volumes of water for which there is insufficient soil suitable for irrigated agriculture in close proximity to the river. Eight digit number refers to model node location. See companion technical report on river modelling (Hughes et al., 2023, for more detail. "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\7_RiverModel\14_river_9\8_waterHarvest\2_output\6_catchReportChap5_plots\catchRep_80annual_residual_flow.png" For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-32 80% annual exceedance (median) streamflow relative to Scenario A in the Roper catchment for a pump start threshold of 1000 ML/day and a pump capacity of 20 days Cross-shading indicates volumes of water for which there is insufficient soil suitable for irrigated agriculture in close proximity to the river. Eight digit number refers to model node location. See companion technical report on river modelling (Hughes et al., 2023, for more detail. "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\7_RiverModel\14_river_9\8_waterHarvest\2_output\6_catchReportChap5_plots\plot2_pumpRate_v_alloc_400eos_1000MLthresh.png" For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-33 Annual reliability of diverting annual system and reach targets for varying pump rates assuming a pump start flow threshold of 1000 ML/day End-of-system flow requirement before pumping can commence is 400 GL. Cross-shading indicates volumes of water for which there is insufficient soil suitable for irrigated agriculture in close proximity to the river. Eight digit number refers to model node location. See companion technical report on river modelling (Hughes et al., 2023, for more detail. Evaporation and seepage losses Losses from a farm-scale dam occur through evaporation and seepage. When calculating evaporative losses from farm dams it is important to calculate net evaporation (i.e. evaporation minus rainfall) rather than just evaporation. Strategies to minimise evaporation include liquid and solid barriers, but these are typically expensive per unit of inundated area (e.g. $12 to $40 per m2). In non-laboratory settings, liquid barriers such as oils are susceptible to being dispersed by wind and have not been shown to reduce evaporation from a water body (Barnes, 2008). Solid barriers can be effective in reducing evaporation but are expensive, at approximately two to four times the cost of constructing the ringtank. Evaporation losses from a ringtank can also be reduced slightly by subdividing the storage into multiple cells and extracting water from each cell in turn so as to minimise the total surface water area. However, constructing a ringtank with multiple cells requires more earthworks and incurs higher construction costs than outlined in this section. A study of 138 farm dams ranging in capacity from 75 to 14,000 ML from southern NSW to central Queensland by the Cotton Catchment Communities CRC (2011) found mean seepage and evaporation rates of 2.3 and 4.2 mm/day, respectively. Of the 138 dams examined, 88% had seepage values of less than 4 mm/day and 64% had seepage values less than 2 mm/day. These results largely concur with IAA (2007), which states that reservoirs constructed on suitable soils will have seepage losses equal to or less than 1 to 2 mm/day and seepage losses will be greater than 5 mm/day if sited on less suitable (i.e. permeable) soils. Ringtanks with greater average water depth lose a lower percentage of their total storage capacity to evaporation and seepage losses, however, they have a smaller storage capacity to excavation ratio. In Table 5-11, effective volume refers to the actual volume of water that could be used for consumptive purposes after losses due to evaporation and seepage. For example, if water is stored in a ringtank with average water depth of 3.5 m from April until January and the average seepage loss is 2 mm/day, more than half the stored volume (i.e. 58%) would be lost to evaporation and seepage. The example provided in Table 5-11 is for a 4000 ML storage but the effective volume expressed as a percentage of the ringtank capacity is applicable to any storage (e.g. ringtanks or gully dams) of any capacity for average water depths of 3.5, 6.0 and 8.5 m. Table 5-11 Effective volume after net evaporation and seepage for ringtanks of three average water depths and under three seepage rates near the Jalboi River in the Roper catchment Effective volume refers to the actual volume of water that could be used for consumptive purposes as a result of losses due to net evaporation and seepage, assuming the storage capacity is 4000 ML. For storages of 4000 ML capacity and average water depths of 3.5, 6.0 and 8.5 m, reservoir surface areas are 110, 65 and 45 ha, respectively. S:E ratio is the storage capacity to excavation ratio. Effective volumes calculated based on the 20% exceedance net evaporation. For more details see companion technical report on surface water storage (Petheram et al., 2022). AVERAGE WATER DEPTH† (m) S:E RATIO SEEPAGE LOSS (mm/day) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENTAGE OF CAPACITY (%) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENTAGE OF CAPACITY (%) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENTAGE OF CAPACITY (%) 5 months (April to August) 7 months (April to October) 10 months (April to January) 3.5 14:1 1 2970 74 2447 61 2002 50 14:1 2 2803 70 2213 55 1666 42 14:1 5 2301 58 1510 38 660 16 AVERAGE WATER DEPTH† (m) S:E RATIO SEEPAGE LOSS (mm/day) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENTAGE OF CAPACITY (%) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENTAGE OF CAPACITY (%) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENTAGE OF CAPACITY (%) 6.0 7.5:1 1 3388 85 3077 77 2811 70 7.5:1 2 3289 82 2938 73 2613 65 7.5:1 5 2993 75 2523 63 2018 50 8.5 5:1 1 3574 89 3358 84 3173 79 5:1 2 3506 88 3262 82 3036 76 5:1 5 3301 83 2975 74 2624 66 †Average water depth above ground surface. Capital, operation and maintenance costs of ringtanks Construction costs of a ringtank may vary considerably, depending on its size and the way the storage is built. For example, circular storages have a higher storage volume to excavation cost ratio than rectangular or square storages. As discussed in the section on large farm-scale gully dams (Section 5.4.5) it is also considerably more expensive to double the height of an embankment wall than double its length due to the low angle of the walls of the embankment (often at a ratio of 3 horizontal to 1 vertical). Table 5-12 provides a high-level breakdown of the capital and operation and maintenance (O&M) costs of a large farm-scale ringtank, including the cost of the water storage, pumping infrastructure, up to 100 m of pipes and O&M of the scheme. In this example it is assumed that the ringtank is within 100 m of the river and pumping infrastructure. The cost of pumping infrastructure and conveying water from the river to the storage is particularly site-specific. In flood-prone areas where flood waters move at moderate-to-high velocities, riprap protection may be required, and this may increase the construction costs presented in Table 5-12 and Table 5-13 by 10 to 20% depending upon volume of rock required and proximity to a quarry with suitable rock. For a more detailed breakdown of ringtank costs see the Northern Australia Water Resource Assessment technical report on large farm-scale dams (Benjamin, 2018). Table 5-12 Indicative costs for a 4000-ML ringtank Assumes a 4.25-m wall height, 0.75-m freeboard, 3:1 ratio on upstream slope, 3:1 ratio on downstream slope and crest width of 3.1 m, approximately 60% of material can be excavated from within storage, and cost of earthfill and compacted clay is $5.4/m3 and $7/m3, respectively. Earthwork costs include vegetation clearing, mobilisation/demobilisation of machinery and contractor accommodation. Costs indexed to 2021. Pump station operation and maintenance (O&M) costs assume cost of diesel of $1.49/L. SITE DESCRIPTION/ CONFIGURATION EARTHWORKS ($) GOVERNMENT PERMITS AND FEES ($) INVESTIGATION AND DESIGN FEES ($) PUMP STATION ($) TOTAL CAPITAL COST ($) O&M OF RINGTANK ($/y) O&M OF PUMP STATION ($/y) TOTAL O&M ($/y) 4000-ML ringtank 1,725,000 38,200 81,800 1,100,000 2,945,000 18,300 107,000 125,300 The capital costs can be expressed over the service life of the infrastructure (assuming a 7% discount rate) and combined with O&M costs to give an equivalent annual cost for construction and operation. This enables infrastructure with differing capital and O&M costs and service lives to be compared. The total equivalent annual costs for the construction and operation of a 1000-ML ringtank with 4.25-m high embankments and 55 ML/day pumping infrastructure is about $149,000 (Table 5-13). For a 4000-ML ringtank with 4.25-m high embankments and 160 ML/day pumping infrastructure, the total equivalent annual cost is about $374,400. For a 4000-ML ringtank with 6.75-m high embankments and 160 ML/day pumping infrastructure, the total equivalent annual cost is about $510,900. Table 5-13 Annualised cost for the construction and operation of three ringtank configurations Assumes freeboard of 0.75 m, pumping infrastructure can fill ringtank in 25 days and assumes a 7% discount rate. Costs based on those provided for 4000 ML provided in Northern Australia Water Resource Assessment technical report on large farm-scale dams (Benjamin, 2018). Costs indexed to 2021. Pump station O&M costs assume cost of diesel of $1.49/L. CAPACITY AND EMBANKMENT HEIGHT ITEM CAPITAL COST ($) LIFE SPAN (y) EQUIVALENT ANNUAL CAPITAL COST ($) ANNUAL O&M COST ($) 1000 ML and 4.25 m Ringtank 925,000 40 69,400 9,250 Pumping infrastructure† 380,000 15 41,700 7,600 Pumping cost (diesel) NA NA NA 21,000‡ 4000 ML and 4.25 m Ringtank 1,725,000 40 129,400 17,250 Pumping infrastructure† 1,100,000 15 120,800 22,000 Pumping cost (diesel) NA NA NA 85,000‡ 4000 ML and 6.75 m Ringtank 3,330,000 40 249,800 33,300 Pumping infrastructure† 1,100,000 15 120,800 22,000 Pumping cost (diesel) NA NA NA 85,000‡ NA = not available. †Costs include rising main, large-diameter concrete or multiple strings of high density polypipe, control valves and fittings, concrete thrust-blocks and head-walls, dissipater, civil works and installation. ‡Value assumes water is piped between river pumping infrastructure and ringtank. Although ringtanks with an average water depth of 3.5 m (embankment height of 4.25 m) lose a higher percentage of their capacity to evaporative and seepage losses than ringtanks of equivalent capacity with average water depth of 6 m (embankment height of 6.75 m) (Table 5-11), their annualised unit costs are lower (Table 5-14) due to the considerably lower cost of constructing embankments with lower walls (Table 5-13). In Table 5-14 the levelized cost, or the equivalent annual cost per unit of water supplied from the ringtank takes into consideration net evaporation and seepage from the storage, which increase with the length of time water is stored (i.e. crops with longer growing seasons will require water to be stored longer). In this table, the results are presented for the equivalent annual cost of water yield from a ringtank of different seepage rates and lengths of time for storing water. Table 5-14 Levelized cost for two different capacity ringtanks under three seepage rates Assumes a 0.75-m freeboard, 3:1 ratio on upstream slope, 3:1 ratio on downstream slope. Crest widths are 3.1 m and 3.6 m for embankments with heights of 4.25 m and 6.75 m, respectively, and assumes earthfill and compacted clay costs $5/m3 and $6.50/m3, respectively. Earthwork costs include vegetation clearing, mobilisation/demobilisation of machinery and contractor accommodation. 1000-ML ringtank reservoir has surface area of 27 ha and storage volume to excavation ratio of about 7:1. 4000-ML ringtank and 4.25-m embankment height reservoir has surface area of 110 ha and storage volume to excavation ratio of about 14:1. 4000-ML ringtank with 6.75-m embankment height reservoir has surface area of 64 ha and storage volume to excavation ratio of about 7.5:1. CAPACITY AND EMBANKMENT HEIGHT ANNUAL- ISED COST† ($) SEEPAGE LOSS (mm/day) UNIT COST ($/ML) EQUIVALENT ANNUAL UNIT COST ($/y per ML/y) UNIT COST ($/ML) EQUIVALENT ANNUAL UNIT COST ($/y per ML/y) UNIT COST ($/ML) EQUIVALENT ANNUAL UNIT COST ($/y per ML/y) 5 months (April to August) 7 months (April to October) 10 months (April to January) 1000 ML and 4.25 m 117,100 1 1758 201 2133 244 2607 298 117,100 2 1862 213 2359 269 3133 358 117,100 5 2269 259 3457 395 7909 903 4000 ML and 4.25 m 284,500 1 951 126 1154 153 1411 187 284,500 2 1008 133 1277 169 1696 224 284,500 5 1228 163 1871 248 4280 567 4000 ML and 6.75 m 402,000 1 1492 172 1810 209 2213 255 402,000 2 1580 182 2002 231 2659 307 402,000 5 1925 222 2934 338 6712 774 †Assumes a 7% discount rate 5.4.5 Large farm-scale gully dams Large farm-scale gully dams are generally constructed of earth or earth and rockfill embankments with compacted clay cores and usually to a maximum height of about 20 m. Dams with a crest height of over 10 or 12 m typically require some form of downstream batter drainage incorporated into embankments. Large farm-scale gully dams typically have a maximum catchment area of about 30 km2 due to the challenges in passing peak floods from large catchments (large farm-scale gully dams are generally designed to pass an event with an annual exceedance probability (AEP) of 1%), unless a site has an exceptionally good spillway option. Like ringtanks, large farm-scale gully dams are a compromise between best-practice engineering and affordability. Designers need to follow accepted engineering principles relating to important aspects of materials classification, compaction of the clay core and selection of appropriate embankment cross-section. However, costs are often minimised where possible; for example, by employing earth bywashes and grass protection for erosion control rather than more expensive concrete spillways and rock protection as found on major dams. This can compromise the integrity of the structure during extreme events and the longevity of the structure, as well as increase the ongoing maintenance costs, but can considerably reduce the upfront capital costs. In this section the following assessments are reported: • suitability of the land for large farm-scale gully dams • indicative capital and O&M costs of large farm-scale gully dams. Net evaporation and seepage losses also occur from large farm-scale gully dams. The analysis presented in Section 5.4.4 is also applicable to gully dams. Suitability of land for large farm-scale gully dams Figure 5-34 provides an indication of where it may be more economical to construct large farm- scale gully dams in the Roper catchment and the likely density of options. This analysis takes into consideration those sites likely to have more favourable topography. It does not explicitly take into consideration those sites that are underlain by soil suitable for the construction of the embankment and to minimise seepage from the reservoir base. This is shown in Figure 5-35. In reality, dams can be constructed on eroded or skeletal soils provided there is access to a clay borrow pit nearby for the cut-off trench and core zone. However, these sites are likely to be less economically viable. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-34 Most economically suitable locations for large farm-scale gully dams in the Roper catchment Gully dam data overlaid on agricultural versatility data (see Section 4.3). Agricultural versatility data indicate those parts of the catchment that are more or less versatile for irrigated agriculture. For the gully dam analysis soil and subsurface data were only available to a depth of 1.5 m, hence this Assessment does not consider the suitability of subsurface material below this depth. Sites with catchment areas greater than 30 km2 or yield to excavation ratio less than 10 are not displayed. The results presented in this figure are modelled and consequently only indicative of the general locations where siting a gully dam may be most economically suitable. This analysis may be subject to errors in the underlying digital elevation model, such as effects due to the vegetation removal process. An important factor not considered in this analysis was the availability of a natural spillway. Site-specific investigations by a suitably qualified professional should always be undertaken prior to their construction. These figures indicate that those parts of the Roper catchment that are more topographically suitable for large-scale gully dam sites generally do not coincide with areas with soils that are moderately suitable for irrigated agriculture. Furthermore, in many areas topographically suitable for gully dams, dam walls would need to be constructed from rockfill, cement and imported clay soils, increasing the cost of their construction. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-35 Suitability of soils for construction of gully dams in the Roper catchment Capital, operation and maintenance costs of large farm-scale gully dams The cost of a large farm-scale gully dam, will vary depending upon a range of factors, including the suitability of the topography of the site, the size of the catchment area, quantity of runoff, proximity of site to good quality clay, availability of durable rock in the upper bank for a spillway and the size of the embankment. The height of the embankment, in particular, has a strong influence on cost. An earth dam to a height of 8 m is about 3.3 times more expensive to construct than a 4-m high dam, and a dam to a height of 16 m will require 3.6 times more material than the 8-m high version, but the cost may be more than 5 times greater, due to design and construction complexity. As an example of the variability in unit costs of gully dams, actual costs for four large farm-scale gully dams in northern Queensland are presented in Table 5-15. Table 5-15 Actual costs of four gully dams in northern Queensland Sourced from Northern Australia Water Resource Assessment technical report on farm-scale design and costs, Benjamin (2018). Costs indexed to 2021. DAM NAME LOCATION CAPACITY (ML) YIELD (ML/y) COST ($) UNIT COST ($/ML) COMMENT Sharp Rock Dam Lakelands 3300 1070 345,400 323 Chimney filter and drainage under-blanket. Two-stage concrete sill spillway. No fishway. Pump station not included Dump Gully Dam Lakelands 1450 420 841,000 2002 Deep and wet cut-off. Chimney filter and downstream under drainage. No fishway. Pump station was $91,000 Spring Dam #2 Lakelands 2540 1377 958,300 696 Chimney filter and drainage under-blanket. Two-stage rock excavation. Spillway with fishway. Fishway was $36,500. Pump station not included Ronny’s Dam Georgetown 9975 1700 479,250 281 Very favourable site. Low embankment and 450-ha ponded area. Natural spillway. No pump station, gravity supply via pipe Performance and cost of three hypothetical farm-scale gully dams in northern Australia A summary of the key parameters for three hypothetical 4-GL (i.e. 4000 ML) capacity farm-scale gully dam configurations is provided in Table 5-16 and a high-level breakdown of the major components of the capital costs for each of the three configurations is provided in Table 5-17. Detailed costs for the three hypothetical sites are provided in the Northern Australia Water Resource Assessment technical report on large farm-scale dams (Benjamin, 2018). Table 5-16 Cost of three hypothetical large farm-scale gully dams of capacity 4 GL Costs include government permits and fees, investigation and design and fish passage. For a complete list of costs and assumptions see Northern Australia Water Resource Assessment technical report on farm-scale dams (Benjamin, 2018). Costs indexed to 2021. SITE DESCRIPTION/ CONFIGURATION CATCHMENT AREA (km2) EMBANK- MENT HEIGHT (m) EMBANK- MENT LENGTH (m) S:E RATIO AVERAGE DEPTH (m) RESERVOIR SURFACE AREA (ha) TOTAL CAPITAL COST ($) O&M COST ($) Favourable site with large catchment, suitable topography and simple spillway (e.g. natural saddle) 30 9.5 1100 29:1 5.0 80 1,380,000 60,000 Unfavourable site with small catchment, challenging topography and limited spillway options (e.g. steep gully banks, no natural saddle) 15 14 750 21:1 6.3 63 1,590,000 38,000 Unfavourable site with moderate catchment, challenging topography and limited spillway options (e.g. steep gully banks, no natural saddle) 20 14 750 21:1 6.3 63 1,670,000 43,000 Table 5-17 High-level breakdown of capital costs for three hypothetical large farm-scale gully dams of capacity 4 GL Earthworks include vegetation clearing, mobilisation/demobilisation of equipment and contractor accommodation. Investigation and design fees include design and investigation of fish passage device and failure impact assessment (i.e. investigation of possible existence of population at risk downstream of site). Costs indexed to 2021. SITE DESCRIPTION/CONFIGURATION EARTHWORKS ($) GOVERNMENT PERMITS AND FEES ($) INVESTIGATION AND DESIGN FEES ($) TOTAL CAPITAL COST ($) UNIT COST ($/ML) Favourable site with large catchment, suitable topography and simple spillway (e.g. natural saddle) 1,247,000 40,000 93,000 1,380,000 345 Unfavourable site with small catchment, challenging topography and limited spillway options (e.g. steep gully banks, no natural saddle) 1,446,000 43,000 101,000 1,590,000 398 Unfavourable site with moderate catchment, challenging topography and limited spillway options (e.g. steep gully banks, no natural saddle) 1,526,000 43,000 101,000 1,670,000 418 Table 5-18 presents calculations of the effective volume for three configurations of 4-GL capacity gully dams (varying average water depth/embankment height) for combinations of three seepage losses and water storage capacities over three time periods in the Roper catchment. Table 5-18 Effective volumes and cost per ML for a 4-GL storage with different average depths and seepage loss rates at Wildman in the Roper catchment Time periods of 4, 6 and 9 months refer to length of time water is stored or required for irrigation. AVERAGE DEPTH AND RESERVOIR SURFACE AREA CON- STRUCTION COST ($) COST ($/ML) SEEPAGE LOSS (mm/d) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENT- AGE OF CAPACITY (%) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENT- AGE OF CAPACITY (%) EFFECTIVE VOLUME (ML) EFFECTIVE VOLUME AS PERCENT- AGE OF CAPACITY (%) 4 months (April to July) 6 months (April to September) 9 months (April to December) 3 m and 133 ha 1,067,000 250 1 3131 78 2693 67 2394 60 1,067,000 250 2 2990 75 2495 62 2111 53 1,067,000 250 5 2566 64 1901 48 1260 31 6 m and 66 ha 1,614,000 375 1 3567 89 3348 84 3198 80 1,614,000 375 2 3497 87 3250 81 3058 76 1,614,000 375 5 3287 82 2956 74 2637 66 9 m and 44 ha 2,152,000 500 1 3707 93 3559 89 3457 86 2,152,000 500 2 3660 91 3493 87 3362 84 2,152,000 500 5 3518 88 3295 82 3078 77 Based on the information presented in Table 5-16, an equivalent annual unit cost including annual O&M cost for a 4-GL gully dam with an average depth of about 6 m is about $186,900 (Table 5-19 and Table 5-20). Table 5-19 Cost of construction and operation of three hypothetical 4-GL gully dams Assumes operation and maintenance (O&M) cost of 3% of capital cost and a 7% discount rate. Figures have been rounded. AVERAGE DEPTH AND RESERVOIR SURFACE AREA ITEM CAPITAL COST ($) EQUIVALENT ANNUAL CAPITAL COST ($) ANNUAL O&M COST ($) EQUIVALENT ANNUAL COST ($) 3 m and 133 ha Low embankment wide gully dam 1,076,000 92,300 32,300 124,600 6 m and 66 ha Moderate embankment gully dam 1,614,000 138,500 48,400 186,900 9 m and 44 ha High embankment narrow gully dam 2,152,000 184,700 64,600 249,200 Table 5-20 Equivalent annualised cost and effective volume for three hypothetical 4-GL gully dams Dam details are in Table 5-19. Annual cost assumes a 7% discount rate. Time periods of 4, 6 and 9 months refer to length of time water is stored or required for irrigation. AVERAGE DEPTH AND RESERVOIR SURFACE AREA EQUIVALENT ANNUAL COST ($/y) SEEPAGE LOSS (mm/d) UNIT COST ($/ML) EQUIVALENT ANNUAL UNIT COST ($/y PER ML/y) UNIT COST ($/ML) EQUIVALENT ANNUAL UNIT COST ($/y PER ML/y) UNIT COST ($/ML) EQUIVALENT ANNUAL UNIT COST ($/y PER ML/y) 4 months (March to June) 6 months (March to August) 9 months (April to December) 3 m and 133 ha 124,600 1 344 40 400 46 449 52 124,600 2 360 42 431 50 510 59 124,600 5 419 49 566 66 854 99 6 m and 66 ha 186,900 1 453 52 482 56 505 58 186,900 2 462 53 497 58 528 61 186,900 5 491 57 546 63 612 71 9 m and 44 ha 249,200 1 581 67 605 70 623 72 249,200 2 588 68 616 71 640 74 249,200 5 612 71 653 76 699 81 Where the topography is suitable for large farm-scale gully dams and a natural spillway is present, large farm-scale gully dams are typically cheaper to construct than ringtanks. 5.4.6 Natural water bodies Wetland systems and waterholes that persist throughout the dry season are natural water bodies characteristic of large parts of the northerly draining catchments of northern Australia. Many property homesteads in northern Australia use natural waterholes for stock and domestic purposes. However, the quantities of water required for stock and domestic supply are orders of magnitude less than that required for irrigated cropping, and it is partly for this reason that naturally occurring persistent water bodies in northern Australia are not used to source water for irrigation. For example, a moderately sized 5-ha rectangular water body of average depth of 3.5 m may contain about 175 ML of water. Based on the data presented in Table 5-11 and assuming minimal leakage (i.e. 1 mm/day) approximately 74, 61 and 50% of the volume would be available if a crop were to be irrigated until August, October and January, respectively. Assuming a crop or fodder with a 6-month growing season requires 5 ML/ha of water before losses, and assuming an overall efficiency of 80% (i.e. the waterhole is adjacent to land suitable for irrigation, 95% conveyance efficiency and 85% field application efficiency), a 175-ML waterhole could potentially be used to irrigate about 20 ha of land for half a year if all the water was able to be used for this purpose. A large natural water body of 20 ha and average depth of 3.5 m could potentially be used to irrigate about 80 ha of land if all the water was able to be used for this purpose. Although the areas of land that could be watered using natural water bodies are likely to be small, the costs associated with storing water are minimal. Consequently, where these waterholes occur in sufficient size and adjacent to land suitable for irrigated agriculture, they can be a very cost- effective source of water. It would appear that where natural water bodies of sufficient size and suitable land for irrigation coincide, natural water bodies may be effective in staging a development (Section 6.3), where several hectares could potentially be developed, enabling lessons learned and mistakes made on a small-scale area before more significant capital investments are undertaken (note staging and learning is best to occur over multiple scales). In a few instances it may be possible to enhance the storage potential of natural features in the landscape such as horseshoe lagoons or cut-off meanders adjacent to a river. The main limitations to the use of wetlands and persistent waterholes for the consumptive use of water is that they have considerable ecological significance (e.g. Kingsford, 2000; Waltham et al., 2013), and in many cases there is a limited quantity of water contained within the water bodies. In particular, water bodies that persist throughout the dry season are considered key ecological refugia (Waltham et al., 2013). It should also be noted that where a water body is situated in a sandy river, the waterhole is likely to be connected to water within the bedsands of the river. Hence, during and following pumping water within the bedsands of a river, the bedsands may in part replenish the waterhole and vice versa. While water within the bedsands of the river may in part replenish a depleted waterhole, in these circumstances it also means that pumping from a waterhole will have a wider environmental impact than just on the waterhole from which water is being pumped. Figure 2-51 indicates the location of (1 km) river reaches containing waterholes that persist more than 90% of the time in the Landsat TM data archive in the Roper catchment. For the purposes of this Assessment they are referred to as ‘persistent’ waterholes. 5.5 Water distribution systems – conveyance of water from storage to crop 5.5.1 Introduction In all irrigation systems, water needs to be conveyed from the water source through artificial and/or natural water distribution systems, before ultimately being used on-field for irrigation. This section discusses water losses during conveyance and application of water to the crop and the associated costs. 5.5.2 Conveyance and application efficiencies Some water diverted for irrigation is lost during conveyance to the field before it can be used by a crop. These losses need to be taken into account when planning irrigation systems and developing likely irrigated areas. The amount of water lost during conveyance depends on the: • river conveyance efficiency, from the water storage to the re-regulating structure or point of extraction • channel distribution efficiency, from the river offtake to the farm gate • on-farm distribution efficiency, in storing (using balancing storages) and conveying water from the farm gate to the field • field application efficiency, in delivering water from the edge of the field and applying it to the crop. The overall or system efficiency is the product of these four components. Little research on irrigation systems has been undertaken in the Roper catchments. The time frame of the Assessment did not permit on-ground research into irrigation systems. Consequently, a brief discussion on the components listed above is provided based on relevant literature from elsewhere in Australia and overseas. Table 5-21 summarises the broad range of efficiencies associated with these components. The total conveyance and application efficiency of the delivery of water from the water storage to the crop (i.e. the overall or system efficiency) is dependent upon the product of the four components listed in Table 5-21. For example, if an irrigation development has a river conveyance efficiency of 80%, a channel distribution efficiency of 90%, an on-farm distribution efficiency of 90% and a field application efficiency of 85%, the overall efficiency is 55% (i.e. 80% * 90% * 90% * 85%). This means only 55% of all water released from the dam can be used by the crop. Table 5-21 Summary of conveyance and application efficiencies COMPONENT TYPICAL EFFICIENCY (%) River conveyance efficiency 50–90† Channel distribution efficiency 50–95 On-farm distribution efficiency 80–95 Field application efficiency 60–90 †River conveyance efficiency varies with a range of factors (including distance) and may be lower than the range quoted here. Under such circumstances, it is unlikely that irrigation would proceed. It is also possible for efficiency to be 100% in gaining rivers. Achieving higher efficiencies requires a re-regulating structure (see Section 5.4.3). River conveyance efficiency The conveyance efficiency of rivers is difficult to measure and even more difficult to predict. Although there are many methods for estimating groundwater discharge to surface water, there are few suitable methods for estimating the loss of surface water to groundwater. In the absence of existing studies for northern Australia, conveyance efficiencies as nominated in water resource plans and resource operation plans for four irrigation water supply schemes in Queensland were examined collectively. The results are summarised in Table 5-22. The conveyance efficiencies listed in Table 5-22 are from the water storage to the farm gate and are nominated efficiencies based on experience delivering water in these supply schemes. These data can be used to estimate conveyance efficiency of similar rivers elsewhere. Table 5-22 Water distribution and operational efficiency as nominated in water resource plans for four irrigation water supply schemes in Queensland WATER SUPPLY SCHEME IN QUEENSLAND TOTAL ALLOCATION VOLUME (ML) RIVER AND CHANNEL CONVEYANCE EFFICIENCY† (%) COMMENT Burdekin Haughton 928,579 78 The primary storage is the Burdekin Falls Dam (1860 GL), approximately 100 km upstream of Clare weir, the major extraction point. The Bowen River, a major unregulated tributary of the Burdekin River, joins the Burdekin River downstream of Burdekin Falls Dam. This may assist in reducing transmission losses between the dam and Clare weir. Lower Mary 34,462 94‡ The Lower Mary Irrigation Area is supplied from two storages, a barrage on the Mary River and a barrage on Tinana Creek. Water is drawn directly from the barrage storages to irrigate land riparian to the streams. Water distribution is predominantly via pipelines. Proserpine River 87,040 72 The scheme has a single source of supply, Peter Faust Dam (491 GL). At various distances downstream of the dam, water is extracted from the river bedsands and is distributed to urban communities, several irrigation water supply boards and individual irrigators. Upper Burnett 26,870 68 The Upper Burnett is a long run of river scheme with one major storage (Wuruma Dam (165 GL)) and four weir storages. The total river length supplied by the scheme is 165 km. †Ignores differences in efficiency between high and medium priority users and variations across the scheme zone areas. ‡Channel conveyance efficiency only. Channel distribution efficiency Across Australia, the average water conveyance efficiency from the river to the farm gate has been estimated to be 71% (Marsden Jacob Associates, 2003). For heavier textured soils and well- designed irrigation distribution systems, conveyance efficiencies are likely to be higher. In the absence of larger scheme-scale irrigation systems in the Roper catchment, it is useful to look at the conveyance efficiency of existing irrigation developments to estimate the conveyance efficiency of irrigation developments in the study area. Australian conveyance efficiencies are generally higher than those found in similarly sized overseas irrigation schemes (Bos and Nugteren, 1990; Cotton Catchment Communities CRC, 2011). The most extensive review of conveyance efficiency in Australia was undertaken by the Australian National Commission on Irrigation and Drainage, which tabulated system efficiencies across irrigation developments in Australia (ANCID, 2001). Conveyance losses were reported as the difference between the volume of water supplied to irrigation customers and the water delivered to the irrigation system. For example, if 10,000 ML of water was diverted to an irrigation district and 8,000 ML was delivered to irrigators, then the conveyance efficiency was 80% and the conveyance losses were 20%. Figure 5-36 shows reported conveyance losses across irrigation areas of Australia between 1999 and 2000, along with the supply method used for conveying irrigation water and associated irrigation deliveries. There is a wide spread of conveyance losses both between years and across the various irrigation schemes. Factors identified by Marsden Jacob Associates (2003) that affect the variation include delivery infrastructure, soil types, distance that water is conveyed, type of agriculture, operating practices, infrastructure age, maintenance standards, operating systems, in- line storage, type of metering used and third-party impacts such as recreational, amenity and environmental demands. Differences across irrigation seasons are due to variations in water availability, operational methods, climate and customer demands. Based on these industry data, Marsden Jacob Associates (2003) concluded that, on average, 29% of water diverted into irrigation schemes is lost in conveyance to the farm gate. However, some of this ‘perceived’ conveyance loss may be due to meter underestimation (about 5% of water delivered to provider (Marsden Jacob Associates, 2003)). Other losses were from leakage, seepage, evaporation, outfalls, unrecorded usage and system filling. For more information on this figure please contact CSIRO on enquiries@csiro.au 0% 15% 30% 45% 60% 0100,000200,000300,000400,000500,000600,000700,000800,000Losses 1999 to 2000 (percent) Irrigation deliveries 1999 to 2000 (ML) NSWQldSATasVicWA Figure 5-36 Reported conveyance losses from irrigation systems across Australia The shape of the marker indicates the supply method for the irrigation scheme: square (▪) indicates natural carrier, circle (•) indicates pipe, and diamond (♦) indicates channel. The colour of the marker indicates the location of the irrigation system (by state), as shown in the legend. Source: ANCID (2001) On-farm distribution efficiency On-farm losses are losses that occur between the farm gate and delivery to the field. These losses usually take the form of evaporation and seepage from on-farm storages and delivery systems. Even in irrigation developments where water is delivered to the farm gate via a channel, many farms have small on-farm storages (i.e. less than 250 ML for a 500-ha farm). These on-farm storages enable the farmer to have a reliable supply of irrigation water with a higher flow rate, and also enable recycling of tailwater. Several studies have been undertaken in Australia for on- farm distribution losses. Meyer (2005) estimated an on-farm distribution efficiency of 78% in the Murray and Murrumbidgee regions, while Pratt Water (2004) estimated on-farm efficiency to be 94% and 88% in the Coleambally Irrigation and Murrumbidgee Irrigation areas, respectively. For nine farms in these two irrigation areas, however, Akbar et al. (2000) measured channel seepage to be less than 5%. Field application efficiency Once water is delivered to the field, it needs to be applied to the crop using an irrigation system. The application efficiency of irrigation systems typically varies between 60 and 90%, with more expensive systems usually resulting in higher efficiency. There are three types of irrigation systems that can potentially be applied in the Roper catchment: surface irrigation, spray irrigation and micro irrigation (Figure 5-37). Irrigation systems applied in the Roper catchment need to be tailored to the soil, climate and crops that may be grown in the catchments and matched to the availability of water for irrigation. This is taken into consideration in the land suitability assessment figures presented in Section 4.2. System design will also need to consider investment risk in irrigation systems as well as likely returns, degree of automation, labour availability, and operation and maintenance costs (e.g. the cost of energy). Irrigation systems have a trade-off between efficiency and cost. Table 5-23 summarises the different types of irrigation systems, including their application efficiency, indicative cost and limitations. Across Australia the ratio of areas irrigated using surface, spray and micro irrigation is 83:10:7, respectively. Irrigation systems that allow water to be applied with greater control, such as micro irrigation, cost more (Table 5-23) and as a result are typically used for irrigating higher value crops such as horticulture and vegetables. For example, although only 7% of Australia’s irrigated area uses micro irrigation, it generates about 40% of the total value of produce grown using irrigation (Meyer, 2005). Further details on the three types of irrigation systems follow Table 5-23. (a) (b) (c) Figure 5-37 Efficiency of different types of irrigation systems (a) In bankless channel surface irrigation systems, application efficiencies range from 60 to 85%. (b) In spray irrigation systems, application efficiencies range from 75 to 90%. (c) For pressurised micro irrigation systems on polymer- covered beds, application efficiencies range from 80 to 90%. Photo: CSIRO Table 5-23 Application efficiencies for surface, spray and micro irrigation systems Application efficiency is the efficiency with which water can be delivered from the edge of the field to the crop. Costs indexed to 2021. IRRIGATION SYSTEM TYPE APPLICATION EFFICIENCY (%) CAPITAL COST ($/ha)† LIMITATIONS Surface Basin 60–85 4,350 Suitable for most crops; topography and surface levelling costs may be limiting factor Border 60–85 4,350 Suitable for most crops; topography and surface levelling costs may be limiting factor Furrow 60–85 4,350 Suitable for most crops; topography and surface levelling costs may be limiting factor Spray Centre pivot 75–90 3,200–7,000 Not suitable for tree crops; high energy requirements for operation Lateral move 75–90 3,200–6,400 Not suitable for tree crops; high energy requirements for operation Micro Drip 80–90 7,700–11,500 High energy requirement for operation; high level of skills needed for successful operation Adapted from Hoffman et al. (2007), Raine and Baker (1996) and Wood et al. (2007). †Source: DEEDI (2011a, 2011b, 2011c). Surface irrigation systems Surface irrigation encompasses basin, border strip and furrow irrigation, as well as variations on these themes such as bankless channel systems. In surface irrigation, water is applied directly to the soil surface with check structures (banks or furrows) used to direct water across a field. Control of applied water is dictated by the soil properties, soil uniformity and the design characteristics of the surface system. Generally, fields are prepared by laser levelling to increase the uniformity of applied water and allow ease of management of water and adequate surface drainage from the field. The uniformity and efficiency of surface systems are highly dependent on the system design and soil properties, timing of the irrigation water and the skill of the individual irrigator in operating the system. Mismanagement can severely degrade system performance and lead to systems that operate at poor efficiencies. Surface irrigation has the benefit that it can generally be adapted to almost any crop and usually has a lower capital cost compared with alternative systems. Surface irrigation systems perform better when soils are of uniform texture as infiltration characteristics of the soil play an important part in the efficiency of these systems. Therefore, surface irrigation systems should be designed into homogenous soil management units and layouts (run lengths, basin sizes) tailored to match soil characteristics and water supply volumes. High application efficiencies are possible with surface irrigation systems, provided soil characteristic limitations, system layout, water flow volumes and high levels of management are applied. On ideal soil types and with systems capable of high flow rates, efficiencies can be higher than 85%. On poorly designed and managed systems on soil types with high variability, efficiencies can be below 60%. Generally, the major cost in setting up a surface irrigation system is land grading and levelling, with costs directly associated with the volume of soil that must be moved. Typical earth-moving volumes are in the order of 800 m3/ha but can exceed 2500 m3/ha. Volumes greater than 1500 m3/ha are generally considered excessive due to costs (Hoffman et al., 2007). Surface irrigation systems are the dominant form of irrigation type used throughout the world. Their potential suitability in the Roper catchment would be due to their generally lower setup costs and adaptability to a wide range of irrigated cropping activities. They are particularly suited to the heavier textured soils found on the alluvial soils adjacent to the Roper River and its major tributaries, which reduce setup or establishment costs of these systems. With surface irrigation, little or no energy is required to distribute water throughout the field and this ‘gravity-fed’ approach reduces energy requirements of these systems. Surface irrigation systems generally have lower applied irrigation water efficiency than spray or micro systems when compared across an industry and offer less control of applied water; however, well-designed and well-managed systems can approach efficiencies found with alternative irrigation systems in ideal conditions. Spray irrigation systems In the context of the Roper catchment, spray irrigation refers specifically to lateral move and centre pivot irrigation systems. Centre pivot systems consist of a single sprinkler, laterally supported by a series of towers. The towers are self-propelled and rotate around a central pivot point, forming an irrigation circle. Time taken for the pivot to complete a full circle can range from as little as half a day to multiple days depending on crop water demands and application rate of the system. Lateral or linear move systems are similar to centre pivot systems in construction, but rather than move around a pivot point the entire line moves down the field in a direction perpendicular to the lateral. Water is supplied by a lateral channel running the length of the field. Lateral lengths are generally in the range of 800 to 1000 m. They are advantageous over surface irrigation systems as they can be utilised on rolling topography and generally require less land forming. Both centre pivot and lateral move irrigation systems have been extensively used for irrigating a range of annual broadacre crops and are capable of irrigating most field crops. They are generally not suitable for tree crops or vine crops or for saline irrigation water applications in arid environments, which can create foliage damage. Centre pivot and lateral move systems usually have higher capital costs but are capable of very high efficiencies of water application. Generally, application efficiencies for these systems range from 75 to 90% (Table 5-23). They are used extensively for broadacre irrigated cropping situations in high evaporative environments in northern NSW and south-west Queensland. These irrigation developments have high irrigation crop water demand requirements, which are similar to those found in the Roper catchment. A key factor in the suitable use of spray systems is sourcing the energy needed to operate these systems, which are usually powered by electricity or diesel depending on available costs and infrastructure. Where available, electricity is considerably cheaper than diesel for powering spray systems. For pressurised systems such as spray or micro irrigation systems, water can be more easily controlled, and potential benefits of the system through fertigation (the application of crop nutrients through the irrigation system (i.e. liquid fertiliser)), are also available to the irrigator. Micro irrigation systems For high-value crops, such as horticultural crops where yield and quality parameters dictate profitability, micro irrigation systems should be considered suitable across the range of soil types and climate conditions found in the Roper catchment. Micro irrigation systems use thin-walled polyethylene pipe to apply water to the root zone of plants via small emitters spaced along the drip tube. These systems are capable of precisely applying water to the plant root zone, thereby maintaining a high level of irrigation control and applied irrigation water efficiency. Historically, micro irrigation systems have been extensively used in tree, vine and row crops, with limited applications in complete-cover crops such as grains and pastures due to the expense of these systems. Micro irrigation is suitable for most soil types and can be practised on steep slopes. Micro irrigation systems are generally of two varieties: above ground and below ground (where the drip tape is buried beneath the soil surface). Below- ground micro irrigation systems offer advantages in reducing evaporative losses and improving trafficability. However, below-ground systems are more expensive and require higher levels of expertise to manage. Properly designed and operated micro irrigation systems are capable of very high application efficiencies, with field efficiencies of 80 to 90% (Table 5-23). In some situations, micro irrigation systems offer water and labour savings and improved crop quality (i.e. more marketable fruit through better water control). Management of micro irrigation systems, however, is critical. To achieve these benefits requires a much greater level of expertise than other traditional systems such as surface irrigation systems, which generally have higher margins of error associated with irrigation decisions. Micro irrigation systems also have high energy requirements, with most systems operating at pressure ranges from 135 to 400 kPa with diesel or electric pumps most often used. 5.6 Potential broad-scale irrigation developments in the Roper catchment 5.6.1 Introduction This section explores the feasibility and likely capital costs of potential broad-scale irrigation development in the Roper catchment. In order to do this, key lessons from other northern Australia irrigation developments are highlighted in Section 5.6.2. In Section 5.5.3, nominal conceptual reticulation scheme configurations were prepared for areas with soils potentially suitable for irrigated agriculture downstream of two potential dam sites with comparatively high yield per unit cost values. The per hectare costs of broad-scheme irrigation development calculated in this section are representative of the more cost-effective locations due to the selected areas having larger contiguous units of soil in close proximity to each other than most other potential locations in the Roper catchment. On-farm costs are discussed in Chapter 6. Generalised information (including costs) about water losses during conveyance and the application of water to the crop are provided in the companion technical report on surface water storage (Petheram et al., 2022). 5.6.2 Learnings from other northern Australian irrigation developments relevant to potential Roper catchment A number of larger scale irrigation developments in northern Australia in recent decades hold potential lessons for any potential irrigation development in the Roper catchment or elsewhere across northern Australia. The following discussion summaries experiences from four schemes: • Emerald Irrigation Scheme, Central Queensland – both in-situ derived basaltic soils and associated alluvial deposits along the Nogoa River • Burdekin Irrigation Scheme, north Queensland – a range of soil types on the Burdekin and Haughton river floodplains and associated upslope areas • Ord Stage 2, Kimberley region of WA – mostly clay alluvium deposits on the Weaber Plain • cane supplementation schemes, in particular Pioneer Valley Water Board and Proserpine Water Board – pumping from rivers and piped reticulation. Since the total river flow from a dam for a good proportion of the year will only comprise irrigation releases, providing adequate submergence for any river re-lift pumps will normally mean either a flow constriction or a constructed re-regulating weir. As some options could potentially be served by each parcel of irrigated land having its own pump site, this submergence requirement will be a major limitation. Infrastructure must be aligned to cater for flood flows in internal and adjacent catchments. This is more of an issue for schemes involving open-flow reticulation, rather than piped and pumped schemes, but will apply to some degree to all schemes. Farm units are best shaped by existing topography and soils distribution. This is especially the case for irrigation using spray systems, as spray system design can cater for reasonably irregular layouts. Hydrogeology is critical to long-term sustainability. That is, any irrigation system must have a mechanism to cater for the increased accessions to groundwater that are an unavoidable part of irrigation. This is mainly because accessions from rainfall are greater in the areas under irrigation than in dryland, due to the higher mean antecedent water in the profile of the soil. In this situation, riparian lands, above but adjacent to a river system, are normally better for irrigated agriculture than isolated lands without drainage incisions. Natural country slope also plays a part in this requirement. Water use efficiency needs to be designed at the start. For example, an open reticulation system should be designed with control structures, overflows and be implemented with Total Channel Control technology etc. Long systems involving substantial travel time can be inefficient and waste valuable water in operational overflows if these components are not included. 5.6.3 Exploration of feasibility and likely capital costs of potential broad-scale irrigation development in the Roper catchment To investigate the feasibility and likely costs of potential broad-scale irrigation development in the Roper catchment, nominal reticulated scheme configurations were developed for areas serviced by the two potential dam sites listed in Section 5.4.2. A more detailed description of the nominal reticulated scheme configurations and costs is provided in the companion technical report on surface water storage (Petheram et al., 2022). The digital soil modelling and land suitability analysis undertaken by the Assessment (Section 4.2 to Section 4.4) indicates that relatively limited soils are serviced directly by either potential storage site, so selection of the area served has been undertaken using the following principles: • Aggregation of suitable soils. The focus has been on areas with aggregations of suitable soils rather than isolated patches of suitable soils. The main target of potential development will be the alluvium adjacent to the streams being impounded, downstream of the storage site. • Proximity to source. The two potential storages are relatively modest in size. Proximity is important for two main reasons: (i) it limits the capital cost of transfer infrastructure to get the water from the source impoundment, whether by connector pipeline or channel, or downstream regulating structure and re-lift, and (ii) it limits losses in transferring the water from source to point of use in all cases other than the fully piped option. • Compatibility to topography. Both potential storages are in sections of the river where the stream is relatively incised, and hence distribution of water by releasing it downstream or by channel conveyance will only potentially serve areas further down the catchment. Distribution of water from the storages by pipeline has the potential to reach adjacent catchments, but at the expense of additional re-lift pumping. • Compatibility with a range of crop types. Preference will be given to soils suitable for a range of crop types rather than soils suitable for a limited suite of crops. The inescapable conclusion for both potential dam sites is that the potential for irrigation development in the immediate proximity to the dam sites is limited. Therefore, the means of conveyance of water from the storage to the development site will be the most crucial consideration in both cases. Areas serviced by the potential dam site on the Waterhouse River ATMD 70.5 km The potential dam site on the Waterhouse River and some of the hypothetical area downstream suited to irrigated agriculture are situated on Aboriginal land scheduled under the Commonwealth Aboriginal Land Rights (Northern Territory) Act 1976. It is near the community of Beswick and is classified under ‘inalienable freehold title’, which means that it cannot be bought, acquired or mortgaged, and is held by an Aboriginal land trust for traditional owners. Consequently the following analysis is speculative and is indicative of the best scheme-scale irrigated agriculture opportunity in the Roper catchment. The soils downstream of the potential Waterhouse River dam site can be characterised as follows: • Soils suitable for a broad range of cropping options are limited in the immediate vicinity of the dam site. The closest large contiguous areas of suitable soils are immediately to the north and east of Mataranka, some 55 km from the potential dam site. • Areas closer to the dam site exist in two main configurations: some areas adjacent to the river are in reasonably contiguous parcels, and a significant block of land along the Waterhouse River west branch is geographically close to the dam but in an adjacent catchment. • Soils targeted for development are mapped by the digital soils mapping as predominantly SGG 4.1 red loamy soils and SGG 2 friable non-cracking clay or clay loam soils. These are rated as suitability class 2 or 3 for a broad range of dry-season crops under spray, with less suitability to wet-season cropping or furrow application. The soils are particularly suited to perennial tree crops, dry-season cultivation of intensive horticulture and root crops under trickle and to a lesser extent spray, grain and fibre crops under spray, small-seeded crops, pulse crops and forage crops under spray. Three potential development themes are possible for use of the water from this potential dam site and are detailed in the companion technical report on water storage (Petheram et al., 2022). The adopted theme is centred on riparian development of suitable soils, extending as far as required to achieve the targeted gross areas. However, as the majority of the areas potentially suitable for irrigated agriculture are a long way downstream, significant conveyance losses would arise if reticulation were not piped. The design of the pipe reticulation was based on the following assumptions: • fully piped reticulation, based on 98% efficiency, to allow for initial filling and some minor system losses • spray irrigation, with assumed efficiency of 85% • net land usage of 95%, to allow for on-farm infrastructure etc. (it has been assumed that more detailed soils surveys may change the configuration of the suitable soils, but not the overall availability) • annual crop demand of 8 ML/ha allowing for a range of cropping options as outlined above. For the dam yield of ~90 GL/year, a net area of 9,580 ha will be targeted. This corresponds to a gross area of 10,100 ha, made up of areas 1 to 11 in Figure 5-38. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-38 Potential piped reticulated layout along the Waterhouse River A boosted pipeline reticulation arrangement at the dam outlet means that the storage operates by gravity at higher storage levels but is boosted by pumping at the dam outlet at lower levels. There is a practical limit to the extent this approach can be used, as the pipeline velocities get higher for higher levels of pressure boost, and consequent water hammer issues will become difficult to resolve. A 10 m boost was found to result in pipeline velocities below 2.5 m/second, where the surge issues would be controllable. The boosted arrangement at the dam outlet would operate so that it is only activated when pipeline pressures at the outlets fell below their prescribed value of 2 m residual head. A variable frequency drive (VFD) would allow tuning of the pump boost to the minimum required, subject to limits of the VFD’s operation. The pump would feature a non-return valve in the bypass, allowing gravity operation at all other times. A boosted arrangement allows the use of smaller pipe diameters than would be used in a gravity case, which reduces the overall cost of the reticulated scheme by about 12%. A preliminary costing for this design is about $13,233/ha for the 9560 ha of irrigated area. A breakdown of the conceptual layout is provided in Table 5-24. Table 5-24 Preliminary costs for nominal conceptual layout ITEM COST ($) Pipes supply and installation 91,195,500 Structures 3,705,500 Contractor overheads 5,030,000 Design and construction overheads 9,993,000 Contingency 16,488,500 Total 126,412,500 Area serviced by the potential dam site on upper Flying Fox Creek ATMD 105 km The major difference between the potential dam sites on the Waterhouse River and upper Flying Fox Creek is that the target areas for the potential upper Flying Fox Creek site are likely to feature clay soils, and hence soils that are less versatile in terms of the potential range of irrigated crops compared to the sandy loamy soils downstream of the potential Waterhouse River dam site. The soils downstream of the potential upper Flying Fox Creek dam site can be characterised as follows: • A limited area of suitable soils exists some 10 km south of the Central Arnhem Highway. However, this is a section where the river is braided into at least six channels, and most of the suitable area is within the braided sections, limiting their usefulness due to flooding risk. • The largest aggregation of suitable soils is much further downstream, some 42 km south of the Central Arnhem Highway, where the creek runs to the northern side of a large area of clay soils. • Soils targeted for development are mapped by the companion technical report on digital soil mapping (Thomas et al., 2022) as predominantly SGG 2 friable non-cracking clay or clay loam soils and SGG 9 cracking clay soils. These are rated as suitability Class 3 for a broad range of dry- season crops under spray and furrow, with less suitability to wet-season cropping. They are particularly suited to dry-season cultivation of intensive horticulture under trickle and to a lesser extent spray, grain and fibre crops under spray and furrow, small-seeded crops, pulse crops, and forage crops under spray. Some of the soils are also rated as suitable for wet-season cultivation of rice and industrial crops. The major challenge in serving this area of soils is the fact that the suitable soils are some 53 km below the potential dam site. Distribution of water this far, without substantial demand on the way, will not be economical. This only leaves river distribution with a re-regulating structure as the likely mode of development. While this will result in significant losses in transmission, there is the potential to pick up additional yield from inflow from the intervening catchment, in particular that from Derim Derim Creek and Maori Creek. The potential re-regulating structure is assumed to be located on Flying Fox Creek ATMD 36 km. Other elements of the potential scheme for service of this area are: • a pump station at the re-regulating structure able to meet the full demand of the channel distribution network • an associated rising main, of sufficient length to (i) reach an elevation where the required area can be served by a gravity channel system, and (ii) extend far enough that the cross slope, which is very steep near the river, is flat enough to allow practical construction of an open-channel system. In practical terms, this indicates a cross slope below about one in six. • an open-channel distribution system along the western edge of the serviced area. A significant feature of this channel system will be allowance for cross drainage from the upslope catchments. This will be by way of both cross-drainage culverts and drainage overpasses featuring inverted siphons. The former will allow the gradeline to be maintained, whereas the latter will involve a reduction in gradeline due to the head loss associated with the piped section of the inverted siphon under the overpass. Since the main limitation to this area will be wetness related to river flooding, there will be incentive to maintain the gradeline as high as possible. Areas were chosen for development based on the following: • suitability for a broad range of crop types; however, the base case chosen for the selection was Crop Type 7 (Table 4-2) under dry-season spray irrigation • allowance for the drainage network that will be required to get flows from above the channel through the developed area • a preference for regular-shaped areas that may be suited to both spray and furrow irrigation. Note that this implies the inclusion of some minor areas of Class 4 soils. The area able to be irrigated is calculated as 5200 ha, based on the following assumptions: • Available water yield is assumed as 80% of 68 GL. Note that this assumes the contribution from the intervening catchments is minor, with the majority of flows from the intervening catchment released for environmental flow purposes. • Crop demand is assumed as 8 ML/ha. • Irrigation efficiency is assumed as 85% for spray. • Channel distribution efficiency is assumed as 90%. Note that this reflects the relatively short system and assumes some supervisory control system, such as Total Channel Control, is implemented. This corresponds to a gross area of some 5485 ha, assuming a 95% land usage factor. The location of the serviced areas and the alignment of the channel distribution system are shown in Figure 5-39. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 5-39 Nominal conceptual layout of potential irrigation area on Flying Fox Creek Cross drainage will be a major consideration for the channel distribution system, since it is essentially aligned to accrue water draining into the river system. The channel will also require a number of control points to allow the water level to be maintained at a minimum level in the channel at all times of operation, to minimise both erosion potential during increases in flow and weed growth. Contributing catchments that need to be safely passed across the channel alignment and align with downstream drainage lines are shown in Figure 5-39. The combined cost of both the earthworks components and the rising main and pump station is some $29.9 million, equivalent to some $5700/ha of the 5200 ha of serviced land (Table 5-25). 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