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 Part II Resource information for assessing potential development opportunities Chapters 2 and 3 provide baseline information that readers can use to understand what soils and water resources are present in the Roper catchment and the current living and built environment of the Roper catchment. This information covers: •the physical environment (Chapter 2) •the people, ecology and institutional context (Chapter 3). Cattle grazing on rangelands Photo: CSIRO – Nathan Dyer 2 Physical environment of the Roper catchment Authors: Justin Hughes, Andrew R Taylor, Seonaid Philip, Steve Marvanek, Peter Wilson, David McJannet, Shaun Kim, Bill Wang, Cuan Petheram, Russell Crosbie and Ian Watson Chapter 2 examines the physical environment of the catchment of the Roper River and seeks to identify the available soil and water resources. It provides fundamental information about the geology, soil, climate and the river and groundwater systems of the catchment. These resources underpin the natural environment and existing industries, providing physical bounds to the potential scale of irrigation development. Key components and concepts are shown in Figure 2-1. Figure 2-1 Schematic diagram of key natural components and concepts in the establishment of a greenfield irrigation development C Petheram 3D 2_5_2018 For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Numbers in blue refer to sections in this report. 2.1 Summary This chapter provides a resource assessment of the geology, soil, climate, groundwater and surface water resources of the Roper catchment. No attempt is made in this chapter to calculate physically plausible areas of land or volumes of water that could potentially be used for agriculture or aquaculture developments. These analyses are reported in chapters 4 and 5. 2.1.1 Key findings Soils The soils with potential for agriculture in the Roper catchment are dominated by red loamy soils (35% of the catchment), which are principally found on the Sturt Plateau. These well-drained soils have moderate to high agricultural potential with spray or trickle irrigation, although low to moderate water-holding capacity and hard-setting surface soils are common constraints. Cracking clay soils represent just over 10% of the catchment. These are mostly found on floodplains and other alluvial plains. They typically have a moderate to high agricultural potential, although flooding risk and high salt levels within the profile are common constraints. Friable, non-cracking clay loam soils (9% of catchment) and brown, yellow and grey loamy soils (8% of catchment) also make up substantial areas. The former of these have generally high agricultural potential while the latter have moderate to high agricultural potential. Shallow and/or rocky soils make up just over 35% of the catchment. Climate The Roper catchment has a hot and arid climate. The catchment has a highly seasonal climate with an extended dry season. It receives, on average, 792 mm of rain per year, 96% of which falls during the wet season. Mean daily temperatures and potential evaporation are high relative to other parts of Australia. On average, potential evaporation is approximately 1900 mm/year. Overall, the climate of the Roper catchment generally suits the growing of a wide range of crops, though in most years rainfall would need to be supplemented with irrigation. The variation in rainfall from one year to the next is moderate compared to elsewhere in northern Australia yet is high compared to other parts of the world with similar mean annual rainfall. The length of consecutive dry years is not unusual in the Roper catchment and the intensity of the dry years is similar to many centres in the Murray–Darling Basin and east coast of Australia. Since 1969–1970, the Roper catchment experienced one tropical cyclone in 40% of cyclone seasons and two tropical cyclones in 8% of seasons. Approximately 16% of the global climate models (GCMs) from the sixth Coupled Model Intercomparison Project (CMIP6) project an increase in mean annual rainfall by more than 5% for a 1.6 C increase in temperature relative to approximately 1990 global temperatures, 28% project a decrease in mean annual rainfall by more than 5% and 56% indicate ‘little change’ (i.e. within 5%). Surface water and groundwater The timing and event-driven nature of rainfall events and high potential evaporation rates across the Roper catchment have important consequences for the catchment’s hydrology. Approximately 97% of runoff occurs during the wet season, with 80% of all runoff occurring during the 3-month period from January to March, which is very high compared to southern Australia. This means that in the absence of groundwater, water storages are essential for dry-season irrigation. The major aquifers in the Roper catchment occur within dissolution features in the Cambrian Limestone Aquifer (CLA) in the south-west and the Dook Creek Aquifer (DCA) in the north-east of the catchment. The CLA is a complex, interconnected and highly productive regional-scale groundwater system (area of about 460,000 km2) and it extends for hundreds of thousands of square kilometres west, south and east of the south-western boundaries of the Roper catchment. Mean annual volumetric recharge over the entire CLA and that part of the CLA within the Roper catchment is calculated to be 995 and 243 GL/year respectively. Bore yields are variable given the complex nature of the karstic aquifer but yields often range between 15 and 45 L/second. The DCA is an intermediate-scale groundwater system and like the CLA is complex, due to the variability and interconnectivity between fractures, fissures and karsts. The DCA extends for over a thousand square kilometres to the north-east of the Roper catchment boundary and relative to the CLA, little information exists. Currently about 31.8 GL of water is licensed to be extracted from the CLA (see Section 3.3.4) and the only water extracted from the Dook Creek Formation is for stock and domestic and town water supply. The median and mean annual discharge from the Roper catchment into the Gulf of Carpentaria is 4341 and 5560 GL, respectively. The majority of streamflow, however, occurs below Red Rock, where the Wilton and Hodgson rivers flow into the Roper River. At Red Rock on the Roper River, the median and mean annual flow is 1925 and 2414 GL, respectively. Current surface water licences total about 0.1 GL (i.e. 0.002% of median annual flow). Many rivers in the catchment are ephemeral, particularly those in the southern parts of the catchment and are reduced to a few scarce and vulnerable waterholes during the dry season. Some waterholes and river reaches, particularly those in the main Roper channel downstream of Mataranka, are permanent and are replenished by groundwater (see Section 3.2). 2.1.2 Introduction This chapter seeks to address the question ‘What soil and water resources are available for irrigated agriculture in the Roper catchment?’ The chapter is structured as follows: • Section 2.2 examines the geology of the Roper catchment, which is important in understanding the distribution of groundwater, soil and areas of low and high relief, which influences flooding and the deposition of soil. • Section 2.3 examines the distribution of soils in the Roper catchment, their attributes and discusses management considerations. • Section 2.4 examines the climate of the Roper catchment, including historical and future projections of patterns in rainfall. • Section 2.5 examines the groundwater and surface water hydrology of the Roper catchment, including groundwater recharge, streamflow and flooding. 2.2 Geology and physical geography of the Roper catchment 2.2.1 Geological history Geological history represents the major periods of deposition and tectonics (i.e. major structural changes), as well as weathering and erosion. These processes are closely linked to the physical environment that influences the evolution and formation of resources such as valuable minerals, coal, groundwater and soil. Geology also controls topography, which in turn is a key factor in the location of potential dam sites, flooding and deposition of soil. These resources are all important considerations when identifying suitable locations for large water storages and understanding past and present ecological systems and patterns of human settlement. The oldest rocks in the area are of Proterozoic eon (2500 to 540 million years old) and consist of repeated thick sequences of sediments and volcanics that include numerous prominent beds of sandstone (Figure 2-2). They were deposited in a series of basins extending across the area and then folded, faulted and intruded by igneous rocks to form mountain chains. Towards the end of the Proterozoic the mountain chains had been eroded down to a level not far above that of the current topography. During the Cambrian period (540 to 485 million years ago) there was widespread extrusion of basalt lava, which was followed by deposition of limestones and dolomites. The Cambrian strata only occur south-west of the Roper River where the limestones and dolomites are affected by karst (solution effects producing underground cave systems) and provide an important regional groundwater source. Erosion recommenced after the Cambrian and continued to the mid-Cretaceous period (about 100 million years ago) when subsidence and high global sea levels resulted in deposition of a thin succession of Cretaceous shallow marine sandstone, conglomerate and mudstone across the Roper catchment. The present landscape has been produced by warping and dissection of a series of erosion surfaces formed during several cycles of erosion that started in the Late Cretaceous about 70 million years ago and ended in the mid-Cenozoic era about 25 million years ago. During this time, stable crustal conditions and subaerial exposure led to patchy erosion of the Cretaceous rocks and prolonged subaerial weathering of the remaining Cretaceous and Proterozoic rocks and resulted in the formation of deep weathering profiles and associated iron-cemented capping. Between the mid-Cenozoic and the present day, there has been gentle uplift and warping of the various surfaces and their weathered capping. Continued erosion has led to the emergence of the present-day landscape, which involved the removal of Cretaceous strata from most of the region and the etching out of structures in the underlying Proterozoic rocks (mainly Roper Group). Extensive floodplains and coastal deposits were built up on the margins of modern drainage systems and the coastline, respectively, in the region. For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-2 Surface geology of the Roper catchment Adapted from Raymond (2012) 2.2.2 Surface geology of the Roper catchment The geological controls outlined in Section 2.2.1 have resulted in four physiographic regions described in Plumb and Roberts (1992), (i) the Cretaceous Tableland (henceforth, Sturt Plateau) in the south-west, (ii) the Gulf Fall in the centre (Figure 2-3), (iii) Wilton River Plateau in the north, and (iv) the Coastal Plain, which are shown in Figure 2-4. The Sturt Plateau is a tableland dominated by Cretaceous sediments and Tertiary lateritic surfaces with interspersed red earthy colluvium and localised clay alluvium (Abbott et al., 2001; Aldrick and Wilson, 1992; Burgess et al., 2015; Day et al., 1984). The dissected Gulf Fall physiographic region occupies most lands from the eastern edge of the Sturt Plateau to the estuarine Coastal Plain and comprises residual rises and hills, strike ridges, mesas and plateaux and intervening fluvial valleys (Abbott et al., 2001). This province is a complex landscape composed of sandstones, mudstones, siltstone and dolerite lithologies along with extensive areas of colluvium and alluvium (Andrews and Burgess, 2021). The Gulf Fall physiographic region has the most suitable topography for instream water storage structures in the Roper catchment. The Wilton River Plateau, located in the northern part of the catchment, is composed of a level to gently undulating sandstone plateau, and the Coastal Plain extends east of the Gulf Fall as an extensive area of salt flats, tidal flats and mangroves. For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-3 The Gulf Fall comprises residual rises and hills, strike ridges, mesas and plateaux and intervening fluvial valleys Source: CSIRO – Nathan Dyer Physiographic unit map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\10_Reporting\3_Roper\1_GIS\1_Map_docs\CR-R-Ch2_500_physiographic_v1-10_10-8.mxd For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-4 Physiographic provinces of the Roper catchment Physiographic areas based on Plumb and Roberts (1992). Significant settlements and roads overlaid on hillshaded terrain relief. Potential dam sites occur where resistant ridges of Proterozoic sandstone beds that have been incised by the river systems outcrop on both sides of river valleys (Petheram et al., 2023). 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 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 site is the extent and depth of 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 (Petheram et al., 2023). Where potentially soluble dolomites occur within the Proterozoic sequences (soluble over a geological timescale), it is possible that potentially leaky dam abutments and reservoir rims may be present, which will require specialised and costly foundation treatment such as extensive grouting. 2.2.3 Major hydrogeological provinces of the Roper catchment In terms of groundwater in the Roper catchment, three major hydrogeological provinces exist: (i) the McArthur Basin, which underlies the centre, north and east of the catchment; (ii) the connected Daly, Wiso and Georgina basins, which overlie the McArthur Basin in the south to south-west of the catchment; and (iii) the Carpentaria Basin, which overlies most of the Daly, Wiso and Georgina basins in the south to south-west of the catchment but also part of the McArthur Basin in the west (Figure 2-5). The broad major rock types associated with each geological province include igneous and sedimentary rocks (McArthur Basin, connected Daly, Georgina and Wiso basins) and unconsolidated (surficial regolith) to consolidated sediments (Carpentaria Basin) (Figure 2-5). The most significant groundwater resource occurs in the connected Daly, Wiso and Georgina basins, which underlie approximately 27,500 km2 of the Roper catchment. Collectively though, the Daly, Wiso and Georgina basins extend west, south and east of the Roper catchment covering a total area of approximately 460,000 km2 (see Section 2.5.2). The basins all vary in thickness of between generally about 80 and 300 m in the Roper catchment, though the Georgina Basin south of Daly Waters can be up to about 400 m thick. The upper fractured, fissured and karstic parts of the carbonate rocks host a regional interconnected groundwater system that is complex but highly productive. This groundwater system, often referred to as the CLA, is the largest and most important groundwater resource and has been partly developed for groundwater-based irrigated agriculture and town and community water supplies (for example, Mataranka). All three basins that host these carbonate rocks are almost entirely overlain by the Carpentaria Basin except for a very small part of the limestone that outcrops at the surface around Mataranka (Figure 2-2). The McArthur Basin is a geological province underlain by about a 10-km thick sequence of sedimentary rocks that in places are intruded (i.e. broken through) by minor igneous rocks of Precambrian age (Paleoproterozoic to Mesoproterozoic). The McArthur Basin extends well beyond the Roper catchment and is bound to the north and east by the Arafura Sea and Gulf of Carpentaria, respectively. To the south it is bound by the Tomkinson Province and to the west by the Pine Creek Orogen. In the Roper catchment, the McArthur Basin is undulating with isolated ranges of quartzite and igneous rocks dissected by river valleys. Topographic features include the Shadforth and McKay hills in the north; the Strangman and Bold ranges and Collara Mountains in the centre; and the Hartz, Downers and High Black ranges south of the Roper River. The rocks of the McArthur Basin have been intruded with dolerite, folded, faulted and uplifted, and subject to long periods of erosion (both physical and chemical weathering) since they were formed. Most of the sedimentary and igneous rocks of the McArthur Basin have very low primary porosity (<2%), with pores that are very small and not interconnected. Consequently, they do not hold or yield much groundwater and can be impermeable across large areas. Where the upper parts of the sedimentary and igneous rocks are weathered and fractured, they can contain volumes of water that, while not large, can have local importance for stock and domestic use as well as community water supplies (for example, Minyerri). However, the fractured, fissured and karstic carbonate rocks of the Mount Rigg Group (Dook Creek Formation) present in the northern part of the McArthur Basin (Proterozoic dolostone) do contain intermediate-scale aquifers with small volumes of good quality groundwater currently used for community water supplies (Beswick, Barunga and Bulman) (Figure 2-2). The fractured, fissured and karstic carbonate rocks and the porous sandstone of the Nathan Group (Knuckey Formation and Mount Birch Sandstone) and the Bukalara Sandstone of the McArthur Basin (Proterozoic dolostone and sandstone) contain local- scale aquifers of good quality water. The Carpentaria Basin sediments are mostly sandstone, siltstone and claystone, which can be up to 100 m thick in the Georgina Basin. The uppermost part (i.e. top 5 m below the ground surface) of the Carpentaria Basin sediments forms a blanket of surficial unconsolidated sediments (regolith) that cover the rocks of most of the Daly, Wiso and Georgina basins as well as small parts of the McArthur Basin in the north and east (Figure 2-5). Most of the rocks and sediments of the Carpentaria Basin also have very low primary porosity and do not hold much groundwater. Where parts of these rocks and sediments mostly comprise sand, sandstone or gravel, however, they can contain volumes of water that, while not large, can have local importance. Unconsolidated alluvial sediments (i.e. sand, silt or clay transported and deposited at some stage by flowing surface water) are sparse across most of the Roper catchment. Occasionally, they are present in conjunction with the Roper River and its tributaries, their channels and floodplains. However, the largest occurrence of unconsolidated alluvial sediments occurs at the mouth of the Roper River in association with the Limmen Bight Tidal Wetlands (see Section 3.2.2). There are very few groundwater bores and little information associated with these sediments but given their limited extent and thickness, and proximity to the coast, they appear to host little groundwater suitable for potential use. For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-5 Major geological provinces of the Roper catchment Source: Adapted based on Raymond (2018) 2.3 Soils of the Roper catchment 2.3.1 Introduction Soils in a landscape occur as complex patterns resulting from the interplay of five key factors: parent material, climate, organisms, topography and time (Fitzpatrick, 1986). Consequently, soils can be highly variable across a landscape, with different soils having different attributes that determine their suitability for growing different crops and guide how they need to be managed. The distribution of these soils and their attributes closely reflect the geology and landform of the catchments. Hence data and maps of soil and soil attributes, which provide a spatial representation of how soils vary across a landscape, are fundamental to regional-scale land use planning. This section briefly describes the spatial distribution of soil groups (Section 2.3.2) and soil attributes (Section 2.3.3) in the Roper catchment. The management considerations for irrigated agriculture are also summarised (Table 2-1). Maps showing the suitability for different crops under different irrigation types in different seasons are presented in Chapter 4. Unless otherwise stated, the material in Section 2.2 is based on findings described in the companion technical report on digital soil mapping and land suitability (Thomas et al., 2022). Soils and their attributes were collected and described adhering to Australian soil survey standards (National Committee on Soil and Terrain, 2009). 2.3.2 Soil characteristics The soils of the Roper catchment were classified into soil generic groups (SGGs) (Figure 2-6; Table 2-1; Table 2-2). These groupings provide a means of aggregating soils with broadly similar properties and management considerations. The different soils have different potential for agriculture, some with almost no potential, such as the shallow and/or rocky soils (SGG 7, Table 2-1) and some with moderate to high potential (e.g. SGG 9) depending on other factors such as flooding and the amount of salt in the profile. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-6 The soil generic groups (SGGs) of the Roper catchment produced by digital soil mapping The inset map shows the data reliability, which for SGG mapping is based on the confusion index as described in the companion technical report on digital soil mapping and land suitability (Thomas et al., 2022). Table 2-1 Soil generic groups (SGGs), descriptions, management considerations and correlations to Australian Soil Classification (ASC) for the Roper catchment Figure 2-6 shows the distribution of the SGGs within the Roper catchment while Table 2-2 provides the areas, in hectares, within the catchment. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au The Roper catchment contains soils from all ten SGGs with the exception of peaty soils (SGG 5). Of those nine SGGs found in the catchment, only three of them occupy more than 10% of the area and together these soils represent 81% of the catchment (Table 2-2). The three SGGs which make up this 81% are the red loamy soils, principally of the Sturt Plateau (SGG 4.1, 35.1%), shallow and/or rocky soils principally found throughout the central parts of the catchment (SGG 7, 35.3%), and cracking clay soils typically found along the rivers and other alluvium (SGG 9, 10.1%). Soil colour is, in general, a useful indicator of historical drainage status. Red soils are generally well-drained, whereas yellows, greys and even bluey-greens indicate increasingly persistent wetness, and ultimately, permanent waterlogging. Mottles indicate cycling between wetting and drying soil conditions, indicating the presence of imperfect drainage and seasonal inundation. Table 2-2 Area and proportions covered by each soil generic group (SGG) for the Roper catchment For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au na = not applicable, not found in the catchment SGG 4 are the moderately deep to very deep loamy soils, divided by colour reflecting their landscape position and soil properties (Table 2-1; Table 2-2; Figure 2-6). The red loamy soils (SGG 4.1, 35.1% of the study area) and brown, yellow and grey loamy soils (SGG 4.2, 8.2%), occur extensively on the Sturt Plateau (Figure 2-7) and a variety of other sandstone geologies and landforms throughout the Roper catchment. The level to gently undulating deeply weathered sediments in the Roper catchment usually have sandy to loamy surfaced well-drained red soils (SGG 4.1) on the lower slopes and level infilled plains, while moderately well-drained to imperfectly drained brown and yellow soils (SGG 4.2) occur on the plains or drainage depressions where water tends to accumulate, and on lower slopes to upper landscape positions due to subsurface duricrusts restricting internal drainage. The depth to iron pans and the amount of iron nodules relates to position in the landscape. Exposed laterite is common. Deeper soils with little rock or ironstone gravels that have resulted from the redistribution of erosion products into the lower landscape positions are highly suited to irrigated agriculture and horticulture. In some locations, narrow or small areas in the landscape may limit infrastructure layout and consequently agricultural opportunities. SGG 4 soils are usually nutrient deficient, hence irrigated cropping requires very high fertiliser inputs when soils are initially cultivated. After the initial high application, fertiliser rates follow recommended crop requirements. Irrigation potential is limited to spray and trickle-irrigated crops on the moderately deep to deep soils with low to high soil water storage (70 to 140 mm) and fewer iron nodules. Narrow levees adjacent to the major rivers, tributaries and prior streams on the alluvial plains throughout the catchment have very deep (>1.5 m) well-drained massive soils with sandy and loamy surfaces over red (SGG 4.1), brown and yellow (SGG 4.2) loam to clay subsoils. Soils are highly suited to irrigated agriculture but the narrow, ribbon form in the landscape may limit infrastructure layout and consequently agricultural opportunities. The lower slopes (<5%) of pediments derived from sandstones and siltstones in the upper catchment usually have moderately deep (0.5 to 1 m), moderately well-drained to imperfectly drained, sandy to loamy surfaced, yellow and brown (SGG 4.2) massive soils with abundant rock fragments occurring frequently throughout the profile. Moderately deep to very deep (0.5 to >1.5 m), well-drained to imperfectly drained, red (SGG 4.1) and mottled yellow (SGG 4.2), loose sandy to hard-setting loamy surfaced massive soils occur in association with friable loamy soils (SGG 2) on the gently undulating rises and plains developed over the Tindall Limestone around Mataranka, extending north-west towards Katherine. Soils occur as a mosaic over the landscape, probably reflecting the depth to the underlying rock with red soils on the deeper areas. This group of soils overlying limestone probably originated from the redistribution of erosion products from the Sturt Plateau. These moderately permeable soils have moderate to high soil water storage and are highly suited to a broad range of irrigated crops. SGG 4.1 soil landscape photo For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Description automatically generated Figure 2-7 Red loamy soil (SGG 4.1) on the Sturt Plateau Source: CSIRO SGG 9, the slowly permeable cracking clay soils (10%), occur on the alluvial plains associated with the Roper River and major rivers draining to the Roper in the north (Table 2-1; Table 2-2; Figure 2-8) and as relict alluvium throughout the Sturt Plateau, often occurring as internal drainage depressions. These very deep (>1.5 m), predominantly imperfectly drained, slowly permeable brown to grey cracking clays are usually strongly sodic at depth with soft self-mulching or hard- setting surfaces. Soils have high to very high water-holding capacity but may have a restricted rooting depth due to very high salt levels in the subsoil. The self-mulching and structured brown and grey cracking clay soils are suitedto a varietyofdry-season grain,forage and pulse crops(Figure2-8). The clayplains of the Ropercatchment are subject to regularflooding and frequentlyhave small (<0.3m) gilgai depressions andnumerous flood channels.These soils onthe alluvial plains grade to seasonally wet soils (SGG3) inthe lower partsof thecatchment below Ngukurr. TheTertiary clay plainsof theSturtPlateau oftenhave large deep gilgai (>0.3m)up to 0.8m deep that willlimit development potential duetotheexcessivelevelling that is required forefficient irrigationpractices. Gravels frequently occur onthe mounds. SGG 9 landscape photo For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Description automatically generated Figure2-8Large areas of brown Vertosols (SGG 9) on alluvial plains along the major rivers are suited to irrigatedgrain and pulse crops, forage crops, sugarcane and cotton Source: CSIRO SGG 7 is the shallow (<0.5m) and/or stony soilsoccurring extensively(35%of thecatchment) throughoutthe mid-to upper catchment on sandstones, siltstones, mudstones,basalts, doleritesand limestones,and exposed lateritic and duricrust surfaces of the deeplyweathered sedimentson the Sturt Plateau(Table2-1; Table2-2; Figure2-6). All shallow and gravelly/stony soilshavevery low to low soil water storage,often onslopes subject to erosionrisk, and areoften found infragmented landscapesdueto intensedrainagepatterns and have limited potential foragricultural development. Very shallow (<0.25m) soilsalso occur extensively as sandy and loamysoils withabundant sandstone or lateritic gravelsandrock outcrop on therises and scarp areasof the dissected quartz sandstonehills and dissected plateauxof the deeplyweathered sediments. Similarly, very shallow (<0.25m) soils with abundant iron nodules, ironpans and exposed lateritealso occuron the eroded edges of theplains and upper slopesof rises of thedeeply weathered Tertiary sediments. Most of the Calcarosols on the limestones are shallow (<0.5m), with abundantrock outcrop, includingthe mound springsassociated with the Tindall Limestone around Mataranka. SGG 2, the friable clays and clay loam soils, occupy 9%of the catchment (Table2-1; Table2-2; Figure2-6). Deep(1.0–1.5m) hard-setting loamysurfaced soilsover friable mottled yellow andbrown clay subsoils occur inthe Elsey Creek andHodgson Creek subcatchmentsand to a limited 36|Water resource assessmentforthe Ropercatchment extent on the Sturt Plateau. Large areas of seasonally wet brown friable clay loam soils occur in the north of the catchment on alluvial plains. The soils are suitable for irrigated agriculture and horticultural crops, depending on soil wetness, slope and amount of rock. Moderately deep (0.5–1.0 m) red friable clays are limited to basic rocks (basalt and dolerite) in the undulating to steep rises and hills of the southern catchment. Scattered stone and boulders often occur. The soils are suitable for cropping and horticultural tree crops. Moderately deep to deep soils (0.5–1.5 m) with few stones or boulders occur on gentle plains, rises and pediments but are usually highly fragmented due to drainage lines and short slope lengths between rock outcrops. Relatively large areas (e.g. 100 ha) are usable for cropping and horticultural land uses. Very deep gilgaied soils (>1.5 m) with clay loam to clay surfaces over mottled structured brown vertic (shrink–swell properties) clay subsoils also occur adjacent to and in association with the alluvial clay plains on the Sturt Plateau. These gilgaied soils, often with sink holes (small vertical depressions), frequently have large deep (>0.3 m) gilgai depressions that limit development due to the excessive levelling that is required for efficient irrigation practices. SGG 3, the seasonally wet or permanently wet soils (1.5%), occur extensively on a range of swamps, drainage lines, internal drainage depressions and low-lying alluvial coastal and marine plains (Table 2-1; Table 2-2; Figure 2-6). The low-lying seasonally wet non-saline alluvial plains of the lower Roper River downstream of Ngukurr are suited to dry-season irrigated agriculture. All other seasonally wet to permanently wet soils have limited potential for agricultural development. The coastal alluvial plains and very poorly drained saline coastal marine plains subject to tidal inundation have the potential for acid sulfate deposits in the profile and are subject to storm surge from cyclones. Closed drainage depressions in the deeply weathered Tertiary plains often have sands and loams deposited over poorly drained clay. The dark clay surfaced grey clay soil associated with the drainage lines and mound springs of the Tindall Limestone around Mataranka usually have no agricultural development potential due to extremely high salt levels and prolonged waterlogging. Sand or loam over relatively friable red (SGG 1.1), brown yellow and grey (SGG 1.2) clay subsoils; red (SGG 6.1) and brown, yellow and grey (SGG 6.2) sandy soils; sand or loam over sodic clay subsoils (SGG 8); and highly calcareous soils (SGG 10) have all been modelled as very small areas (<1%, Table 2-1; Table 2-2; Figure 2-6) but due to the resolution of the mapping these areas may be underestimated. Note that 1% of the area is equivalent to 77,400 ha and that some of these soils do have agricultural potential (Table 2-1 and Thomas et al., 2022). 2.3.3 Soil attribute mapping Using a combination of field sampling (Figure 2-9) and digital soil mapping techniques, the Assessment mapped 16 attributes affecting the agricultural suitability of soil for the Roper catchment as described in the companion technical report on digital soil mapping and land suitability (Thomas et al., 2022). Descriptions and maps for six key attributes are presented below: 1. Surface soil pH 2. Soil thickness 3. Soil surface texture 4. Permeability 5. Available water capacity (AWC) in the upper 100 cm of the soil profile – referred to as AWC 100 6. Rockiness. An important feature of the predicted attributes map is the companion reliability map indicating the relative confidence in the accuracy of the attribute predictions, noting that mapping is only provided here for regional-scale assessment. Areas of high reliability allow users to be more confident in the quality of mapping, whereas areas of low reliability show where users should be cautious. SGG 4.1 soil profile photo For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-9 The very deep, well-drained, sandy surfaced red massive loamy soils (Kandosol, SGG 4.1) overlying limestone in the Mataranka area are suited to a wide range of irrigated crops Source: CSIRO Surface soil pH The pH value of a soil reflects the extent to which the soil is alkaline or acidic. This is important because pH affects the extent to which nutrients are available to the plant and, hence, plant growth. For the majority of plant species, most soil nutrients are available in the pH range 5.5 to 6.5. Nutrient imbalances are common for soils with pH greater than 8.5 and less than 5.5. The surface of most soils in the Roper catchment are in the pH range 5.5 to 7.0 (Figure 2-10) and would not present a limitation to crop growth in almost all instances. There are instances of alkaline soils (pH >8.5) associated with limestone formations and associated springs in the Mataranka area on the Sturt Plateau, and in the central north at the Gulf Fall and Wilton River Plateau transition. Some acidic soils (pH <5.5) are found with the shallow, rockier soils associated with the hills and ranges in the west, centre and east in the Assessment area, typically SGG 7 (shallow and/or rocky) soils. Mapping reliability is strongest in the Roper River alluvium and the Sturt Plateau. Soil surface pH \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\3_Roper\1_GIS\1_Map_docs\LL-R-513-518_DSM_1x2_v4_ArcGIS10_8.mxd For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-10 Surface soil pH of the Roper catchment (a) Surface soil pH as predicted by digital soil mapping, and (b) reliability of the prediction. Surface soil pH is the pH in the top 10 cm. Soil thickness Soil thickness defines the potential root space and the amount of soil from which plants obtain their water and nutrients. Typically, the deepest soils in the catchment are on the Sturt Plateau (Figure 2-11) where soils often exceed 1.5 m depth, especially in lower landscape positions on the plains, and on the Quaternary clay deposits (SGG 9, cracking clay soils). Soils are also particularly deep near the mouth of the river in the coastal plain (SGG 3, seasonally or permanently wet soils) and on the alluvial plains of the Sturt Plateau (SGG 9, cracking clay soils). The shallower soils strongly coincide with SGG 7 (shallow and/or rocky soils) in the Gulf Fall country. Mapping reliability is generally strongest on the central Roper catchment and parts of the Sturt Plateau. Soil thickness \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\3_Roper\1_GIS\1_Map_docs\LL-R-513-518_DSM_1x2_v4_ArcGIS10_8.mxd For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-11 Soil thickness of the Roper catchment (a) Soil thickness as predicted by digital soil mapping, and (b) reliability of the prediction. Soil surface texture Soil texture refers to the proportion of sand, silt and clay-sized particles that make up the mineral fraction of a soil. Surface texture influences soil water-holding capacity, soil permeability, soil drainage, water and wind erosion, workability and soil nutrient levels. Light soils are generally those high in sand and heavy soils are dominated by clay. The Roper catchment is dominated by sandy textured surface soils (Figure 2-12), especially the sandstone geologies of the Sturt Plateau, Proterozoic geologies in the central parts of the catchment, and massive sandstones of the Wilton River Plateau. The alluvial plains of the Wilton River Plateau show localised examples of loamy soils (SGG 2, friable non-cracking clay or clay loam soils) and clay alluvium (SGG 9, cracking clay soils) associated with drainage lines of the Roper River and major tributaries and coastal marine plains. Cracking clay soils (SGG 9) are also associated with fine-grained sedimentary rocks and basalts in the Hodgson River catchment in the south and east of Mataranka. Sandy surfaced soils dominate the northern part of the Sturt Plateau, whereas the southern part is dominated by loamy surfaced soils (SGG 4.1, red loamy soils). This southern area of the catchment also features Quaternary clay deposits (SGG 9, cracking clay soils) in drainage depressions on the plateau. In terms of mapping reliability, reliability is strongest in the northern Sturt Plateau, western Gulf Fall country and the northern Wilton River Plateau. Soil surface texture \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\3_Roper\1_GIS\1_Map_docs\LL-R-513-518_DSM_1x2_v4_ArcGIS10_8.mxd For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-12 Soil surface texture of the Roper catchment (a) Surface texture of soils as predicted by digital soil mapping and (b) reliability of the prediction. Permeability The permeability of the profile is a measure of how easily water moves through a soil. Flood and furrow irrigation is most successful on soils with low and very low permeability to reduce root zone drainage (i.e. water that passes below the root zone of a plant), rising watertables and nutrient leaching. Spray or trickle irrigation is more efficient than flood and furrow irrigation on soils with moderate to high permeability. Most of the Roper catchment has been categorised as moderately permeable (Figure 2-13). Notably the Sturt Plateau is dominated by these soils, coinciding with the red loams of SGG 4.1 (red loamy soils). Highly permeable soils dominate much of the Wilton River Plateau with distributions aligned to shallow sandy soils (SGG 7, shallow and/or rocky soils) developed on quartz sandstones. There are significant areas of slowly permeable soils in the central Gulf Fall areas associated with the cracking clays of SGG 9 (cracking clay soils) of the Roper River and major tributaries and the Quaternary clay deposits of the Sturt Plateau, and also around the mouth of the Roper River in the coastal plain where seasonally wet or permanently wet soils dominate (SGG 3, seasonally or permanently wet soils). Mapping reliability is patchy throughout but strongest in parts of the Sturt Plateau and some areas of the Gulf Fall region. Soil permeability \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\3_Roper\1_GIS\1_Map_docs\LL-R-513-518_DSM_1x2_v4_ArcGIS10_8.mxd For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-13 Soil permeability of the Roper catchment (a) Soil permeability as predicted by digital soil mapping, and (b) reliability of the prediction. Available water capacity (AWC) to 100 cm AWC is the maximum volume of water the soil can hold for plant use. AWC 100 is the maximum volume of water that the top 100 cm of soil can hold for plant use. The higher the AWC 100 value, the greater the capacity of the soil to supply plants with water. For irrigated agriculture, it is one factor that determines irrigation frequency and volume of water required to wet up the soil profile. Low AWC 100 soils require more frequent watering and lower volumes of water per irrigation. For dryland agriculture, AWC 100 determines the capacity of crops to grow and prosper during dry spells. The largest AWC values are associated with the cracking clays soils (SGG 9) on the alluvium around the main watercourses and tributaries, especially in the Gulf Fall country (Figure 2-14). Also, extensive areas of larger AWC values are evident on the deep loams (SGG4.1, red loamy soils) and clay deposits (SGG 9, cracking clay soils) of the Sturt Plateau. The lowest value AWC areas coincide with the shallow and/or rocky soils (SGG 7) along ridges and rises in the Gulf Fall and Wilton River Plateau. Mapping reliability is generally strongest throughout the Sturt Plateau and weaker throughout the remaining areas. Soil available water capacity to 100 cm \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\3_Roper\1_GIS\1_Map_docs\LL-R-513-518_DSM_1x2_v4_ArcGIS10_8.mxd For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-14 Available water capacity in the Roper catchment (a) Available water capacity (AWC) in the upper 100 cm of the soil profile as predicted by digital soil mapping, and (b) reliability of the prediction. Rockiness The rockiness of the soil has both an effect on agricultural management and on the growth of some crops, particularly root crops. Coarse fragments (e.g. pebbles, gravel, cobbles, stones and boulders), hard segregations and rock outcrop in the plough zone can damage and/or interfere with the efficient use of agricultural machinery. Surface gravel, stone and rock are particularly important and can interfere significantly with planting, cultivation and harvesting machinery used for root crops, small crops, annual forage crops and sugarcane. The distribution of rocky soils in the Roper catchment closely matches the more freshly exposed lithologies, that is the hills and ranges mantled by SGG 7 (shallow and/or rocky soils) in the Gulf Fall and areas of the Wilton River Plateau (Figure 2-15). Non-rocky soils are associated with parts of the deeply weathered soils (SGG 4.1, red loamy soils) on the Sturt Plateau, and along river and tributary margins (SGG 9, cracking clays) and friable non-cracking clay or clay loam soils (SGG 2) and on the coastal marine plains (SGG 3, seasonally or permanently wet soils). Mapping reliability is strongest over much of the Sturt Plateau, large areas of the mid-catchment and on the alluvium associated with the Roper River. Soil rockiness \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\3_Roper\1_GIS\1_Map_docs\LL-R-513-518_DSM_1x2_v4_ArcGIS10_8.mxd For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-15 Rockiness in soils of the Roper catchment (a) Rockiness represented by presence or absence as predicted by digital soil mapping, and (b) reliability of the prediction. 2.4 Climate of the Roper catchment 2.4.1 Introduction Weather is the key source of uncertainty affecting hydrology and crop yield. It influences the rate and vigour of crop growth, while catastrophic weather events can result in extensive crop losses. Key climate parameters controlling plant growth and crop productivity include rainfall, temperature, radiation, humidity and wind speed and direction. Of all the climate parameters affecting hydrology and agriculture in water-limited environments, rainfall is usually the most important. Rainfall is the main determinant of runoff and recharge and is a fundamental requirement for plant growth. For this reason, reporting of climate parameters is heavily biased towards rainfall data. Other climate variables affecting crop yield are discussed in the companion technical report on climate (McJannet et al., 2023). Climate data presented in this report were calculated using SILO data (Jeffrey et al., 2001) unless stated otherwise. Unless otherwise stated, the material in Section 2.4 is based on findings described in the companion technical report on climate (McJannet et al., 2023). 2.4.2 Weather patterns over the Roper catchment The Roper catchment is characterised by distinctive wet and dry seasons due to its location in the Australian summer monsoon belt (Figure 2-16). During the build-up months (typically September to December) the Roper catchment typically experiences low-level easterly winds, which can carry pockets of dry or humid air, and can result in short-lived thunderstorm activity under favourable conditions. During the wet season, low-level westerly winds dominate. ‘Shallow westerly’ regimes are typical of an ‘inactive monsoon’ period (when the monsoon trough temporarily weakens or retreats north of Australia), and favour early morning thunderstorms along the coast, while afternoon thunderstorm activity is more common inland. ‘Deep westerly’ regimes correspond to an ‘active monsoon’ period, where storms typically have low cloud-top heights and showers and thunderstorms can be gusty and cause heavy rainfall due to the large water content of the maritime air mass. The mean annual rainfall, averaged over the Roper catchment for the 109-year historical period (1 September 1910 to 31 August 2019), is 792 mm. Annual rainfall is highest in the northern part of the catchment and lowest in the most southerly part the catchment (Figure 2-16). This is because the more northerly regions of the catchment receive more wet-season rainfall as a result of active monsoon episodes. The Roper catchment is relatively flat, and consequently there is no noticeable topographic influence on climate parameters such as rainfall or temperature. Approximately 96% of rain falls in the Roper catchment during the wet-season months (1 November to 30 April). The spatial distribution of rainfall during the wet and dry seasons is shown in Figure 2-16. Median wet-season rainfall exhibits a very similar spatial pattern to median annual rainfall, while median dry-season rainfall exhibits a west–east gradient, with only a slight north–south gradient evident. The highest monthly rainfall totals typically occur during January, February and March (Figure 2-17). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-16 Historical rainfall, potential evaporation and rainfall deficit Median (a) annual, (b) wet-season and (c) dry-season rainfall; median (d) annual, (e) wet-season and (f) dry-season potential evaporation; and median (g) annual, (h) wet-season and (i) dry-season rainfall deficit in the Roper catchment. Rainfall deficit is rainfall minus potential evaporation. The lack of rainfall during the dry season is largely due to the predominance of dry continental south-easterlies and the significant dry air aloft that inhibits shower and thunderstorm formation. During the months where the climate is transitioning to the wet season (i.e. typically mid- September to mid-December) strong sea breezes pump moist air inland, fuelling the steady growth of shower and thunderstorm activity over a period of weeks to months. This can result in highly variable rainfall during these months. Tropical cyclones and tropical lows contribute a considerable proportion of total annual rainfall, but the actual amount is highly variable from one year to the next (see the companion technical report on climate (McJannet et al., 2023)), since tropical cyclones do not affect the Roper catchment in more than half of years. For the 53 tropical cyclone seasons from 1969–70 to 2021– 22, 53% of seasons registered no tropical cyclones tracking over the region, 40% experienced one tropical cyclone, and 8% experienced two (BOM, 2023). 2.4.3 Potential evaporation and potential evapotranspiration Evaporation is the process by which water is lost from open water, plants and soils to the atmosphere; it is a ‘drying’ process. It has become common usage to also refer to this as evapotranspiration. There are three major ways in which evaporation affects the potential for irrigation: 1. Losses that reduce runoff and deep drainage and, hence, the ability to fill water storages (Section 2.5) 2. Influence on crop water requirements (Section 4.3) 3. Losses from water storages (Section 5.3). Potential evaporation (PE), or potential evapotranspiration (PET), is defined as the amount of evaporation that would occur if an unlimited source of water was available. The Roper catchment has a mean annual PE (Morton’s Wet) of 1883 mm (1910 to 2019) and like rainfall, has a relatively strong north–south gradient across the catchment (Figure 2-16). Preliminary estimates of mean annual irrigation demand and net evaporation from water storages are sometimes calculated by subtracting the mean annual (seasonal) PE from the mean annual (seasonal) rainfall. This is commonly referred to as the mean annual (seasonal) rainfall deficit (Figure 2-16). The rainfall deficit or mean annual net evaporative water loss from potential open storages at Mataranka in the Roper catchment is about 1065 mm. Two common methods for characterising climates are the United Nations Environment Program aridity index and the Köppen-Geiger classification (Köppen, 1936; Peel et al., 2007). The aridity index classifies the Roper catchment as mainly ‘Semi-arid’ and the Köppen-Geiger classification classifies it as ‘Tropical savanna’ (see the companion technical report on climate (McJannet et al., 2023)). 2.4.4 Variability and long-term trends in rainfall and potential evaporation Climate variability is a natural phenomenon that can be observed in many ways, for example, warmer than average dry seasons, low and high rainfall wet seasons. Climate variability can also operate over long-term cycles of decades or more. Climate trends represent long-term, consistent directional changes such as warming or increasingly higher average rainfall. Separating climate variability from climate change is difficult, especially when comparing climate on a year-to-year basis. In the Roper catchment, 96% of rain falls during the wet season (November to April). The highest monthly rainfall in the Roper catchment typically occurs during January and February (Figure 2-17). The months with the lowest rainfall are June through to September. In Figure 2-17, the blue shading, A range, represents the range under Scenario A (i.e. 1 September 1910 to 31 August 2019). The upper limit of the A range is the value at which rainfall (or PE) is exceeded 1 year in 5 and is known as the 20% exceedance. The lower limit of the A range is the value at which rainfall (or PE) is exceeded 4 years in 5 and is known as the 80% exceedance. The difference between the upper and lower limits of the A range provides a measure of the potential variation in monthly values from one year to the next. Chart, histogram. Monthly rainfall plots. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\1_Climate\0_Working\1_Ang\plot\rainfall_month_variation.png Figure 2-17 Monthly rainfall in the Roper catchment at Mataranka and Ngukurr under Scenario A (a) Monthly rainfall at Mataranka, and (b) monthly rainfall at Ngukurr. Scenario A is the historical climate (1910 to 2019). A range shows the range in values between the 20% and 80% monthly exceedance rainfall. PE also exhibits a seasonal pattern. During the month of October, mean PE is about 205 mm (Figure 2-18). It is at its lowest during June (110 mm). Months where PE is high correspond to those months where the demand for water by plants is also high. Mean wet-season and dry- season PE in the Roper catchment are shown in Figure 2-16. Compared to rainfall, the variation in monthly PE from one year to the next is small (Figure 2-18). Chart, line chart. Monthly evaporation. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au "file://fs1-cbr.nexus.csiro.au/{lw-rowra}/work/1_Climate/0_Working/1_Ang/plot/evap_month_variation.png" Figure 2-18 Monthly potential evaporation in the Roper catchment at Mataranka and Ngukurr under Scenario A (a) Monthly potential evaporation at Mataranka, and (b) monthly potential evaporation at Ngukurr. Scenario A is the historical climate (1910 to 2019). A range shows the range in values between the 20% and 80% exceedance potential evaporation. Relative to other catchments in southern and northern Australia, the Roper catchment has a low variability in rainfall from one year to the next. Nevertheless, under Scenario A, rainfall for the Roper catchment still exhibits considerable variation from one year to the next (Figure 2-19). The highest annual rainfall at Mataranka (1779 mm) occurred in the 2010–11 wet season, which was six times the lowest annual rainfall (297 mm in 1951–52) and more than twice the median annual rainfall value (i.e. 784 mm). The 10-year running mean provides an indication of the sequences of wet or dry years (i.e. variability at decadal timescales). For an annual time series, the 10-year running mean is the average of the last 10 years of data including the current year. The 10-year running mean varied from 647 to 1141 mm. This figure illustrates that the period between 2000 and 2010 was particular wet relative to the historical record. Under Scenario A, PE exhibits much less inter-annual variability than rainfall (not shown, see the companion technical report on climate (McJannet et al., 2023)). Chart, histogram. Annual rainfall. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au "file://fs1-cbr.nexus.csiro.au/{lw-rowra}/work/1_Climate/0_Working/1_Ang/plot/rainfall_annual.png" Figure 2-19 Annual rainfall at Mataranka and Ngukurr under Scenario A (a) Annual rainfall at Mataranka, and (b) annual rainfall at Ngukurr. Scenario A is the historical climate (1910 to 2019). The blue line represents the 10-year running mean. The coefficient of variation (CV) provides a measure of the variability of rainfall from one year to the next, where the larger the CV value, the larger the variation in annual rainfall relative to a location’s mean annual rainfall – it is calculated as the standard deviation of mean annual rainfall divided by the mean annual rainfall. In Figure 2-20, the CV of annual rainfall is shown for rainfall stations with a long-term record around Australia. This figure shows that the inter-annual variation in rainfall in the Roper catchment is about average for northern Australia catchments but is more variable than stations in southern Australia with similar mean annual rainfall. (a) (b) For more information on this figure please contact CSIRO on enquiries@csiro.au For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-20 (a) Coefficient of variation of annual rainfall, and (b) the coefficient of variation of annual rainfall plotted against mean annual rainfall for 99 rainfall stations around Australia (a) The grey polygon indicates the extent of the Roper catchment. (b) Rainfall station in the Roper catchment (Roper) is indicated by red symbol. The light blue diamonds indicate rainfall stations from the rest of northern Australia and hollow squares indicate rainfall stations from southern Australia (SA). Furthermore, Petheram et al. (2008) observed that the inter-annual variability of rainfall in northern Australia is about 30% higher than that observed at rainfall stations from the rest of the world for the same type of climate as northern Australia. Hence, caution should be exercised before drawing comparisons between the agricultural potential of the Roper catchment and other parts of the world with a similar climate. There are several factors driving this high inter-annual variation in Australia’s climate, including the El Niño – Southern Oscillation (ENSO), the Indian Ocean Dipole, the Southern Annular Mode, the Madden–Julian Oscillation and the Inter-decadal Pacific Oscillation. Of these influences, the ENSO is a phenomenon that is considered to be the primary source of global climate variability over the 2- to 6-year timescale (Rasmusson and Arkin, 1993) and is reported as being a significant cause of climate variability for much of eastern and northern Australia. One of the modes of ENSO, El Niño, has come to be a term synonymous with drought in the western Pacific and eastern and northern Australia (though El Niño does not necessarily mean a ‘drought’ will occur). Rainfall stations along eastern and northern Australia have been observed to have a strong correlation (0.5 to 0.6) with the Southern Oscillation Index (SOI), a measure of the strength of ENSO, during spring, suggesting that ENSO plays a key role in between-year rainfall variability (McBride and Nicholls, 1983). Another known impact of ENSO in northern Australia is the tendency for the onset of useful rains after the dry season to be earlier than normal in La Niña years and later than normal in El Niño years. For all years between 1910 and 2019, the mean rainfall onset date (defined as being the accumulation of 50 mm of rain after the dry season) for the Roper catchment is the last 10 days of October (see the companion technical report on climate (McJannet et al., 2023)). The mean SOI for the September to December period in each year was used to define if given years were in negative (SOI <–8, El Niño), positive (SOI >8, La Niña), or neutral SOI (–8< SOI <8). Using this method, in El Niño, neutral and La Niña years, the median rainfall onset dates for the Roper catchment are the start of December, late November and early November, respectively. Trends Previously, CSIRO (2009) found that rainfall in northern Australia between 1997 and 2007 was statistically different to that between 1930 and 1997. In other work, Evans et al. (2014) found a strong relationship between monsoon active periods and the Madden–Julian Oscillation, and that the increasing rainfall trend observed at Darwin Airport was related to increased frequency of active monsoon days rather than increased intensity during active periods. Runs of wet and dry years The rainfall-generating systems in northern Australia and their modes of variability combine to produce irregular runs of wet and dry years. In particular, length and magnitude (intensity) of dry spells strongly influence the scale, profitability and risk of water resource related investments. The Roper catchment is likely to experience dry periods of similar severity to many centres in the Murray–Darling Basin and east coast of Australia. The Roper catchment is characterised by irregular periods of consistently low rainfall when successive wet seasons fail, as well as the typical annual dry season. Runs of wet and dry years occur when consecutive years of rainfall occur that are above or below the median, respectively. These are shown in Figure 2-21 at Mataranka and Ngukurr stations as annual differences from the median rainfall. A run of consistently dry years may be associated with drought (though an agreed definition of drought continues to be elusive). Analysis of annual rainfall at stations in the Roper catchment indicate equally long runs of wet and dry years and nothing unusual about the length of the runs of dry years. A picture containing timeline. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-21 Runs of wet and dry years at (a) Mataranka, and (b) Ngukurr stations under Scenario A Wet years are shown by the blue columns and dry years by the red columns. Scenario A is the historical climate (1910 to 2019). Palaeoclimate records for northern Australia The instrument record is very short in a geological sense, particularly in northern Australia, so a brief review of palaeoclimate data is provided. The literature indicates that atmospheric patterns approximating the present climate conditions in northern Australia (e.g. Pacific circulation responsible for ENSO) are thought to have been in place from about 3 to 2.5 million years ago (Bowman et al., 2010), which would suggest many ecosystems in northern Australia have experienced monsoonal conditions for many millions of years. However, past climates have been both wetter and drier than the instrument record for northern Australia, and the influence of ENSO has varied considerably over recent geological time. Several authors have found that present levels of tropical cyclone activity (i.e. over the instrumental record) in northern Australia are low (Denniston et al., 2015, Forsyth et al., 2010, Nott and Jagger 2013) and possibly unprecedented over the past 550 to 1500 years (Haig et al., 2014) and that the recurrence frequencies of high-intensity tropical cyclones (Category 4 to Category 5 events) may have been an order of magnitude higher than that inferred from the current short instrumental records. 2.4.5 Changes in rainfall and evaporation under a future climate The effects of projected climate change on rainfall and PE are presented in Figure 2-22, Figure 2-23 and Figure 2-24. This analysis used 32 GCMs downloaded from the sixth Coupled Model Intercomparison Project (CMIP6) website (https://pcmdi.llnl.gov/CMIP6/) to represent a world where the global mean surface air temperatures are 1.6 °C higher relative to approximately 1990 global temperatures. Under the adopted Shared Socioeconomic Pathway (SSP) scenario, SSP2-4.5 (IPCC, 2022), a 1.6 °C increase in temperature relative to approximately 1990 global temperatures occurs at ~2060. This SSP scenario was adopted because it is considered the more likely scenario based on current projections and global commitments to emission abatement (Hausfather and Peters 2020). Because the scale of GCM outputs is too coarse for use in catchment and point-scale hydrological and agricultural computer models, they were transformed to catchment-scale variables using a simple scaling technique (PS) and referred to as GCM-PSs. See the companion technical report on climate (McJannet et al., 2023) for further details. In Figure 2-22 the rainfall and PE projections of the 32 GCM-PSs are spatially averaged across the Roper catchment and the GCM-PSs are ranked in order of increasing mean annual rainfall. This figure shows that five (or 16%) of the projections for GCM-PSs indicate an increase in mean annual rainfall by more than 5%, nine (or 28%) of the projections indicate a decrease in mean annual rainfall by more than 5%, and 14 (or 56%) of the projections indicate a change in future mean annual rainfall of less than 5% under a 1.6 °C warming scenario. The spatial distribution of mean annual rainfall under Scenario C is shown in Figure 2-23. In this figure only the third ‘wettest’ GCM-PS (i.e. Scenario Cwet), the middle or 17th wettest GCM-PS (i.e. Scenario Cmid), and the third ‘driest’ (i.e. Scenario Cdry) GCM-PS are shown. Figure 2-24a shows mean monthly rainfall under scenarios A and C. The data suggest that under Scenario Cmid, mean monthly rainfall will be similar to the mean monthly rainfall under Scenario A. Under scenarios Cwet, Cmid and Cdry the seasonality of rainfall in northern Australia is similar to that under Scenario A. A graph of a graph showing the number of data Description automatically generated with medium confidence Figure 2-22 Percentage change in mean annual rainfall and potential evaporation under Scenario C relative to under Scenario A Simple scaling of rainfall and potential evaporation have been applied to global climate model output (GCM-PS). GCM-PSs are ranked by increasing rainfall. "\\fs1-cbr\{lw-rowra}\work\1_Climate\3_Roper\1_GIS\1_Map_docs\1_Exports\Cl-R-514-annualRain-Cwet-Cmid-Cdry.png" For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 2-23 Spatial distribution of mean annual rainfall across the Roper catchment under scenarios Cwet, Cmid and Cdry A graph of a graph of a graph Description automatically generated with medium confidence Figure 2-24 Monthly rainfall and potential evaporation for the Roper catchment under scenarios A and C (a) Monthly rainfall and (b) monthly potential evaporation. C range is based on the computation of the 10% and 90% monthly exceedance values separately – the lower and upper limits in C range are therefore not the same as scenarios Cdry and Cwet. Potential evaporation The majority of GCM-PS show a projected increase in PE of about 5 to 10% (Figure 2-22). Under scenarios Cwet, Cmid and Cdry, PE exhibits a similar seasonality to that under Scenario A (Figure 2-24b). However, different methods of calculating PE give different results. Consequently, there is considerable uncertainty on how PE may change under a warmer climate. See Petheram et al. (2012) and Petheram and Yang (2013) for a more detailed discussion. Sea-level rise and sea surface temperature projections Global mean sea levels have risen at a rate of 1.7 ± 0.2 mm/year between 1900 and 2010, a rate in the order of ten times faster than the preceding century. Australian tide gauge trends are similar to the global trends (CSIRO and Bureau of Meteorology, 2015). Sea-level projections for the Roper catchment are summarised in Table 2-33. This information may be considered in coastal aquaculture developments and flood inundation of coastal areas. Table 2-3 Projected sea-level rise for the coast of the Roper catchment Values are median of Coupled Model Intercomparison Project (CMIP) Phase 5 GCMs. Numbers in parentheses are the 5 to 95% range of same. Projected sea-level rise values are relative to a mean calculated between 1986 and 2005. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au RCP = Representative Concentration Pathway Source: CoastAdapt (2017) Sea surface temperature (SST) increases around Australia are projected with very high confidence for all emissions scenarios, with warming of around 0.4 to 1.0 °C in 2030 under Representative Concentration Pathway (RCP) 4.5, and 2 to 4 °C in 2090 under RCP 8.5, relative to a 1986 to 2005 baseline (CSIRO and Bureau of Meteorology, 2015). There will be regional differences in SST warming due to local hydrodynamic responses, however, there is only medium confidence in coastal projections as climate models do not resolve local processes (CSIRO and Bureau of Meteorology, 2015). For Roper and Gulf catchments, the corresponding projected SST increases are 0.7 °C (range across climate models is 0.5 to 1.0 °C) for 2030 and 2.9 °C (2.4 to 3.9 °C) for 2090. These changes are relative to a 1986 to 2005 baseline (CSIRO and Bureau of Meteorology, 2015). 2.4.6 Establishment of an appropriate hydroclimate baseline The allocation of water and the design and planning of water resources infrastructure and systems require great care and consideration and need to take a genuine long-term view. A hydroclimate baseline from 1910 to 2019 (i.e. current) was deemed the most suitable baseline for the Assessment. A poorly considered design can result in an unsustainable system or preclude the development of a more suitable and possibly larger system, thus adversely affecting existing and future users, industries and the environment. Once water is overallocated it is economically, financially, socially and politically difficult to reduce allocations in the future, unless water allocations are only assigned over short time frames (e.g. <15 years) and then reassessed. However, many water resource investments, particularly agricultural investments, require time frames longer than 30 years as there are often large initial infrastructure costs and a long learning period before full production potential is realised. Consequently, investors require certainty that over their investment time frame (and potentially beyond), their access to water will remain at the level of reliability initially allocated. A key consideration in the development of a water resource plan, or in the assessment of the water resources of a catchment, is the time period over which the water resources will be analysed, also referred to as the hydroclimate ‘baseline’ (e.g. Chiew et al., 2009). If the hydroclimate baseline is too short it can introduce biases in a water resource assessment, for various reasons. Firstly, the transformation of rainfall to runoff and rainfall to groundwater recharge is non-linear. For example, averaged across the Flinders catchment in northern Australia the mean annual rainfall is only 8% higher than the median annual rainfall, yet the mean annual runoff is 59% higher than the median annual runoff (Lerat et al., 2013). Similarly, between 1895 and 1945 the median annual rainfall was the same as the median annual rainfall between 1948 and 1987 (less than 0.5% difference), yet there was a 21% difference in the median annual runoff between these two time periods (and a 40% difference in the mean annual runoff) (Lerat et al., 2013). Consequently, great care is required if using rainfall data alone to justify the use of short periods over which to analyse the water resources of a catchment. In developing a water resource plan, the volume of water allocated for consumptive purposes is usually constrained by the drier years (referred to as dry spells where consecutive dry years occur) in the historical record (see Section 2.4.4). This is because it is usually during dry spells that water extraction most adversely affects existing industries and the environment. All other factors being equal (e.g. market demand, interest rates), consecutive dry years are usually also the most limiting time periods for new water resource developments/investments, such as irrigated agricultural enterprises, particularly if the dry spells coincide with the start of an investment cycle. Consequently, it is important to ensure a representative range of dry spells (i.e. of different durations, magnitudes and sequencing) are captured over the Assessment time period. For example, it is possible that two time periods may have very similar median annual runoffs, but the duration, magnitude and sequencing of the dry spells may be sufficiently different that they pose different risks to investors and result in different modelled ecological outcomes. In those instances where there is the potential for a long memory, such as in intermediate- and regional-scale groundwater systems or in river systems with large reservoirs, long periods of record are preferable to minimise the influence of initial starting conditions (e.g. assumptions regarding initial reservoir storage volume), to properly assess the reliability of water supply from large storages and to encapsulate the range of likely conditions (McMahon and Adeloye, 2005). All these arguments favour using as long a time period as practically possible. However, there may be some circumstances in which a shorter period may be preferable on the basis that it is a more conservative option. For example, in south-western Australia, water resource assessments to support water resource planning are typically assessed from 1975 onwards (Chiew et al., 2012; McFarlane et al., 2012). This is because since the mid-1970s there has been a marked reduction in runoff in south-western Australia, and this declining trend in rainfall is consistent with the majority of GCM projections, which project reductions of rainfall into the future (McJannet et al., 2023). Although there were few rainfall stations in the study area at the turn of the 20th century relative to 2019 (McJannet et al., 2023), an exploratory analysis of rainfall statistics of the early period of instrument record does not appear to be anomalous when compared to the longer term instrument record. In deciding upon an appropriate time period over which to analyse the water resources of the Roper catchment, consideration was given to the above arguments, as well as palaeoclimate records, observed trends in the historical instrumental rainfall data and future climate projections. For the Roper catchment, although 56% of GCM-PSs project no change in mean annual rainfall for a 1.6 °C warming scenario, 28% of GCM-PSs project a drier future climate and all GCM-PSs project an increase in potential evaporation. Furthermore, palaeoclimate records indicate multiple wetter and drier periods have occurred in the recent geological past (Northern Australia Water Resource Assessment technical report on climate, Charles et al., 2017). There are very few climate data available in the region prior to 1910. For these reasons the baseline that was adopted for this assessment was from 1910 to 2019. It should be noted, however, that as climate is changing on a variety of timescales, detailed scenario modelling and planning (i.e. the design of major water infrastructure) should be broader than just comparing a single hydroclimate baseline to an alternative future. 2.5 Hydrology of the Roper catchment 2.5.1 Introduction The timing and event-driven nature of rainfall events and high PE rates across the Roper catchment have important consequences for the catchment’s hydrology. The spatial and temporal patterns of rainfall and PE across the Roper catchment are discussed in Section 2.4. Rainfall can be broadly broken into evaporated and non-evaporated components (also referred to as ‘excess water’). The non-evaporated component can be broadly broken into overland flow and recharge (Figure 2-25). Recharge replenishes groundwater systems, which in turn discharge into rivers and the ocean. Overland flow and groundwater discharged into rivers combine to become streamflow. Streamflow in the Assessment is defined as a volume per unit of time. Runoff is defined as the millimetre depth equivalent of streamflow. Flooding is a phenomenon that occurs when the flow in a river exceeds the river channel’s capacity to carry the water, resulting in water spilling onto the land adjacent to the river. Section 2.5 covers the remaining terms of the terrestrial water balance (accounting for water inputs and outputs) of the Roper catchment, with particular reference to those processes and terms that are relevant to irrigation at the catchment scale. Information is firstly provided on groundwater, groundwater recharge and surface water – groundwater connectivity. Runoff, streamflow, flooding and persistent waterholes in the Roper catchment are then discussed. Figure 2-25 shows a schematic diagram of the water balance of the Roper catchment, along with estimates of the mean annual value spatially averaged across the catchment and an estimate of the uncertainty for each term. The ‘water balance’ comprises all the water inflows and outflows to and from a particular catchment over a given time period. Unless stated otherwise, the material in sections 2.5.2 to 2.5.4 is based on findings described in the companion technical report on hydrogeological assessment (Taylor et al., 2023). Similarly, the material in Section 2.5.5 draws on the findings of the companion technical report on river modelling (Hughes et al., 2023), unless stated otherwise. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-25 Simplified schematic diagram of terrestrial water balance in the Roper catchment Runoff is the millimetre depth equivalent of streamflow. Overland flow includes shallow subsurface flow. Numbers indicate mean annual values spatially averaged across the catchment under Scenario A. Numbers will vary locally. 2.5.2 Groundwater Within the Roper catchment the distribution, availability and quality of groundwater resources are heavily influenced by the physical characteristics of the sediments and rocks of the major geological divisions (see Section 2.2). In general, several aquifer (rocks and sediments in the subsurface that store and transmit groundwater) types exist: • fractured and weathered rocks • sedimentary dolostones, limestones, sandstones and siltstones • surficial sediments that predominantly include siltstone, claystone, regolith and alluvium. The sedimentary limestones of the interconnected Daly, Wiso and Georgina basins – in particular, the Tindall Limestone and its lithological and age equivalent hydrogeological units (Montejinni Limestone and Gum Ridge Formation) – host the largest groundwater resource in the Roper catchment (referred to as the Cambrian Limestone Aquifer – CLA) (Figure 2-26). The CLA is a complex, interconnected and highly productive regional-scale groundwater system. That is, the distance between the recharge (inflow of water through the soil, past the root zone and into an aquifer) and discharge (outflow of water from an aquifer into a water body or evaporated from the soil or vegetation) areas can be tens of kilometres to hundreds of kilometres, and the time taken for groundwater to discharge following recharge can be in the order of tens of years to hundreds of years. For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-26 Simplified regional geology of the Roper catchment To show the spatial extent of key regional geological 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 on the lower right inset. Geology data sources adapted from: Department of Industry, Tourism and Trade (2014) and Department of Environment, Parks and Water Security (2008) Geological faults data source: Department of Industry, Tourism and Trade (2010) The CLA extends for tens to a few hundred kilometres to the west, south and east of the Roper catchment. This means that the surface water catchment boundary is not the groundwater flow boundary (or groundwater divide). Groundwater in parts of the CLA flows from areas outside of the Roper catchment with higher groundwater levels, to areas inside the catchment with lower groundwater levels (Figure 2-28)(see Section 5.2.2). The sedimentary dolostone aquifers – in particular, the Dook Creek Formation (Figure 2-27) of the Mount Rigg Group in the McArthur Basin – also host a productive intermediate-scale groundwater system (Figure 2-26). That is, the distance between the recharge and discharge areas can be a few kilometres to tens of kilometres, and the time taken for groundwater to discharge following recharge can be in the order of hundreds of years to thousands of years. Where information exists for the DCA, it extends for tens of kilometres to the north-east of the Roper catchment and has a mapped extent just over twenty thousand square kilometres. Similar to the CLA, the surface water catchment boundary is not the groundwater flow boundary. Groundwater in the DCA flows from areas inside the Roper catchment with higher groundwater levels, to areas outside of the catchment with lower groundwater levels (Figure 2-28). The sedimentary dolostone and sandstone aquifers of the Nathan Group – in particular the Knuckey Formation and Mount Birch Sandstone – also host productive but local-scale groundwater systems. The sedimentary sandstone aquifers of the Bukalara Sandstone and Roper Group, and the fractured and weathered rock aquifers of the Derim Derim Dolerite of the McArthur Basin, host local-scale groundwater systems that are low yielding and poorly characterised (Figure 2-26). That is, the distance between the recharge and discharge areas is in the order of 1 to 10 km. The surficial alluvial and regolith aquifer systems of the Carpentaria and McArthur basins in the catchment have a limited extent, are only partially saturated and are poorly characterised. For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-27 Groundwater from the Dook Creek Formation Photo: CSIRO For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-28 Simplified regional geology for the entire spatial extent of the Mount Rigg Group of the McArthur Basin and the Tindall Limestone and equivalents of the Daly, Wiso and Georgina basins To show the spatial extent of key regional geological 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 on the lower right inset. Geology data sources adapted from: Department of Industry, Tourism and Trade (2014) and Department of Environment, Parks and Water Security (2008) Geological faults data source: Department of Industry, Tourism and Trade (2010) Hydrogeological units Hydrogeological units of the Roper catchment are shown in Figure 2-29. The rocks and sediments of these geological units host a diverse range of aquifers that vary in extent, storage and productivity. The major aquifers in the Roper catchment are found in the Cambrian-age limestone of the interconnected Daly, Wiso and Georgina basins and the Proterozoic-age dolostones and sandstones of the McArthur Basin. For this Assessment, major aquifer systems are considered to be aquifers that contain regional and intermediate-scale groundwater systems, with adequate storage volumes (i.e. gigalitres) that could potentially yield water at a sufficient rate (i.e. >10 L/second) and be of a sufficient water quality (i.e. <1000 mg/L total dissolved solids (TDS)) for a range of irrigated cropping. Minor aquifers are considered to be aquifers that contain local-scale groundwater systems with lower storage (i.e. megalitres). The yields from minor aquifers are variable but are often low yielding (i.e. <5 L/second) and have variable water quality ranging from fresh (~500 mg/L TDS) to brackish (~3000 mg/L TDS). The distribution and characteristics of these rocks is covered in Section 2.2. Unless otherwise stated, the material in Section 2.5.2 is based on findings described in the companion technical report on hydrogeological assessment by Taylor et al. (2023). Only the major aquifers relevant to potential opportunities for future groundwater resource development are discussed in detail. Limestone aquifers The Cambrian-age limestone aquifers (CLA) of the interconnected Daly, Wiso and Georgina basins occur beneath the south to south-west of the Roper catchment, occupying about 36% of the total catchment area (Figure 2-29). They consist mostly of three equivalent hydrogeological units (Tindall Limestone, Montejinni Limestone and Gum Ridge Formation) which extend far to the west, south and east of the Roper catchment (Figure 2-30). The combined total area of the CLA beyond the Roper catchment is approximately 460,000 km2 though this includes several other equivalent hydrogeological units. Within the south-west of the Roper catchment the CLA underlies an area of approximately 27,500 km2. The CLA is a highly productive interconnected and complex regional-scale aquifer, which is the largest groundwater resource in this physiographic region (both inside and beyond the Roper catchment). The complexity of the system arises from the variability and interconnectivity between fractures, fissures and karsts across the spatial extent of the formation (similar to the dolostone aquifers). In addition, the presence and thickness of the veneer of overlying Cretaceous rocks combined with temporal and spatial variability in rainfall influence groundwater recharge. Groundwater resources from the aquifer have mostly been developed for groundwater-based irrigated agriculture but also for community water supplies at Mataranka, Larrimah and Daly Waters. For more information on current groundwater use see Section 3.3.4. The CLA is a complex aquifer system because: • The variability in karstic features affects permeabilities and bore yields across the aquifer. • Recharge to the aquifer occurs where the aquifer either directly outcrops or it is unconfined beneath overlying Cretaceous sandstone, siltstone and claystone. Inside the Roper catchment, the aquifer only outcrops around Mataranka (see inset of Figure 2-29) but is mostly unconfined beneath the spatially variable veneer of overlying Cretaceous rocks. The aquifer is confined in places by the Cambrian siltstone (mostly outside of the Roper catchment), which influences spatial variability in recharge to the aquifer. Recharge processes include a combination of localised preferential infiltration of rainfall and streamflow via sinkholes or stream channels directly in the aquifer outcrop, or via both broad diffuse infiltration or spatially variable preferential infiltration of rainfall through the overlying sandstone, siltstone and claystone where the aquifer is unconfined. • The aquifer is partly intruded by the igneous basalt of the Antrim Plateau Volcanics, which partly interrupts its continuous spatial extent and influences the directions of regional groundwater flow. • The aquifer discharges via a combination of diffuse seepage to streams (Roper Creek, upper Roper River, Waterhouse River and Elsey Creek), localised spring discharge (Bitter, Rainbow, Botanic Walk and Fig Tree springs) including a few instream springs in the bed of the upper Roper River and its tributaries, transpiration from riparian and spring-fed vegetation, and extraction of groundwater. The sources of groundwater discharge to the upper Roper River and its tributaries and springs comes from a combination of regional discharge from the Georgina Basin in the south, intermediate to local discharge from the Daly Basin to the west and localised discharge from the aquifer outcrop around Mataranka. Groundwater flow in the aquifer system is complex due to a combination of the variability in the amount and connectivity of karstic features across the aquifer, as well as spatial variability in seasonal recharge and discharge across large areas. At a local scale, groundwater flow can be via preferential flow in connected holes and caverns but across the aquifer extent regional flow occurs via the interconnected nature of the karstic features acting as a porous media (one with sufficient spaces between rocks for groundwater flow to occur across large areas). Regional groundwater flow in the aquifer is generally from south to north. In the Georgina Basin, regional flow is from north to south into the Daly Basin and toward the Roper River. Whereas in the Wiso Basin, regional flow is from south to north towards the Flora River just east of the catchment boundary. Some intermediate to local scale flow also comes from the Daly Basin in the west and north toward the Roper River. Bore (defined here as a hole in the ground for extracting groundwater) yields are variable given the complex nature of the karstic aquifer, but yields often range between 15 and 45 L/second, with appropriately constructed production bores (Figure 2-31), and groundwater quality is generally fresh (<500 mg/L TDS, Figure 2-33). The productive (high-yielding) part of the limestone aquifer occurs in the weathered, fractured and karstic zone, above the unweathered (solid) limestone (Figure 2-32). For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-29 Simplified regional hydrogeology of the Roper catchment To show the spatial extent of key regional geological 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 on the lower right inset. Geology data sources adapted from: Department of Industry, Tourism and Trade (2014) and Department of Environment, Parks and Water Security (2008) Spring and sinkhole data source: Department of Environment, Parks and Water Security (2013, 2014) Dolostone aquifers The Proterozoic-age dolostones aquifers are associated with the Mount Rigg and Nathan groups of the McArthur Basin and predominantly occur beneath the northern part of the Roper catchment (Figure 2-26). These dolostones host productive intermediate to local-scale aquifers. The most significant dolostone aquifer is hosted in the Dook Creek Formation of the Mount Rigg Group in the north-east of the catchment where the outcropping and subcropping extent occurs from Barunga to Bulman (Figure 2-26 and Figure 2-29). The aquifer also dips steeply in the subsurface beneath the Roper Group rocks to the east of the Central Arnhem Road (see Chapter 5). This aquifer system is referred to as the Dook Creek Aquifer (DCA). The DCA also extends to the north- east outside of the Roper catchment occupying an area of approximately 21,800 km2. Within the Roper catchment the DCA occupies an area of approximately 14,100 km2 (about 18% of the Roper catchment). The DCA is complex, due to the variability and interconnectivity between fractures, fissures and karsts (the formation of holes and caverns from the dissolving of soluble rocks) across the spatial extent of the formation. Groundwater resources from the aquifer have mostly been developed for community water supplies at Barunga, Beswick and Bulman. For more information on current groundwater use see Section 3.3.4. The Proterozoic dolostone also occurs around Ngukurr (Knuckey Formation, part of the Nathan Group) where a localised karstic dolostone aquifer similar in characteristics to the DCA but of much smaller extent occurs (Figure 2-29). Groundwater resources from the aquifer have mostly been developed for community water supplies at Urapunga and Ngukurr. For more information on current groundwater use see Section 3.3.4. The DCA is a complex aquifer system because: • The variability in karstic features affects permeabilities (the ability of a porous rock, sediment or soil to transmit water) and bore yields across the aquifer. • Where the aquifer is unconfined in the west (see Figure 2-34), recharge is spatially variable and occurs via a combination of broad diffuse infiltration of rainfall and in some places localised preferential infiltration via sinkholes directly in the outcrop or through the overlying sandstone, siltstone and claystone. • The aquifer is confined (sealed by overlying sandstone so that water cannot infiltrate from the land surface into the aquifer) in the east by the sandstone of the Roper Group (see Figure 2-34), which influences the spatial variability in recharge to the aquifer as well as discharge. • The aquifer discharges via a combination of diffuse seepage to streams (Flying Fox Creek, Mainoru and Wilton rivers), localised spring discharge (Weemol and Emu springs), transpiration from riparian and spring-fed vegetation, and extraction of groundwater. Groundwater flow in the aquifer system is complex due to a combination of the variability in the amount and connectivity of karstic features across the aquifer, and spatial and temporal variability in annual recharge and discharge. Groundwater flow is generally in a north-easterly direction, though groundwater-level data for the aquifer are sparse. Bore yields are variable given the complex nature of the karstic aquifer, but yields can range between 15 and 45 L/second, with appropriately constructed production bores (Figure 2-31), and groundwater quality is generally fresh (<500 mg/L) (Figure 2-33). The productive (high-yielding) part of the dolostone aquifer occurs in the upper (top 10to 20m)weathered, fractured and karstic zone, above theunweathered (solid) dolostone (Figure2-32). For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure2-30Full extent of both the Cambrian Limestone Aquifer and Dook Creek Aquifer To show the spatial extent ofkeyregional geological units in the subsurface, the blanket of surficial Cretaceous toQuaternary rocks andsediments has been removed. The extent of the surficial Cretaceous to Quaternary rocks andsediments is shown on the lower right inset. Geology data sources adaptedfrom: Department of Industry, Tourism and Trade (2014) and Department of Environment,Parks and Water Security(2008) Chapter2 Physical environment ofthe Roper catchment|67 Sandstone aquifers The Proterozoic-age sandstones of the McArthur Basin are mostly located across large parts of the centre and north of the Roper catchment and include rocks of the Roper Group, Nathan Group and Bukalara Sandstone (Figure 2-26 and Figure 2-29). These aquifers host local-scale flow systems, which provide an important source of groundwater in places for community water supply and stock and domestic use. The most productive sandstone aquifers in the catchment are hosted in the Nathan Group around Ngukurr. Individual bore yields can often be ~15 L/second with maximum yields of up to 30 L/second where production bores have been constructed and pump tested. The Limmen Sandstone and Bessie Creek Sandstone of the Roper Group also host locally productive aquifers where it is heavily fractured and jointed. Individual bore yields can often be a few litres per second (~3 L/second) for the Bukalara Sandstone but are generally lower for the Limmen Sandstone, ranging from 0.5 to 2 L/second (Figure 2-31). Water quality for these aquifers is variable, ranging between fresh (~500 mg/L TDS) to brackish (~3000 mg/L TDS, Figure 2-33). Groundwater storage and flow occurs via secondary porosity features such as fractures and jointing. Recharge occurs as infiltration of rainfall in the aquifer outcrop and some streamflow (where streams traverse the outcrop of these aquifers) or through overlying sediments and rocks to vertical fractures and joints in the aquifers. The main discharge mechanisms are from bores extracting groundwater for stock and domestic use and from evaporation (through the soil or plants) from shallow watertables (the start of the saturated zone of an aquifer) and as discharge to rivers and creeks. For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-31 Groundwater bore yields for (a) the major aquifers hosted in the Tindall Limestone and equivalents and the Mount Rigg and Nathan groups and (b) other minor aquifers of the Roper catchment Symbol shape indicates different aquifers within which bores are sited, colour indicates bore yield classes. Bore yield data source: Department of Environment, Parks and Water Security (2014) Fractured rock aquifers The Proterozoic igneous and sedimentary (i.e. sandstone, siltstone and mudstone) rocks (some of the oldest rocks across the catchment) combined with the Cambrian basalt occur over approximately 45% of the Roper catchment (Figure 2-29). These include rocks of the Katherine River Group, Roper Group, Derim Derim Dolerite and the Antrim Plateau Volcanics (Figure 2-26). These highly heterogenous rocks host fractured rock aquifer systems that supply small quantities of groundwater mainly used for stock and domestic purposes. These aquifers are highly variable in composition and host local-scale flow systems, with most groundwater storage and flow resulting from the size and connectivity of secondary porosity features such as joints, fractures or faults. Individual bore yields are variable but often low, >2 L/second (Figure 2-31), and water quality is variable, ranging from fresh (~500 mg/L TDS) to brackish (~3000 mg/L TDS, Figure 2-33). Recharge occurs as infiltration of rainfall and some streamflow (where rivers and creeks traverse these hydrogeological units) through the soil to vertical fractures and joints. The main discharge mechanisms are from bores extracting groundwater for stock and domestic use, from evaporation (through the soil or plants) from shallow watertables and as discharge to rivers and creeks. These aquifers offer little potential for future groundwater resource development beyond stock and domestic purposes. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-32 Two-dimensional conceptual schematic of the interconnected aquifer system and its variability Bore yields vary significantly depending on where (which geological unit) a bore is drilled and installed into and at what depth. Adapted from DENR (2016) Siltstone aquifers The Cambrian-age siltstones of the connected Daly, Wiso and Georgina basins host three lithologically equivalent hydrogeological units: (i) the Jinduckin Formation; (ii) the Hooker Creek Formation; and (iii) the Anthony Lagoon Formation. These units are mostly shale, which generally confines the CLA, but where they consist of interbedded lenses of sandstone and dolostone they also host local-scale aquifers of limited extent, and low permeability and yield. These aquifers are highly variable in composition, have a limited extent in the catchment and little information exists for bore yields and water quality. For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-33 Groundwater salinity for (a) the major aquifers hosted in the Tindall Limestone and equivalents and the Mount Rigg and Nathan groups and (b) other minor aquifers of the Roper catchment Symbol shape indicates aquifer formation within which bore is sited, colour indicates total dissolved solids (TDS). Salinity data source: Department of Environment, Parks and Water Security (2014) Surficial aquifers Surficial sediments and rocks include unconsolidated Quaternary-age regolith and alluvium, and consolidated Cretaceous-age sandstone, siltstone and claystone of the Carpentaria Basin. Alluvium predominantly occurs near the mouth of the Roper River, and in association with minor parts of the rivers, creeks and their floodplains and channels throughout the catchment. However, these aquifers have limited extent, are poorly characterised and therefore have very little information on bore yields and water quality. Aquifers hosted in regolith and Cretaceous rocks occur predominantly across the south and north-east of the catchment where they overlie the CLA and DCA, respectively. Aquifers hosted in the Cretaceous rocks are mostly comprised of sandstone. Individual bore yields can often be a few litres per second (~4 L/second) (Figure 2-31), and water quality for these aquifers is generally fresh (<1000 mg/L TDS) (Figure 2-33). Recharge to these aquifers occurs via diffuse rainfall infiltration through overlying regolith. The main discharge mechanisms are from bores extracting groundwater for stock and domestic use and from evaporation (through the soil or plants) from shallow watertables and as discharge to rivers and creeks. These aquifers offer little potential for future groundwater resource development beyond stock and domestic purposes. 2.5.3 Groundwater recharge Groundwater recharge is an important component of the water balance of an aquifer. It can inform how much an aquifer is replenished on an annual basis and therefore how sustainable a groundwater resource may be in the long term, particularly for aquifers with either low storage or that discharge to rivers, streams, lakes and the ocean, or via transpiration from groundwater- dependent vegetation. Recharge is influenced to varying degrees by many factors including spatial changes in soil type (and their physical properties), the amount of rainfall and evaporation, vegetation type (and transpiration), topography and depth to the watertable. Recharge can also be influenced by changes in land use, such as land clearing and irrigation. Directly measuring recharge can be very difficult as it usually represents only a small component of the water balance, can be highly variable spatially and temporally, and can vary depending on the type of measurement or estimate technique used (Petheram et al., 2002). In the Assessment, several independent approaches were used to estimate annual recharge for all aquifers in the Roper catchment. Figure 2-34 provides an example of the recharge estimates using the upscaled chloride mass balance (CMB) method. For more detail on how these estimates were derived, see the companion technical report on hydrogeological assessment (Taylor et al., 2023). For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-34 Annual recharge estimates for the Roper catchment Estimates based on upscaled chloride mass balance (CMB) method for the (a) 50th, (b) 5th and (c) 95th percentiles. Red polygons indicate the spatial extent of the Cambrian Limestone Aquifer (CLA) and delineate the unconfined and confined parts of the aquifer. Green polygons indicate the spatial extent of the Dook Creek Aquifer (DCA) and delineate the unconfined and confined parts of the aquifer. Aquifer data sources: Department of Environment, Parks and Water Security (2008); DCA – Knapton (2009) Figure 2-35 provides a summary of the range in recharge estimates related to the outcropping area of six key hydrogeological units across the Roper catchment. The range in recharge estimates are based on the 5th and 95th percentiles and range from approximately: • 11 to 38 mm/year for the Cambrian basalt • 26 to 67 mm/year for the Proterozoic dolostone and sandstone • 11 to 30 mm/year for the Cretaceous sandstone, siltstone and claystone • 15 to 46 mm/year for the Proterozoic sedimentary and igneous rocks • 2 to 5 mm/year for the Cambrian limestone. The estimates of groundwater recharge in the Assessment represent the spatial variability in recharge across the land surface and are a good starting point for estimating a water balance arithmetically or using a groundwater model. However, none of the methods account for aquifer storage (available space in the aquifer) so it is unclear whether the aquifers can accept these rates of recharge on an annual basis. The methods also do not account for potential preferential recharge from streamflow or overbank flooding, or through karst features, such as dolines and sinkholes that occur across parts of the Roper catchment. Therefore, the key features of an aquifer must be carefully conceptualised before simply deriving a recharge volume based on the surface area of an aquifer outcrop and an estimated recharge rate. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au 020406080100120140Cambrian basaltProterozoic dolostoneand sandstoneCretaceous sandstone, siltstone and claystoneProterozoic sedimentaryand igneousCambrian limestoneMean annual recharge (mm/yr) Hydrogeological unitCMB 5th percentileCMB 50th percentileCMB 95th percentile Figure 2-35 Summary of recharge statistics to outcropping areas of key hydrogeological units across the Roper catchment Error bars represent the standard deviation from the mean. CMB is the chloride mass balance method. 2.5.4 Surface water – groundwater connectivity As discussed in Section 2.5.2, groundwater discharge to surface water features occurs from a variety of aquifers across the Roper catchment. Areas of groundwater discharge are important for sustaining both aquatic and terrestrial groundwater-dependent ecosystems (GDEs). These areas have been mapped in Figure 2-36 as three categories: perennial groundwater discharge, seasonally varying and coastal. Perennial groundwater discharge areas often exhibit springs that occur in a variety of hydrogeological settings, these can involve groundwater flow systems at a variety of scales ranging from hundreds of metres to a few hundreds of kilometres. Areas with seasonally varying groundwater discharge are generally associated with localised alluvial, fractured and weathered rock aquifer systems that are adjacent to streams and are recharged during the wet season. These stores of water may sustain the riparian vegetation through the dry season. Although surface water is thought to be the major source for these systems, groundwater discharge from adjacent aquifers can also occur when river levels fall during the dry season. Coastal discharge occurs within the estuary of the Roper River and is associated with the Limmen Bight (Port Roper) Tidal Wetland System. These areas may have a component of coastal submarine groundwater discharge along with the evapotranspiration of sea water. The largest area of groundwater discharge in the Roper catchment is from the CLA hosted in the Cambrian Limestone to the upper Roper River and its tributaries (Roper Creek, Waterhouse River and Elsey Creek) and the Mataranka Spring Complex in Elsey National Park (Figure 2-36). Discharge occurs via a combination of diffuse seepage to streams (Roper Creek, upper Roper River, Waterhouse River and Elsey Creek), localised spring discharge at the Mataranka Spring Complex (Bitter, Rainbow, Botanic Walk and Fig Tree springs) as well as a few instream springs in the streambed of the upper Roper River. The source of discharge is from a combination of both regional and local groundwater flow within and outside of the catchment. The largest of these springs (Rainbow, Bitter and Fig Tree springs) flow at ~400 L/second through the dry season and combined with the many smaller springs have a combined flow of ~2500 L/second. In most years, this spring flow maintains a perennial flow in the Roper River down to the estuary, the flow continuously decreases in the downstream direction due to losses of water into the alluvium and fractured and weathered rock aquifers that support riparian vegetation. In the south-west of the catchment, where the Cretaceous Carpentaria Basin overlies the CLA hosted in the Cambrian Limestone, there is a large area notable for the absence of groundwater discharge sooth of the Mataranka Spring Complex. In this area, the watertable is generally very deep (up to 100 m) preventing any interaction with the surface. Most of the groundwater in this area flows north and is discharged in the Mataranka Springs Complex (some flows outside the catchment to discharge into the Flora and Daly rivers). There are many smaller springs in the north and east of the Roper catchment mostly associated with the Proterozoic dolostones and sandstones of the McArthur Basin between Baringa and Bulman. These generally support terrestrial GDEs by providing a source of water throughout the dry season. These springs are important locally but do not provide enough flow to maintain connectivity of the river systems with the water consumed within hundreds of metres from the source in most cases. The most notable of these are sourced from the DCA hosted in the Dook Creek Formation (Proterozoic dolostone) and supply some flow into Flying Fox Creek, the Mainoru and Wilton rivers, and outside the catchment to Guyuyu Creek and the Goyder River. Key springs associated with the DCA include Weemol, Lindsay, White Rock, Top and Emu springs. These discrete springs occur either at the change in geology where the DCA (Proterozoic dolostone) meets the Roper Group (Proterozoic sedimentary and igneous) or where large fractures occur in the overlying Limmen Sandstone of the Roper Group (Proterozoic sedimentary and igneous) allowing confined (naturally pressurised) groundwater to flow from the DCA to the surface (see the companion technical report on hydrogeological assessment by Taylor et al. (2023)). For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-36 Spatial distribution of groundwater discharge classes including surface water – groundwater connectivity across the Roper catchment Groundwater discharge classes inferred from remotely sensed estimates of evapotranspiration and open water persistence. Geology data sources adapted from: Department of Industry, Tourism and Trade (2014) and Department of Environment, Parks and Water Security (2008) Spring data source: Department of Environment, Parks and Water Security (2013) 2.5.5 Surface water Streamflow Approximately 60% of Australia’s runoff is generated in northern Australia (Petheram et al., 2010, 2014). Unlike the large internally draining Murray–Darling Basin, however, northern Australia’s runoff is distributed across many hundreds of smaller externally draining catchments (Figure 2-37). Figure 2-37 shows the magnitude of median annual streamflow of major rivers across Australia, prior to water resource development. To place the Roper catchment in a broader context it is useful to compare its size and the magnitude of its median annual streamflow to other river systems across Australia. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-37 Modelled streamflow under natural conditions Streamflow under natural conditions is indicative of median annual streamflow prior to European settlement (i.e. without any large-scale water resource development/extractions) assuming the historical climate (i.e. 1890 to 2015). Source: Petheram et al. (2017) The Roper catchment is 77,432 km2 in area (Figure 2-39) and features flat, tidally affected coastal plains that extend 20 to 60 km inland, that typically lie at less than 10 mAHD and are prone to seasonal flooding (see Figure 2-46). The Roper River extends approximately 300 km inland from the river mouth with major tributaries, the Wilton River and the Hodgson River, entering the river mid-catchment from the north and south, respectively. In headwater areas situated in the north- western part of the Roper catchment, altitudes reach up to 420 mAHD. Tidal influence on streamflow is detectable as far upstream as Roper Bar (around 10 km downstream of gauge 9030250), around 130 km from the Roper mouth. Due to the difficulty of streamflow measurement in tidally affected rivers, the lowermost reliable stream gauge on the Roper River is at Red Rock (Figure 2-38), a further 10 km upstream of Roper Bar. For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-38 Red Rock streamflow gauging station on the Roper River Photo: CSIRO Map - gauge station location "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\1_GIS\1_Map_docs\1_Exports\Hy-R-501_Roper_gauge_status_and_data_draft.png" Figure 2-39 Streamflow observation data availability in the Roper catchment The median and mean annual discharge from the Roper catchment under Scenario A is 4341 and 5560 GL, respectively. Table 2-4 provides a key summary of metrics for all gauging stations in the Roper catchment. The difference between the mean and median is less pronounced in the Roper catchment than in a number of other parts of northern Australia. The cease-to-flow column in Table 2-4 indicates the percentage of time that no streamflow was observed at each of the streamflow gauging stations in the Roper catchment. Gauges in the southern portions of the catchment exhibit higher proportions of cease-to-flow days, sometimes in combination with very low runoff coefficients. This is particularly apparent for the Elsey Creek gauge (9030001) where the runoff coefficient is around 1%. This is considered unusually low given the climate of the contributing land area. Table 2-4 Streamflow metrics at gauging stations in the Roper catchment under Scenario A Annual streamflow data are calculated under Scenario A. These data are shown schematically in Figure 2-40 and Figure 2-41. In the table, 20th, 50th and 80th refer to 20%, 50% and 80% exceedance, respectively. Cease-to-flow percentage (the percentage of all observation days where no streamflow was recorded) is determined using observed data, where streamflow less than 0.1 ML/day was assumed to be equal to zero. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 2-40 shows how median annual streamflow increases towards the coast in the Roper catchment. As an indication of variability, Figure 2-41 shows the 20% and 80% exceedance of annual streamflow in the Roper catchment. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-40 Median annual streamflow (50% exceedance) in the Roper catchment under Scenario A "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\10_Reporting\3_Roper\1_GIS\1_Map_docs\1_Export\CR-R-514_2x1_E20_80_Accumulated_runoff_v04.png" For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-41 20% and 80% exceedance of annual streamflow in the Roper catchment under Scenario A Figure 2-42 illustrates the decrease in catchment area and increase in elevation along the Roper River from its mouth to its source in the Waterhouse River. The large ‘step’ changes in catchment area are where major tributaries join the river. Chart, line chart. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-42 Catchment area and elevation profile along the Roper River from its mouth to the upper Waterhouse River at elevation 270 mAHD Catchment runoff The simulated mean annual runoff averaged over the Roper catchment under Scenario A is 72 mm. Figure 2-43 shows the spatial distribution of mean annual rainfall and runoff under Scenario A (1910 to 2019) across the Roper catchment. Mean annual runoff broadly follows the same spatial patterns as mean annual rainfall; runoff is highest in the north of the study area and lowest in the south. Notably, runoff is very low in the south-western portion of the catchment. This area is associated with the Sturt Plateau. Monthly and annual runoff data in the Roper catchment exhibit less variation from one year to the next compared to other parts of northern Australia. The annual runoff at 20%, 50% (median) and 80% exceedance averaged acrossthe Roper catchment is 113, 56and 23mm, respectively(Figure2-44). That is, runoff spatially averaged across the Roper catchment will exceed113mm 1year in 5, 56mm halfthe time and23mm 4 years in5.Figure2-44showsthe spatial distribution of the annual runoff at 20%,50% and 80% exceedanceunderScenario A. Intra-and inter-annual variability in runoff Rainfall,runoff and streamflow in the Roper catchment are variable between years but alsowithinyears. Approximately 80% of all runoff in the Roper catchment occursin the 3 months fromJanuaryto March, whichis very high compared to rivers in southern Australia (Petheram et al., 2008). While streamflow is ephemeral at manygauge sites,there are someriversin thewestern catchment (near Mataranka) and two adjacentto the Central Arnhem Highwaythat are perennial(Table2-4).Figure2-45b illustrates that during thewet season there isa high variationinmonthlyrunoff from one year tothenext. For example,during the month of March, in 20% of yearsthespatial mean runoffexceeded 49mm and in 20%of years it was less than6mm. The largestcatchment meanannualrunoff under Scenario Awas 300mm in 1975–76 and thesmallestcatchment meanannualrunoff under Scenario Awas2mm in 1951–52 (Figure2-45a). The CV ofannual runoff in the Roper catchment is0.9.Based on data from Petheram et al. (2008), thevariability in annual runoff in the Roper catchment is middleof therangecompared to theannualvariability in runoff of other rivers in northern and southern Australia with a comparable meanannual runoff. 82|Water resource assessmentforthe Ropercatchment \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\1_GIS\1_Map_docs\1_Exports\Hy-R-507_Aust_2x1_mean_annual_rainfall_runoff.png For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-43 Mean annual (a) rainfall and (b) runoff across the Roper catchment under Scenario A Pixel scale variation in mean annual runoff is due to modelled variation in soil type. \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\1_GIS\1_Map_docs\1_Exports\Hy-R-508_Aust_3x1_E20_50_80_runoff.png For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-44 Maps showing annual runoff at (a) 20%, (b) 50% and (c) 80% exceedance across the Roper catchment under Scenario A Pixel scale variation in mean annual runoff is due to modelled variation in soil type. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-45 Runoff in the Roper catchment under Scenario A (a) Time series of annual runoff averaged across the Roper catchment showing the 10 year rolling mean in blue and (b) monthly runoff averaged across the Roper catchment with the range (in blue) representing the 80 to 20% exceedance totals for that month Flooding The inland and coastal floodplains of the Roper catchment regularly flood over large areas, and flooding may extend for many hundreds of kilometres inland (Figure 2-46). Characterising these flood events is important for a range of reasons. Flooding can be catastrophic to agricultural production in terms of loss of stock, fodder and topsoil, and damage to crops and infrastructure; it can isolate properties and disrupt vehicle traffic providing goods and services to people in the catchment. However, flood events also provide opportunity for offstream wetlands to be connected to the main river channel. The high biodiversity found in many unregulated floodplain systems in northern Australia is thought to largely depend on flood events, which allow for biophysical exchanges to occur between the main river channel and wetlands. Unless otherwise stated, the material in this section is based on findings described in the companion technical report on flood modelling (Kim et al., 2023). For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-46 Flood inundation map of the Roper catchment Data captured using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery. This figure illustrates the maximum percentage of each MODIS pixel inundated between 2000 and 2020. Further observations of flooding under the historical climate in the Roper catchment are as follows: • The maximum areas inundated for events of AEP 1 in 2 (1988), AEP 1 in 5 (2008) and AEP 1 in 13 (1991) were 374, 1476 and 1495 km2, respectively (Figure 2-47). • Flood peaks typically take about 3 days to travel from Mataranka Homestead to Red Rock, at a mean speed of 3.3 km/hour. • For flood events of annual exceedance probability (AEP) 1 in 2, 1 in 5 and 1 in 13 the peak discharge at Red Rock on the Roper River is 1100, 1500 and 3000 m3/second, respectively. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-47 Spatial extent and temporal variation of inundation in the Roper catchment Simulated flood events during (a) 1991 (AEP 8%) and (b) 2008 (AEP 20%). AEP = annual exceedance probability • Between 1966 and 2019 (53 years), about 85% of events with a discharge greater than or equal to AEP 1 in 1 occurred between December and March. However, within those years, all months between August and April experienced events of this size at least once. Of the ten largest flood events at Red Rock on the Roper River, four events occurred during December, three in January and one event in each of February, March and April. Flood frequency in the Roper floodplain Flood frequency analysis (FFA) was performed in the Roper catchment to establish streamflow thresholds, above which a flood event would occur. FFA used streamflow observations from gauging station 9030250 (Roper River at Red Rock) as this gauge has a long historical record (>50 years) and has reasonable quality data. Flood extents for discrete flood events determined by assessing Landsat imagery (Landsat 5, 7 and 8) were matched with corresponding streamflow values at gauge 9030250. In general, FFA relies on event peak flow. However, in this study, to help determine the true magnitude of the events, the FFA accounted for total discharge volume as well as peak discharge for each event. This is motivated by the knowledge that the duration of an event can have a great impact on inundated area and not only its maximum discharge. Figure 2-48 displays the relationship between peak flow and AEP for gauge 9030250. This figure shows that total discharge volume is obviously closely linked with peak discharge. For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-48 Peak flood discharge and annual exceedance probability at gauge 9030250 (Red Rock) Instream waterholes during the dry season The rivers in the Roper catchment are largely ephemeral. Most notably, perennial flow is associated with groundwater discharge/springs in the Mataranka area (Figure 2-49) and downstream of this area. In ephemeral reaches, such as the Hodgson River, once streamflow has ceased, the rivers break up into a series of waterholes during the dry season. Waterholes that ‘persist’ from one year to the next are considered to be key aquatic ‘refugia’ and are likely to sustain ecosystems in the Roper catchment (Section 3.2). In some reaches, waterholes may be partly or wholly sustained by groundwater discharge (Section 2.5.2). However, in other reaches there is little evidence that ‘persistent’ waterholes receive water from groundwater discharge and are likely to be replenished following wet-season flows of surface water. For more information on this figure, chart or equation, please contact CSIRO on enquiries@csiro.au Figure 2-49 Groundwater fed waterhole near Bitter Springs, Mataranka Photo: CSIRO - Nathan Dyer The ecological importance and functioning of key aquatic refugia are discussed in more detail in the companion technical report on ecological modelling (Stratford et al., 2023). The formations of waterholes following a cease-to-flow event were captured using satellite imagery for a reach of the Flinders River in northern Australia (Figure 2-50). Figure 2-51 maps 1-km river reaches/segments where water is recorded in greater than 90% of dry-season satellite imagery. It provides an indication of those river reaches containing permanent water. Maps of instream waterhole evolution. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-50 Instream waterhole evolution This figure shows the area of waterholes at a given time after flow ceased and the ability of the water index threshold to track the change in waterhole area and distribution. "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\1_GIS\1_Map_docs\1_Exports\Hy-R-509_Roper_Persistent_Waterholes.png" For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-51 Location of river reaches containing permanent water in the Roper catchment Persistent river reaches are defined as 1-km river reaches where water was identified in greater than 90% of the dry- season Landsat (Landsat 5, 7 and 8) imagery between 1989 and 2018. Mapping of persistent river reaches is confounded by riparian vegetation in the Roper catchment. Surface water quality Observations of various aspects of surface water quality have been made in a series of studies, most notably by Schult and colleagues (i.e. Schult, 2014, 2016, 2018; Schult and Novak, 2017; see also Figure 2-52). These are summarised below: • Schult (2014) measured electrical conductivity (EC), dissolved oxygen (DO), turbidity, pH, nitrate (NO3), ammonia (NH3), filterable reactive phosphorus (FRP), total nitrogen (TN), total phosphorus (TP), major anions and cations and silica at the end of the dry season in 2012 and 2013 at sites across the upper Roper River (approximately 20). Water samples indicate that streamflow chemistry at these sampling times reflects groundwater chemistry of contributing areas. • Schult (2016) sampled surface water in the dry season of 2015 at six sites from Mataranka to near Red Rock. Of the 122 chemicals tested for, ten were detected in this study (i.e. three herbicides (diuron, simazine, tebuthiuron), one insecticide (imidacloprid), one flame retardant (TDCPP), and ingredients of insect repellents and cosmetics (DEET, galaxolide, tonalid, piperonyl butoxide)). Australian guideline values for ecosystem protection were not available for most contaminants, and levels were not exceeded for those that were. The springs around Mataranka were noted as a high source of nitrate to the Roper River. • Schult and Novak (2017) summarised water quality data collected over the years 2008 to 2016. They concluded that dry-season surface water quality is primarily influenced by the discharge of groundwater in the Roper River around Mataranka where high nitrates were also noted. However, nitrates decrease rapidly in a downstream direction. Wet-season flows were turbid, with over 300 nephelometric turbidity units (NTU) observed in peak flows. • Schult (2018) examined the relationships between flow and water quality at Elsey station in the dry season of 2017. Observations showed that EC and turbidity were strongly correlated with flow (EC inversely so). For more information on this figure or equation please contact CSIRO on enquiries@csiro.au Figure 2-52 Location of water quality sampling undertaken by previous studies A river with trees and a blue sky Description automatically generated Figure 2-53 Tranquil reach on the Roper River Photo: CSIRO – Nathan Dyer 2.6 References Abbott ST, Sweet IP, Plumb KA, Young DN, Cutovinos A, Ferenczi PA and Pietsch BA (2001) Roper Region: Urapunga and Roper River, Northern Territory. SD 53-10, 11. 1:250 000 geological map series. Explanatory notes. Northern Territory Geological Survey and Geoscience Australia, Canberra. Aldrick JM and Wilson PL (1992) Land systems of the Roper River catchment, Northern Territory. Conservation Commission of the Northern Territory, Northern Territory Government, Darwin. Andrews K and Burgess J (2021) Soil and land assessment of the southern part of Flying Fox station for irrigated agriculture. Part A: Land resources and general land capability. Department of Environment, Parks and Water Security, Northern Territory Government, Darwin. BOM (2023) Tropical cyclone databases. Bureau of Meteorology, Canberra. Viewed 6 February 2023, http://www.bom.gov.au/cyclone/tropical-cyclone-knowledge-centre/databases/. Bowman DMJS, Brown GK, Braby MF, Brown JR, Cook LG, Crisp MD, Ford F, Haberle S, Hughes J, Isagi Y, Joseph L, McBride J, Nelson G and Ladiges PY (2010) Biogeography of the Australian monsoon tropics. Journal of Biogeography 37(2), 201-216. DOI: 10.1111/j.1365- 2699.2009.02210.x. Burgess J, McGrath N, Andrews K and Wright A (2015) Agricultural land suitability series, report 1. Soil and land suitability assessment for irrigated agriculture in the Larrimah Area, Sturt Plateau. Department of Land Resource Management, Darwin. Charles S, Petheram C, Berthet A, Browning G, Hodgson G, Wheeler M, Yang A, Gallant S, Vaze J, Wang B, Marshall A, Hendon H, Kuleshov Y, Dowdy A, Reid P, Read A, Feikema P, Hapuarachchi P, Smith T, Gregory P and Shi L (2017) Climate data and their characterisation for hydrological and agricultural scenario modelling across the Fitzroy, Darwin and Mitchell catchments. A technical report to the Australian Government from the CSIRO Northern Australia Water Resource Assessment, part of the National Water Infrastructure Development Fund: Water Resource Assessments. CSIRO, Australia. Chiew FHS, Cai W and Smith IN (2009) Advice on defining climate scenarios for use in Murray– Darling Basin Authority Basin Plan modelling. CSIRO report for the Murray–Darling Basin Authority. Chiew F, Post D and Moran R (2012) Hydroclimate baseline and future water availability projections for water resources planning. 34th Hydrology and Water Resources Symposium 2012, Sydney, NSW, 19–22 November 2012. Engineers Australia, Barton, 461–468. CoastAdapt (2017) Sea-level rise and future climate information for coastal councils: Kowanyama, QLD. National Climate Change Adaptation Research Facility and Australian Government Department of Environment and Energy. Viewed 03 May 2018, https://coastadapt.com.au/sea-level-rise-information-all-australian-coastal- councils#NT_DARWIN. CSIRO (2009) Water in the Van Diemen region. In: Water in the Timor Sea Drainage Division. A report to the Australian Government from the CSIRO Northern Australia Sustainable Yields Project. CSIRO Water for a Healthy Country Flagship, Australia, 363–452. CSIRO and Bureau of Meteorology (2015) Climate change in Australia. Information for Australia’s natural resource management regions: technical report. CSIRO and Bureau of Meteorology, Australia. Day K, Sivertsen DP and Torlach DA (1984) Land resources of the Sturt Plateau, Northern Territory: a reconnaissance land system survey. Land Conservation Unit, Conservation Commission of the Northern Territory, Darwin. Denniston RF, Villarini G, Gonzales AN, Wyrwoll K-H, Polyak VJ, Ummenhofer CC, Lachniet MS, Wanamaker AD, Humphreys WF, Woods D and Cugley J (2015) Extreme rainfall activity in the Australian tropics reflects changes in the El Niño/Southern Oscillation over the last two millennia. Proceedings of the National Academy of Sciences 112(15), 4576-4581. DOI: 10.1073/pnas.1422270112. DENR (2016) Bore construction Darwin rural area most common in dolomite country. Department of Natural Resources and Environment. Data downloaded in September 2022, Hyperlink to: Bore construction Darwin rural area most common in dolomite country . Department of Environment, Parks and Water Security (2008) Northern Territory groundwater – aquifers 1:2,000,000. Northern Territory Department of Environment, Parks and Water Security, Darwin. Data downloaded in February 2021, https://data.nt.gov.au/dataset/northern-territory-ground-water- aquifers/resource/2eb67d03-8665-4a2d-8208-79885b8a9316. Department of Environment, Parks and Water Security (2013) Springs of the Northern Territory. Northern Territory Department of Environment, Parks and Water Security, Darwin. Data downloaded in February 2021, https://data.nt.gov.au/dataset/northern-territory- springs/resource/e2473c3c-a682-46fd-9ae7-5542b2c25ec7. Department of Environment, Parks and Water Security (2014) Digital groundwater database for the Northern Territory, supplied by the Department of Environment, Parks and Water Security, Copyright – Northern Territory of Australia. Data downloaded from the Northern Territory Government (NTG) Open Data Portal in September 2021, https://data.nt.gov.au/dataset/. Department of Environment, Parks and Water Security (2014) Sinkholes of the Northern Territory. Northern Territory Department of Environment, Parks and Water Security, Darwin. Data downloaded in February 2021, https://data.nt.gov.au/dataset/sinkholes-of-the-northern- territory/resource/de5d196e-f388-4c28-a97b-9fee24a9d973. Department of Industry, Tourism and Trade (2010) Northern Territory Geological faults 2500k. Northern Territory Department of Industry, Tourism and Trade, Darwin. Data downloaded in March 2021, http://geoscience.nt.gov.au/contents/prod/Downloads/Geology/GEO_FAULTS_2500K_shp.zip. Department of Industry, Tourism and Trade (2014) Northern Territory Geological map (interp) 2500K. Northern Territory Department of Industry, Tourism and Trade, Darwin. Data downloaded in February 2021, https://geoscience.nt.gov.au/downloads/NTWideDownloads.html. Evans S, Marchand R and Ackerman T (2014) Variability of the Australian monsoon and precipitation trends at Darwin. Journal of Climate 27(22), 8487–8500. DOI: 10.1175/JCLI-D- 13-00422.1. Fitzpatrick EA (1986) An introduction to soil science. Longman Scientific and Technical Group, Essex, UK. Forsyth AJ, Nott J and Bateman MD (2010) Beach ridge plain evidence of a variable late-Holocene tropical cyclone climate, North Queensland, Australia. Palaeogeography, Palaeoclimatology, Palaeoecology 297(3–4), 707-716. DOI: http://dx.doi.org/10.1016/j.palaeo.2010.09.024. Jeffrey SJ, Carter JO, Moodie KB and Beswick AR (2001) Using spatial interpolation to construct a comprehensive archive of Australian climate data. Environmental Modelling and Software 16, 309–330. Haig J, Nott J and Reichart G-J (2014) Australian tropical cyclone activity lower than at any time over the past 550-1,500 years. Nature 505(7485), 667-671. DOI: 10.1038/nature12882. http://www.nature.com/nature/journal/v505/n7485/abs/nature12882.html#supplementary -information. Hausfather Z and Peters GP (2020) Emissions–the ‘business as usual’ story is misleading. Nature 577(7792), 618-620. Hughes J, Yang A, Marvanek S, Wang B, Petheram C and Philip S (2023) River model calibration and scenario analysis for the Roper catchment. A technical report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia. IPCC (2022) Climate Change 2022: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. Cambridge University Press, Cambridge, UK and New York, NY, USA. Kim S, Hughes J, Ticehurst C, Stratford D, Merrin L, Marvanek S and Petheram C (2023) Floodplain inundation mapping and modelling for the Roper catchment. A technical report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Knapton A (2009) Gulf Water Study. An integrated surface – groundwater model of the Roper River Catchment, Northern Territory. Part C – FEFLOW Groundwater Model. Department of Natural Resources, Environment, The Arts & Sport Water Resources Branch, Technical Report No. 32/2009D. Available online: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.369.7455&rep=rep1&type=pdf Köppen W (1936) Das geographisca System der Klimate. In: Köppen W and Geiger G (eds), Handbuch der Klimatologie, 1. C. Gebr, Borntraeger, 1–44. Lerat J, Egan C, Kim S, Gooda M, Loy A, Shao Q and Petheram C (2013) Calibration of river models for the Flinders and Gilbert catchments. A technical report to the Australian Government from the CSIRO Flinders and Gilbert Agricultural Resource Assessment, part of the North Queensland Irrigated Agriculture Strategy. CSIRO Water for a Healthy Country and Sustainable Agriculture flagships, Australia. McBride JL and Nicholls N (1983) Seasonal relationships between Australian rainfall and the Southern Oscillation. Monthly Weather Review 111(10), 1998–2004. DOI: 10.1175/1520- 0493(1983)111<1998:SRBARA>2.0.CO;2. McFarlane D, Stone R, Martens S, Thomas J, Silberstein R, Ali R and Hodgson G (2012) Climate change impacts on water yields and demands in south-western Australia. Journal of Hydrology 475, 488–498. DOI: 10.1016/j.jhydrol.2012.05.038. McJannet D, Yang A and Seo L (2023) Climate data characterisation for hydrological and agricultural scenario modelling across the Victoria, Roper and Southern Gulf catchments. A technical report from the CSIRO Victoria, Roper and Southern Gulf Water Resource Assessments for the National Water Grid. CSIRO, Australia. McMahon TA and Adeloye AJ (2005) Water resources yield. Water Resources Publications LLC, Colorado, USA. National Committee on Soil and Terrain (2009) Australian soil and land survey field handbook. CSIRO Publishing, Collingwood. Nott JF and Jagger TH (2013) Deriving robust return periods for tropical cyclone inundations from sediments. Geophysical Research Letters 40(2), 370-373. DOI: 10.1029/2012GL054455. Peel MC, Finlayson BL and McMahon TA (2007) Updated world map of the Köppen-Geiger climate classification. Hydrology and Earth System Science 11(5), 1633–1644. DOI: 10.5194/hess-11- 1633-2007. Petheram C, Walker G, Grayson R, Thierfelder T and Zhang L (2002) Towards a framework for predicting impacts of land-use on recharge: 1. A review of recharge studies in Australia. Soil Research 40, 397–417. DOI: 10.1071/SR00057. Petheram C, McMahon TA and Peel MC (2008) Flow characteristics of rivers in northern Australia: implications for development. Journal of Hydrology 357(1–2), 93–111. DOI: 10.1016/j.jhydrol.2008.05.008. Petheram C, McMahon TA, Peel MC and Smith CJ (2010) A continental scale assessment of Australia's potential for irrigation. Water Resources Management 24(9), 1791–1817. Petheram C, Rustomji P, McVicar TR, Cai W, Chiew FHS, Vleeshouwer J, Van Niel TG, Li L, Cresswell RG, Donohue RJ, Teng J and Perraud J-M (2012) Estimating the impact of projected climate change on runoff across the tropical savannas and semiarid rangelands of northern Australia. Journal of Hydrometeorology 13(2), 483–503. DOI: 10.1175/JHM-D-11-062.1. Petheram C and Yang A (2013) Climate data and their characterisation for hydrological and agricultural scenario modelling across the Flinders and Gilbert catchments. A technical report to the Australian Government from the CSIRO Flinders and Gilbert Agricultural Resource Assessment, part of the North Queensland Irrigated Agriculture Strategy. CSIRO Water for a Healthy Country and Sustainable Agriculture flagships, Australia. Viewed6 February 2018,https://publications.csiro.au/rpr/pub?pid=csiro:EP13826. Petheram C, Gallant J, Wilson P, Stone P, Eades G, Roger L, Read A, Tickell S, Commander P,MoonA, McFarlaneD,Marvanek S (2014) Northern rivers and dams: a preliminary assessment ofsurfacewater storage potential for northern Australia. CSIRO Land and Water FlagshipTechnical Report. CSIRO, Australia. Petheram C, Rogers L, Read A, Gallant J, MoonA,Yang A, GonzalezD, SeoL, Marvanek S, Hughes J, Ponce Reyes R, Wilson P, Wang B,Ticehurst Cand Barber M (2017) Assessment of surfacewater storageoptions inthe Fitzroy,Darwin andMitchell catchments. Atechnicalreport tothe Australian Governmentfrom the CSIRO Northern Australia Water Resource Assessment, part of theNational Water InfrastructureDevelopment Fund: Water Resource Assessments. CSIRO, Australia. Petheram C, Yang A, SeoL, Rogers L, Baynes F,Devlin K,Marvanek S, Hughes J, Ponce Reyes R, Wilson P, Stratford D, Philip S (2022)Assessmentof surface water storageoptions andreticulation infrastructure in the Roper catchment. A technical report fromthe CSIRO RoperRiver Water Resource Assessment for theNational Water Grid. CSIRO, Australia. Plumb KAand RobertsHG(1992)Thegeology of Arnhem Land, Northern Territory.MineralsandLand Use Program.Bureau ofMineral Resources, Geology and Geophysics, Canberra. Rasmusson EM and Arkin PA (1993) A global view of large-scaleprecipitation variability. Journal ofClimate 6, 1495–1522. Raymond OL (2018) Australiangeological provinces 2018.01 edition. Geoscience Australia, Canberra.Viewed 18 October2020, https://ecat.ga.gov.au/geonetwork/srv/eng/catalog.search#/metadata/116823. Schult J (2014) End of dry season waterquality in the Daly and Roper River catchments. Report no. 03/2014D. Northern Territory Government Department of Land ResourceManagement, Palmerston. Schult J (2016) Pesticideand nutrient monitoringin the Roper River region duringthe 2015dryseason. Report no. 20/2016D. Northern Territory Department of Environment andNatural Resources, Palmerston. Schult J (2018)Dry season water qualityatElseystation in the upper reaches of the Roper River, 2017. Reportno. 3/2018D. Northern TerritoryDepartment of Environment and NaturalResources, Palmerston. Schult J and Novak P (2017) Water quality of theRoper River 2012–2016. Reportno. 2/2017D. NorthernTerritoryDepartment of Environmentand Natural Resources, Palmerston. Stratford D, Merrin L, Linke S, Kenyon R,Ponce Reyes R, Buckworth R,Deng R, McGinness H, Pritchard J, Seo L and Waltham N (2024) Assessmentofthe potential ecological outcomes ofwater resourcedevelopment in the Roper catchment.A technical reportfrom the CSIRORoper River Water Resource Assessmentfor the National Water Grid. CSIRO, Australia. Taylor AR, Crosbie RS,Turnadge C, Lamontagne S, Deslandes A,Davies PJ,Barry K, SuckowA, Knapton A,Marshall S, Hodgson G, Tickell S, Duvert C, Hutley L and Dooley K (2023) Chapter2 Physical environment ofthe Roper catchment|97 Hydrogeological assessment of the Cambrian Limestone Aquifer and the Dook Creek Aquifer in the Roper catchment, Northern Territory. A technical report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Thomas M, Philip S, Stockman U, Wilson PR, Searle, R, Hill J, Bui E, Gregory, L, Watson, I, Wilson PL and Gallant G (2022) Soils and land suitability for the Roper catchment, Northern Territory. A technical report from the CSIRO Roper River Water Resource Assessment for the National Water Grid. CSIRO, Australia.