Australia’s National Science Agency Water resource assessment for the Victoria catchment A report from the CSIRO Victoria River Water Resource Assessment for the National Water Grid Editors: Cuan Petheram, Seonaid Philip, Ian Watson, Caroline Bruce and Chris Chilcott ISBN 978-1-4863-2105-6 (print) ISBN 978-1-4863-2106-3 (online) Citation Petheram C, Philip S, Watson I, Bruce C and Chilcott C (eds) (2024) Water resource assessment for the Victoria catchment. A report from the CSIRO Victoria River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Chapters should be cited in the format of the following example: Bruce C, Petheram C, Philip S and Watson I (2024) Chapter 1: Preamble. In: Petheram C, Philip S, Watson I, Bruce C and Chilcott C (eds) (2024) Water resource assessment for the Victoria catchment. A report from the CSIRO Victoria River Water Resource Assessment for the National Water Grid. CSIRO, Australia. Copyright © Commonwealth Scientific and Industrial Research Organisation 2024. 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 Victoria 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 (NT) 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); Northern Land Council; NT Department of Environment, Parks and Water Security; NT Department of Industry, Tourism and Trade; Office of Northern Australia; Queensland Department of Agriculture and Fisheries; Queensland 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 and Water Security; NT Department of Industry, Tourism and Trade; NT Farmers; NT Seafood Council; Office of Northern Australia; Parks Australia; Regional Development Australia; Roper Gulf Regional Council Shire; Watertrust 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 their release. This report was reviewed by Dr Brian Keating (Independent consultant). Individual chapters were reviewed by Dr Rebecca Doble, CSIRO (Chapter 2); Dr Chris Pavey, CSIRO (Chapter 3); Dr Heather Pasley, CSIRO (Chapter 4); Mr Chris Turnadge, CSIRO (Chapter 5); Dr Nikki Dumbrell, CSIRO (Chapter 6); Dr Adam Liedloff, CSIRO (Chapter 7). The material in this report draws largely from the companion technical reports, which were themselves internally and externally reviewed. For further acknowledgements, see page xxv. 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 The Victoria River is the longest singularly named river in the NT with permanent water. Photo: 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 Victoria catchment and the current living and built environment of the Victoria catchment. This information covers: • the physical environment (Chapter 2) • the people, ecology and institutional context (Chapter 3). The Wickham River downstream of Yarralin. Adjacent to the river are red loamy and sandy levee soils potentially suitable for irrigated horticulture. On the break of slope, these soils are susceptible to erosion, as can be seen on the margins of the river banks. Also pictured are contiguous areas of treeless alluvial clay soils that are potentially suitable for irrigated broadacre crops. Photo: CSIRO – Nathan Dyer 2 Physical environment of the Victoria catchment Authors: Andrew R Taylor, Justin Hughes, Seonaid Philip, Jodie Pritchard, Steve Marvanek, Peter R Wilson, David McJannet, Fazlul Karim, Bill Wang, Cuan Petheram, Russell Crosbie Chapter 2 examines the physical environment of the catchment of the Victoria 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 "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\10_Reporting\1_All\9_Graphics_artist\3_Vic and SoG\C Bruce Vic CR Chp2_8_2024.jpg" For more information on this figure 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, and groundwater and surface water resources of the Victoria 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. Those analyses are reported in chapters 4 and 5. 2.1.1 Key findings Soils Soils with potential for agriculture in the Victoria catchment are mostly red loamy soils (17.5% of the catchment), cracking clay soils (11.7%) and other smaller areas of soils such as friable non-cracking clays or clay loams (6.5%). The cracking clay soils with potential are found on the alluvial plains and relict alluvial plains of the Victoria River and tributaries with broad areas along the West Baines River. They are moderately deep to very deep, slowly permeable and have high to very high water-holding capacity, but they may have restricted rooting depth in some areas due to very high salt levels in the subsoil. The alluvial plains on the lower Victoria and Baines rivers are poorly drained and subject to flooding. Cracking clay soils are also common in the south of the catchment on the Basalt gentle plains and the Basalt hills physiographic units but can be too rocky and/or shallow for agricultural development. The red loamy soils are typically found on the deeply weathered sediments in the south-west and south and the edge of the Sturt Plateau in the south-east; they are usually nutrient deficient and have low to high soil water storage. Some of the friable non-cracking clays or clay loams are subject to severe sheet and gully erosion. Some areas of soils highly suited to irrigated agriculture are found in narrow, ribbon-like distributions (e.g. loamy soils along the Wickam River around Yarralin), which may limit infrastructure layout and consequently agricultural opportunities. Over half the catchment (57.4%) is made up of shallow and/or rocky soils. Climate The Victoria catchment has a hot and arid climate that is highly seasonal and has an extended dry season. It receives a mean rainfall of 681 mm/year, 95% 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 Victoria catchment is generally suitable for growing a wide range of crops, though in most years rainfall would need to be supplemented with irrigation. Variation in rainfall from one year to the next is moderate compared to elsewhere in northern Australia but is high compared to other parts of the world with similar mean annual rainfall. The length of consecutive dry years in the Victoria catchment is not unusual when compared to other catchments in northern Australia, and the magnitude of dry spells is similar to many regions in the Murray–Darling Basin and east coast of Australia. Since the 1969–70 water year (1 September to 31 August), the Victoria catchment has experienced one tropical cyclone in 21% of cyclone seasons and two tropical cyclones in 6% of seasons. Approximately 13% of the global climate models (GCMs) project an increase in mean annual rainfall by more than 5%, about half project a decrease in mean annual rainfall by more than 5% and about a third indicate ‘little change’. Surface water and groundwater The timing and event-driven nature of rainfall events and high potential evaporation rates across the Victoria catchment have important consequences for the catchment’s hydrology. Approximately 98% of runoff occurs during the wet season (November to April, inclusive), and 93% of all runoff occurs during the 4-month period from December to March, which is a high concentration of runoff 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 Victoria catchment occur within the fractured and karstic (i.e. having features formed by dissolution) Cambrian limestone along the eastern margin and the Proterozoic dolostone in the centre and south of the catchment. The Montejinni Limestone along the eastern margin hosts the Cambrian Limestone Aquifer (CLA). The CLA is a complex, interconnected and highly productive regional-scale groundwater system (about 460,000 km2 in area). It extends for about 1000 km to the south-east and a couple of hundred kilometres both north and south of the eastern boundary of the Victoria catchment. In the Victoria catchment, the CLA occurs in the subsurface across an area of approximately 12,000 km2. Mean annual volumetric recharge over the entire CLA and over the part of the CLA that falls within the Victoria catchment are calculated to be 995 and 80 GL/year, respectively. Yields from bores are highly variable due to the complex nature of the karstic aquifer and can range up to 10 L/second. However, bore yields for the CLA in the Victoria catchment are currently poorly characterised. East of the catchment boundary where proper testing has been carried out, bore yields are commonly found to range from 10 to 50 L/second. The CLA in the Victoria catchment hosts fresh (<500 mg/L total dissolved solids (TDS)) to slightly brackish (<2500 mg/L TDS) groundwater suitable for a variety of different uses. Proterozoic dolostone aquifers (PDAs) hosted mostly in the Skull Creek and Timber Creek formations in the centre of the catchment, and the Campbell Springs and Pear Tree dolostones in the south of the catchment, host productive intermediate-scale groundwater systems. Like the CLA, the PDAs are complex due to the variability and interconnectivity between fractures, fissures and karsts. The PDAs outcrop and subcrop across an area of about 7000 km2 in the centre (between Timber Creek and Yarralin) and south (near and west of Kalkarindji) of the catchment. Bore yields are highly variable due to the complex nature of the karstic aquifers and commonly range from 5 to 15 L/second. Where appropriately constructed production bores have been installed and pumping tests conducted, bores can yield up to 40 L/second. Water quality in the dolostone aquifers is generally fresh (<500 mg/L TDS) but can be slightly brackish (<2000 mg/L TDS) in places, which is suitable for a variety of uses. Currently, there are no licensed groundwater entitlements in the Victoria catchment. There are three licensed entitlements totalling 7.4 GL/year from the CLA for use in agriculture about 150 km to the north-east and outside the Victoria catchment. Groundwater is the most common water source for stock and domestic use as well as community water supplies. The median and mean annual discharges from the Victoria catchment into the Joseph Bonaparte Gulf are 5730 and 6990 GL, respectively. Annual variation is high, and the annual flow is modelled to range from 800 to 23,000 GL. Flow is highly seasonal: 93% of all flow occurs in the months of December to March, inclusive. Current surface water licensed entitlements in the study area total about 152 GL, across four licenses. However, apart from one license for 0.7 GL which occurs in the Victoria catchment, the three larger licenced entitlements are far downstream and close to the coast (see Section 3.3.4). Many rivers in the catchment, particularly those in the southern parts of the catchment, are ephemeral and reduced to a few scarce and vulnerable waterholes during the dry season. The northern-most reaches of the Victoria River are tidal and can experience high tidal ranges (>8 m). Tidal influence is detectable as far south as near Timber Creek. 2.1.2 Introduction This chapter seeks to address the question: What soil and water resources are available for irrigated agriculture in the Victoria catchment? The chapter is structured as follows: • Section 2.2 examines the geology of the Victoria catchment, which is important in understanding the distribution of groundwater, soil and areas of low and high relief, which in turn influence flooding and the deposition of soil. • Section 2.3 examines the distribution and attributes of soils in the Victoria catchment and discusses management considerations. • Section 2.4 examines the climate of the Victoria catchment, including historical data and future projections of patterns in rainfall. • Section 2.5 examines the groundwater and surface water hydrology of the Victoria catchment, including groundwater recharge, streamflow and flooding. Figure 2-2 Soil sampling in the West Baines catchment A car in a field of trees Description automatically generated Photo: CSIRO – Nathan Dyer 2.2 Geology and physical geography of the Victoria catchment 2.2.1 Geological history The geological history of an area describes 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 determines 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 when understanding past and present ecological systems and patterns of human settlement. The oldest rocks in the area are Proterozoic (2500 to 540 million years old) and consist of repeated thick sequences of sediments, including some units containing significant amounts of dolostone (dolomite-rich rocks that are prone to solution over a geological timescale) (Figure 2-3). These sediments were deposited in a series of basins extending across the area and then gently folded, faulted and uplifted to form highlands. By the end of the Proterozoic, the highlands had been eroded down to a level not far above that of the current topography. During the Cambrian, 540 to 485 Ma (million years ago), there was widespread extrusion of basalt lava onto the eroded surface of the Proterozoic sediments. This event was followed by deposition of a sequence of limestones and dolostones (Figure 2-3). Further gentle folding, faulting and uplift then occurred followed by another cycle of erosion, which started after the Cambrian and continued to the mid-Cretaceous (about 100 Ma), again resulting in erosion down to a level not far above that of the current topography. During the remainder of the Cretaceous (to about 66 Ma), subsidence and high global sea levels resulted in deposition of a thin succession of Cretaceous shallow marine sandstone, conglomerate and mudstone. These layers were probably deposited across the whole area but are now only preserved in the south-east of the 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 Ma and ended in the mid-Cenozoic about 25 Ma. During this time, stable crustal conditions, subaerial exposure and prolonged subaerial weathering of the remaining Proterozoic, Cambrian and Cretaceous rocks resulted in the formation of deep weathering profiles and associated iron-cemented cappings on those rocks. Between the mid-Cenozoic and the present day, there has been gentle uplift and warping of the various surfaces and their weathered cappings. Continued erosion led to the emergence of the present-day landscape, which involved the development of incised valley systems that have been superimposed on the underlying Proterozoic rocks. Erosion has produced broader valleys where the dolomite-rich sediments were exposed and weathering and solution could occur. Extensive floodplains and coastal deposits were built up on the margins of modern drainage systems and the coastline, respectively. Figure 2-3 Surface geology of the Victoria catchment Surface geology map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\2_Victoria\1_GIS\1_Map_docs\1_Export\Gr-V-500_surface_geology_v02CR.png For more information on this figure please contact CSIRO on enquiries@csiro.au Source: adapted from Raymond (2012) 2.2.2 Physiography of the Victoria catchment The geological controls outlined in Section 2.2.1 have resulted in multiple physiographic units within the Victoria catchment as shown in Figure 2-4 and described in Table 2-1. The eight physiographic units, with shortened names in parentheses, are: • coastal marine plains (Marine plains) • alluvial plains of rivers and creeks (Alluvial plains) • level lateritic plains, plateaux and escarpments (Tertiary sedimentary plains) • gently undulating plains and rises on basalt (Basalt gentle plains) • gently undulating plains and pediments on dolomite and limestone, minor shales/mudstones/siltstones (Limestone gentle plains) • undulating rises to steep hills on basalt (Basalt hills) • hills and ridges on limestone (Limestone hills) • dissected plateaux, escarpments, steep hills and ridges on sandstones, siltstones and shales (Sandstone hills). The physiographic units serve as a useful framework to understand the potential agricultural lands and soils in terms of qualities and limitations, as each unit is derived from a distinct group of lithologies and landforms that give rise to a particular set of soil types and geomorphic patterns. In addition, they are useful for characterising sites that may offer potential to store water in the landscape. Potentially feasible dam sites occur where resistant ridges of rock that have been incised by the river systems outcrop on both sides of river valleys. The rocks are generally weathered to varying degrees, and the depth of weathering, the amount of outcrop on the valley slopes, the occurrence of dolomitic rocks that may contain solution features, and the width and depth of alluvium in the base of the valley are fundamental controls on the suitability of the potential dam sites. Where the rocks are relatively unweathered and outcrop on the abutments of the potential dam site, less stripping will be required to achieve a satisfactory founding level for the dam. In general, where stripping removes the more weathered rock, it is anticipated that the Proterozoic sandstones, siltstones, mudstones and conglomerates will form a reasonably watertight dam foundation requiring conventional grout curtains and foundation preparation. However, because dolostones are soluble over a geological timescale, it is possible that, where they occur within the Proterozoic sequences, potentially leaky dam abutments and reservoir rims may be present, which would require specialised and costly foundation treatment such as extensive grouting. Where this condition is possible, based on review of the 250,000 geological map sheets, it has been noted. The extent and depth of the Cenozoic or Quaternary alluvial sands and gravels in the floor of the valley are also important geological controls on dam feasibility, as these materials will have to be removed to achieve a satisfactory founding level for the dam. Table 2-1 provides more information on each physiographic unit shown in Figure 2-4: the area in hectares and as a percentage of the study area. Figure 2-4 Physiographic units of the Victoria catchment Physiographic map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\10_Reporting\2_Victoria\1_GIS\1_Map_Docs\CR-V-514_LL501_location_v5-10_Physiographic_v1.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Physiographic units based on Sweet (1977). Significant settlements and roads are overlaid on hillshaded terrain relief. Table 2-1 Victoria catchment physiographic unit descriptions, shortened names, areas and percentage areas PHYSIOGRAPHIC UNIT DESCRIPTION SHORTENED NAME AREA (HA) % OF STUDY AREA Coastal marine plains Marine plains 143,000 1.8 Alluvial plains of rivers and creeks Alluvial plains 643,000 7.8 Level lateritic plains, plateaux and escarpments Tertiary sedimentary plains 1,320,000 16.0 Gently undulating plains and rises on basalt Basalt gentle plains 1,031,000 12.5 Gently undulating plains and pediments on dolomite and limestone, minor shales/mudstones/siltstones Limestone gentle plains 741,000 9.0 Undulating rises to steep hills on basalt Basalt hills 1,100,000 13.3 Hills and ridges on limestone Limestone hills 540,000 6.6 Dissected plateaux, escarpments, steep hills and ridges on sandstones, siltstones and shales Sandstone hills 2,722,000 33.0 2.2.3 Major hydrogeological basins and provinces of the Victoria catchment Six major hydrogeological provinces with a generally north-east to south-west trending orientation occur across the Victoria catchment (Figure 2-5). From oldest to youngest these are the: (i) Birrindudu Basin, underlying a large portion of the central part of the catchment and outcropping in the centre and south-west, (ii) Fitzmaurice Basin, which outcrops across a small part of the north-west of the catchment and is bound to the south-east by the Victoria River Fault Zone, (iii) Victoria Basin, which overlies the Birrindudu Basin and underlies the central and northern parts of the catchment, outcropping mostly across the north, (iv) Kalkarindji Igneous Province (KIP), which overlies the Wiso, Victoria and Birrindudu basins and occurs most prominently across the east and south of the catchment, (v) Wiso Basin, which underlies and outcrops along the eastern margin of the catchment, and (vi) Bonaparte Basin, which outcrops in the most north-west peninsula. The Palaeo-Mesoproterozoic Birrindudu Basin is a sedimentary basin mostly comprising sedimentary sequences of sandstone, dolostone and siltstone (Dunster and Ahmad, 2013a). The basin overlies metamorphic basement rocks of the Halls Creek and Pine Creek orogens in the Victoria catchment and has a subsurface extent of approximately 37,000 km2 (Dunster and Ahmad, 2013a). The basin extends in the subsurface south and west of the catchment over an area of about 82,000 km2 across the NT, extending beyond the catchment boundary beneath the cover of overlying basins and provinces (Dunster and Ahmad, 2013). Sedimentary sequences of the Birrindudu Basin can be more than 10 km thick, and major outcrops for the basin occur in the centre of the Victoria catchment (Figure 2-5). Where the dolostone rocks have been weathered, they host productive karstic aquifers. Where more resistive sandstone rocks outcrop, they often form mountain ranges. Topographic features associated with the Birrindudu Basin include the Fitzgerald Range in the centre of the catchment, which in places is dissected by the Victoria, Wickham and East Baines rivers (Cutovinos et al., 2013). Figure 2-5 Major geological basins and provinces of the Victoria catchment Australian Geological Provinces data source: Raymond (2018); Regional geological faults data source: Department of Industry, Tourism and Trade (2010) The Palaeo-Mesoproterozoic Fitzmaurice Basin has an outcropping and subcropping area of approximately 2000 km2 in the Victoria catchment and extends north and west of the catchment boundary. It also overlies metamorphic rocks of the Pine Creek and Hall Creek orogens (Dunster, 2013). The Fitzmaurice Basin is mostly comprised of a series of stacked sandstone sequences of interlayered siltstone and shale with a conglomerate base and has a collective thickness of more than 3.5 km (Dunster, 2013). It is bounded to the south-east by the Victoria and Birrindudu basins along the Victoria Fault Zone and in the north-west by the younger Bonaparte Basin (Figure 2-5). Rocks of Geological basins and provinces map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\2_Victoria\1_GIS\1_Map_docs\Gr-V-501_GW_provinces_v07CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Fitzmaurice Basin are heavily faulted and gently folded in places and host partial aquifers with only minor groundwater resources. The sandstone rocks form mountain ranges such as the Pinkerton, Spencer and Yambarran ranges, which are dissected by the lower reaches of the Victoria River (Dunster, 2013). The Neoproterozoic Victoria Basin is a sedimentary basin hosting the Auvergne Group, which is mostly composed of interlayered sandstone and siltstone rocks (Dunster and Ahmad, 2013b). In the Victoria catchment, the Victoria Basin overlies the Pine Creek Orogen in the north-west and the Birrindudu Basin in the south-east. The Victoria Basin outcrops and subcrops mostly across the north of the catchment and has a subsurface extent beneath the catchment of approximately 26,000 km2. Its entire subsurface extent in the NT is approximately 36,000 km2 under the cover of the overlying KIP (Dunster and Ahmad, 2013b). The Victoria Basin is bordered by the Victoria River Fault Zone to the north-west and the Wiso Basin to the south-east (Figure 2-5). Outcropping rocks of the Victoria Basin are faulted in places and host localised fractured and weathered rock aquifers. The resistive sandstones form the Newcastle and Stokes ranges in the north of the catchment, which are in places dissected by the East Baines and Victoria rivers and Timber Creek (Cutovinos et al., 2014). The Neoproterozoic Wolfe Basin is composed of glacial and fluvioglacial sediments. It overlies the Birrindudu Basin and only occurs as a minor outcrop in the west of the Victoria catchment with the remainder obscured by the overlying KIP (Glass et al., 2013) (Figure 2-5). As the Wolfe Basin only intersects a minor part of the Victoria catchment, it is not described in detail here. The Kalkarindji Igneous Province was produced by widespread basaltic lava flows deposited over about 2,000,000 km2 during the early Cambrian (Glass et al., 2013). In the Victoria catchment, the KIP has a subsurface extent of approximately 40,000 km2 and is mostly composed of basalt and basalt breccia with minor sandstone and chert interbeds that can collectively be more than 300 m thick. The KIP overlies the Birrindudu and Victoria basins in the east and south of the catchment and underlies the Wiso Basin along the eastern margin (Figure 2-5). Outcropping rocks of the KIP form gentle basalt hills such as the Tent Hills in the east of the catchment, which are dissected in places by the Armstrong River and its tributaries (Glass et al., 2013). Where the basalt is fractured and faulted or occurs in conjunction with chert and/or sandstone, it hosts localised fractured rock aquifers. The middle Cambrian Wiso Basin is a sedimentary basin occupying approximately 140,000 km2 of the NT and is mostly composed of sandstone, siltstone, limestone and dolostone. The Wiso Basin is interconnected laterally with the Daly and Georgina basins to the north-east and south-east of the Victoria catchment, respectively (Kruse and Munson, 2013a, 2013b). Collectively these basins have a combined total area of about 460,000 km2, of which only a small portion (~12,000 km2) in the north- west coincides with the Victoria catchment. In the Victoria catchment, the Wiso Basin overlies and is bounded to the north-east by the KIP (Glass et al., 2013) (Figure 2-5). The sandstone, siltstone, limestone and dolostone sequences of the Wiso Basin are typically less than 300 m thick and are overlain by Cretaceous siltstone and claystone of the Mesozoic geological Carpentaria Basin (Kruse and Munson, 2013b; Munson et al., 2013). Outcropping limestone rocks of the Wiso Basin sometimes form gentle undulating plains to the east of Top Springs that are incised by tributaries of the Armstrong River (Cutovinos et al., 2013). Where the limestone and dolostone rocks have been weathered, they host productive karstic aquifers. The onshore and offshore parts of the late Palaeozoic to Cenozoic Bonaparte Basin have a total subsurface extent of approximately 270,000 km2, of which the onshore component only occupies an area of approximately 20,000 km2 (Ahmad and Munson, 2013). The basin is mostly composed of siliciclastic rocks and carbonate sedimentary rocks deposited in marine and fluvial environments that have a maximum onshore thickness of about 5 km (Ahmad and Munson, 2013). The onshore part of the basin in the Victoria catchment is small with a subsurface extent of approximately 1000 km2. In the Victoria catchment, the basin overlies the Halls Creek Orogen and is bound to the east by the Pine Creek Orogen and to the south by the Fitzmaurice Basin (Figure 2-5). It is mostly obscured by overlying Cenozoic sediments such as estuarine and delta deposits, black soil plains, sand plains and alluvium. Sandstone rocks of the basin host localised aquifers in places. 2.3 Soils of the Victoria 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; Jenny, 1941). Consequently, soils can be highly variable across a landscape. Different soils have different attributes that determine their suitability for growing different crops and guide how they need to be managed. The distribution of the different soils and their attributes closely reflects the geology and landform of the catchments. Hence, data and maps of soil and soil attributes that 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 Victoria catchment. Management considerations for irrigated agriculture are summarised in Table 2-2. Maps showing the suitability of different areas for different crops under different irrigation types in different seasons are presented in Chapter 4. Unless otherwise stated, the material in Section 2.3 is based on findings described in the companion technical report on digital soil mapping and land suitability (Thomas et al., 2024). Soils and their attributes were collected and described according to Australian soil survey standards (National Committee on Soil and Terrain, 2009). 2.3.2 Soil characteristics The soils of the Victoria catchment are presented in a soil generic group (SGG) classification (Figure 2-6; Table 2-2; Table 2-3). These groupings provide a means of aggregating soils with broadly similar properties and management considerations. The distinctive groupings have different potential for agriculture: some have almost no potential (e.g. the shallow and/or rocky soils – SGG 7), and some have moderate to high potential (e.g. the cracking clay soils – SGG 9), assuming other factors such as flooding and the amount of salt in the profile are not limiting. The SGGs were designed to simultaneously cover a number of purposes: (i) to be descriptive so as to assist non-expert communication regarding soil and resources, (ii) to be relatable to agricultural potential, and (iii) to align, where practical, to the Australian Soil Classification (ASC) (Isbell and CSIRO, 2016). Soil generic groups were first used in Queensland to facilitate extension in the sugar industry, and they have been modified to suit the range of soils encountered in the Assessment area. Figure 2-6 The soil generic groups (SGGs) of the Victoria 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., 2024). Labels on the map relate to the locational description of soils later in this Section (2.3.2). Soil generic group map and identified locations \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\10_Reporting\2_Victoria\1_GIS\1_Map_Docs\CR-V-513_SGGandLocats_v1-10_10-8.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Table 2-2 Soil generic groups (SGGs), descriptions, management considerations and correlations to Australian Soil Classification (ASC) for the Victoria catchment SGG SGG OVERVIEW GENERAL DESCRIPTION LANDFORM MAJOR MANAGEMENT CONSIDERATIONS ASC† CORRELATION 1.1 Sand or loam over relatively friable red clay subsoils Strong texture contrast between the A and B horizons: A horizons generally not bleached; B horizon not sodic and may be acid or alkaline. Moderately deep to deep well-drained red soils Undulating plains to hilly areas on a wide variety of parent materials The non-acid soils are widely used for agriculture; the strongly acid soils are generally used for native and improved pastures Red Chromosols and Kurosols except those with strongly bleached A horizons (the AT, AV, AY, AZ, BA or BB subgroups) 1.2 Sand or loam over relatively friable brown, yellow and grey clay subsoils As above but moderately well-drained to imperfectly drained brown, yellow and grey soils As above As above but may be restricted by drainage- related issues Brown, yellow and grey Chromosols and Kurosols except those with strongly bleached A horizons (the AT, AV, AY, AZ, BA or BB subgroups) 2 Friable non- cracking clay or clay loam soils Moderate to strongly structured, neutral to strongly acid soils with little or only gradual increase in clay content with depth. Grey to red, moderately deep to very deep soils Plains, plateaux and undulating plains to hilly areas on a wide variety of parent materials Generally high agricultural potential because of their good structure, moderate to high chemical fertility and water-holding capacity. Ferrosols on young basalt and other basic landscapes may be shallow and rocky Ferrosols and Dermosols without sodic B horizons (EO, HA, HC, HO, BA or HB subgroups) 3 Seasonally or permanently wet soils A wide variety of soils grouped together because of their seasonal or permanent inundation. No discrimination between saline and fresh water Coastal areas to inland wetlands, swamps and drainage depressions. Mostly unconsolidated sediments, usually alluvium Require drainage works before development can proceed. Acid sulfate soils and salinity are associated problems in some areas Hydrosols and Aquic Vertosols and Podosols with long-term saturation 4.1 Red loamy soils Well-drained, neutral to acid red soils with little, or only gradual, increase in clay content at depth. Moderately deep to very deep red soils Level to gently undulating plains and plateaux, and some unconsolidated sediments, usually alluvium Moderate to high agricultural potential with spray or trickle irrigation due to their good drainage. Low to moderate water- holding capacity; often hardsetting surfaces Red Kandosols 4.2 Brown, yellow and grey loamy soils As above but moderately well-drained to imperfectly drained brown, yellow and grey soils As above As above but may be restricted by drainage- related issues Brown, yellow and grey Kandosols 5 Peaty soils Soils high in organic matter Predominantly swamps Low agricultural potential due to very poor drainage Organosols SGG SGG OVERVIEW GENERAL DESCRIPTION LANDFORM MAJOR MANAGEMENT CONSIDERATIONS ASC† CORRELATION 6.1 Red sandy soils Moderately deep to very deep red sands. May be gravelly Sandplains and dunes. Aeolian, fluvial and siliceous parent material Low agricultural potential due to excessive drainage and poor water-holding capacity. Potential for irrigated agriculture Red Tenosols and Red Rudosols 6.2 Brown, yellow and grey sandy soils Moderately deep to very deep brown, yellow and grey sands. May be gravelly As above Low agricultural potential due to poor water-holding capacity combined with seasonal drainage restrictions. May have potential for irrigated agriculture Brown, yellow and grey Tenosols. Rudosols and Podosols without long-term saturation 7 Shallow and/or rocky soils Very shallow to shallow (<0.5 m). Usually sandy or loamy but may be clayey. Generally weakly developed soils that may contain gravel Crests and slopes of hilly and dissected plateaux in a wide variety of landscapes Negligible agricultural potential due to lack of soil depth, poor water- holding capacity and presence of rock Most soils <0.5 m, mainly very shallow to shallow Rudosols, Tenosols, Calcarosols and Kandosols 8 Sand or loam over sodic clay subsoils Strong texture contrast between the A and B horizons; A horizons usually bleached. Usually alkaline but occasionally neutral to acid subsoils. Moderately deep to deep Lower slopes and plains in a wide variety of landscapes Generally low to moderate agricultural potential due to restricted drainage, poor root penetration and susceptibility to gully and tunnel erosion. Those with thick to very thick A horizons are favoured Sodosols, bleached Chromosols and Kurosols (those with AT, AV, AY, AZ, BA or BB subgroups) and Dermosols with sodic B horizons (EO, HA, HC, HO, BA or HB subgroups) 9 Cracking clay soils Clay soils with shrink–swell properties that cause cracking when dry. Usually alkaline and moderately deep to very deep Floodplains and other alluvial plains. Level to gently undulating plains and rises (formed on labile sedimentary rock). Minor occurrences in basalt landscapes Generally moderate to high agricultural potential. The flooding limitation will need to be assessed locally. Many soils are high in salt (particularly those associated with the treeless plains). Gilgai and coarse structured surfaces may occur Vertosols 10 Highly calcareous soils Moderately deep to deep soils that are calcareous throughout the profile Plains to hilly areas Generally moderate to low agricultural potential depending on soil depth and presence of rock Calcarosols † Isbell and CSIRO (2016). The soil groups and soil characteristics presented below are evaluated in the context of their relationship to physiographic units within the catchment (Figure 2-4). These physiographic units serve as a useful framework to understand the distribution of SGGs and soil characteristics. The Victoria catchment contains soils from nine of the ten SGGs (Figure 2-6) − peaty soils (SGG 5) are not found. Of the nine SGGs found in the catchment, only three occupy more than 10% of the area (Table 2-3). Together these three soils occupy 87% of the catchment: the cracking clay soils (SGG 9, 11.7%) principally associated with the alluvial plains and relict alluvial plains of the West Baines and Victoria rivers and major tributaries – these are likely to be the first soils developed; red loamy soils (SGG 4.1, 17.5%) principally found on the deeply weathered sediments in the south-west, south and south-east – these soils make up the largest area with potential for development; and shallow and/or rocky soils (SGG 7, 57.4%), which make up over half the catchment and are derived from a wide range of geologies and geomorphic processes – these soils have very little or no potential for development. The red sandy soils (SGG 6.1, 1.6%), although a minor unit, are a large contiguous area in the far south-east of the catchment. Red soils are generally well drained, whereas yellow, grey and even bluey-green soils indicate increasingly persistent wetness and, ultimately, permanent waterlogging. Mottles indicate cycling between wetting and drying soil conditions, a sign of imperfect drainage and seasonal inundation. Table 2-3 Area and proportions covered by each soil generic group (SGG) in the Victoria catchment SGG DESCRIPTION AREA (HA) % OF STUDY AREA 1.1 Sand or loam over relatively friable red clay subsoils 780 0.01 1.2 Sand or loam over relatively friable brown, yellow and grey clay subsoils 2,010 0.02 2 Friable non-cracking clay or clay loam soils 536,580 6.5 3 Seasonally or permanently wet soils 295,660 3.6 4.1 Red loamy soils 1,439,840 17.5 4.2 Brown, yellow and grey loamy soils 80,440 0.9 5 Peaty soils 0 0 6.1 Red sandy soils 127,470 1.6 6.2 Brown, yellow and grey sandy soils 46,060 0.56 7 Shallow and/or rocky soils 4,730,850 57.4 8 Sand or loam over sodic clay subsoils 990 0.01 9 Cracking clay soils 962,440 11.7 10 Highly calcareous soils 16,880 0.2 SGG 9 soils are slowly permeable cracking clays (Vertosols) comprising 962,440 ha of the catchment. These occur on the alluvial plains associated with the West Baines (CA1 in Figure 2-6) and Victoria rivers and major tributaries (CA2), as relict alluvial plains throughout the catchment where they are associated with the Alluvial plains physiographic unit (Figure 2-4) (CR1 on the West Baines River and CR2 on tributaries of the Victoria River), and as level to gently undulating plains where they have an association with the Basalt gentle plains (CB1) and the Basalt hills physiographic units (CB2). Collectively these moderately deep to very deep (0.5 to >1.5 m), imperfectly to well-drained, slowly permeable, brown, red or grey, and occasionally black, cracking clay soils are non-sodic to strongly sodic at depth and have soft self-mulching or hardsetting surfaces. Sodicity is inherited from the parent material. The soils have high to very high water-holding capacity (>140 mm) but may have a restricted rooting depth due to very high salt levels in the subsoil. The brown, red, grey and black cracking clay soils are suited to a variety of dry-season grain, forage and pulse crops, sugarcane and cotton. The very deep (>1.5 m) clay plains of the West Baines River (CA1) and Victoria River (CA2) alluvial plains are predominantly imperfectly drained to moderately well-drained grey and brown hardsetting cracking clay soils, frequently with small (<0.3 m) normal gilgai depressions (Figure 2-7). These soils on the lower West Baines River alluvial plains grade to seasonally wet soils (SGG 3), including Aquic Vertosols (W1). Figure 2-7 Brown Vertosol SGG 9 soils on alluvial plains along the West Baines River. Gilgai microrelief is evident Photo: CSIRO The relict alluvial plains shown in Figure 2-8 are dominated by imperfectly drained self-mulching grey cracking clay soils grading to moderately well-drained grey-brown clay soils in the lower-rainfall southern parts of the catchment (CR3). These plains were deposited over a diverse range of geologies and frequently have shallow (0.1 to 0.2 m) normal to linear gilgai and surface gravels or stones of various lithology. These very deep (>1.5 m) grey to grey-brown clay soils are distinctly different to the SGG 9 Vertosols developed from basalt, which tend to be well structured and self-mulching, stonier and often shallower. Soil or landscape photo \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\2_Victoria\2_Reporting\Photos\SGG_PWB For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-8 A plain with grey Vertosol SGG 9 soils on relict alluvial plains near Top Springs. Linear gilgai surface microrelief is evident in the mid-left distance Photo: CSIRO – Nathan Dyer SGGs 4.1 and 4.2 (Kandosols) are the moderately deep to very deep (0.5 to >1.5 m) loamy soils separated by colour that reflects their landscape position. The well-drained red loamy variant SGG 4.1 covering 1,439,840 ha represents a significant area (17.5%) of the catchment, while the yellow loamy (SGG 4.2) variant covers less than 1%. Combined, these soils dominate the deeply weathered sediments of the Sturt Plateau (K1 in Figure 2-6) in the east to south-east and other deeply weathered landscapes to the south and west of Kalkarindji (K2). The deeply weathered character of these soils means that their distribution strongly correlates with the Tertiary sedimentary plains physiographic unit (Figure 2-4). Generally, the intact deeply weathered surface has moderately deep to deep (0.5 to <1.5 m) red soils (SGG 4.1) with moderate amounts of iron nodules (Figure 2-9). The depth to iron pans and the amount of iron nodules in the profile relate to position in the landscape. For example, shallow pans are associated with residual plateaux and residual concentrations of iron nodules on and/or in the soil profile in these positions. Alternatively, iron nodules could have been transported to places lower in the landscape. Soil or landscape photo \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\2_Victoria\2_Reporting\Photos\SGG_PWB For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-9 Well-drained red loamy soils (SGG 4.1) with iron nodules on the Sturt Plateau Photo: CSIRO SGG 4 soils on the deeply weathered landscapes are usually nutrient deficient with low to high soil profile water storage (70 to 140 mm). Irrigation potential is limited to spray and trickle irrigated crops on the moderately deep to deep soils with low to high soil water storage. Water storage is reduced as iron nodule content in these soils increases. SGG 4 Kandosols on the alluvial plains (K3) and minor locations elsewhere in the Victoria catchment are uncommon and often fragmented with narrow flat areas dissected by stream channels and deep gullies. The soils are highly suited to irrigated agriculture, but the characteristically narrow, ribbon- like distribution of these soils in the landscape may limit infrastructure layout and consequently agricultural opportunities. These moderately permeable soils have a moderate to high (100 to 140 mm) soil water storage capacity. Shallow and/or rocky soils (SGG 7; 4,730,850 ha) make up over half the catchment (Table 2-3). This grouping covers a wide range of different shallow (<0.5 m) and/or rocky soil types. They have a range of parent geologies, which means that they correspond to multiple physiographic units, especially upland units like Sandstone hills, Basalt hills, and Limestone hills (Figure 2-4). Soils like these tend to have very low to low soil water storage (<70 mm) and are sometimes found on eroded slopes and where intense gully patterns have fragmented the land surface to make the land agriculturally unviable. Examples of SGG 7 soils include shallow (<0.5 m) Kandosols with abundant iron nodules, iron pans and exposed laterite on the rises and scarp areas of deeply weathered landscapes (Figure 2-10). Soil or landscape photo \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\2_Victoria\2_Reporting\Photos\SGG_PWB For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-10 Shallow and rocky soils (SGG 7) on laterite outcrops and scarps of deeply weathered landscapes Photo: CSIRO SGG 2 soils, the friable clays and clay loams, occur extensively throughout the catchment but represent only 6.5% of the area (Table 2-3; 536,580 ha). The well-drained, moderately permeable, very deep (>1.5 m) red and brown soils associated with the levees of the rivers and major tributaries are subject to severe sheet and gully erosion and moderate wind erosion in the lower-rainfall areas of the southern catchment (F1 in Figure 2-6). These non-sodic soils have very strong slaking properties (breakdown of dry soil aggregates to micro-particles when wet) in the subsoils, making them less resistant to erosion. McCloskey (2010) describes the erosion processes and erosion extent on the riparian zone of the Victoria River. The strong soil slaking, deeply incised river channel with steep slopes in the riparian zone, intensive rainfall events and past land management have all contributed to the severe erosion and very large sediment loads entering the waterways. Extensive areas of these soils (F2) also feature in the Alluvial plains physiographic unit (Figure 2-4). The well-drained, moderately deep (0.5 to 1 m) red friable loams developed on the dolomite and limestone plains and pediments (F3) are subject to severe sheet erosion due to erosion of the thin (predominantly <0.1 m) sandy surface and exposure of the strongly slaking subsoil. The high silt and fine sand in the clay subsoil develop a strongly hardsetting scalded surface when eroded, which results in extensive runoff and rill erosion. In the lower-rainfall southern parts of the catchment (F1), these soils are also subject to wind erosion, leaving exposed scalded subsoils. These sheet-eroded soils are difficult to rehabilitate and have limited development potential. As they occur in association with extensive areas of shallow soils and rock outcrops, the areas suitable for agricultural development are usually small and fragmented. Soil or landscape photo \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\2_Victoria\2_Reporting\Photos\SGG_PWB For more information on this figure please contact CSIRO on enquiries@csiro.au SGG 3 soils include seasonally wet or permanently wet soils (Hydrosols and Aquic Vertosols). These soils comprise 295,660 ha (3.6%) of the catchment (Table 2-3) and occur extensively on the lower Baines (W1 in Figure 2-6), lower Victoria (W2) and Angalarri rivers (W3), and the low-lying alluvial coastal and marine plains (W4). The soils typically have a mottled grey clay subsoil, often with debil-debil microrelief. The low-lying seasonally wet non-saline alluvial plains of the lower Victoria River (W5) are suited to a limited number of dry-season irrigated crops. All other seasonally wet to permanently wet soils have limited potential for agricultural development. The coastal alluvial plains and very poorly drained saline marine plains subject to tidal inundation (W4) have very deep strongly mottled grey non-cracking and cracking clay soils subject to storm surge from cyclones. These near- coastal areas have potential for acid sulfate soils and are best represented in the Marine plains physiographic unit (Figure 2-4). SGG 6.1 soils are the deep red sandy, highly permeable soils (Tenosols; 127,470 ha of the catchment) on the sandplains and sand dunes of the northern extent of the Tanami Desert (S1 in Figure 2-6) coinciding with the Tertiary sedimentary plain physiographic unit in the far south of the catchment. Soils have a very low soil water storage (<70 mm) with potential for irrigated horticulture using trickle or drip systems. In the absence of irrigation, agricultural potential of these soils is low. 2.3.3 Soil attribute mapping Using a combination of new field sampling, pre-existing field data and digital soil mapping techniques, the Assessment mapped 18 attributes affecting the agricultural and aquaculture suitability of soil in the Victoria catchment, as described in the companion technical report on digital soil mapping and land suitability (Thomas et al., 2024). Descriptions and maps are presented below for six key attributes: •surface soil pH •soil thickness •soil surface texture •permeability •available water capacity (AWC) in the upper 100 cm of the soil profile (referred to as AWC 100) •rockiness. An important feature of each predicted attributes map (e.g. Figure 2-11a) is the companion reliability map showing the relative confidence in the accuracy of the attribute predictions (e.g. Figure 2-11b). Reliability statistical methods are described in the companion technical report on digital soil mapping and land suitability (Thomas et al., 2024). Note 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. Attributes are evaluated in terms of their relationship with the physiographic units (Figure 2-4). Surface soil pH The pH value of a soil reflects the degree to which the soil is alkaline or acidic, which affects the extent to which nutrients are available to plants for growth. Surface soil pH is the pH in the top 10 cm of the soil. For the majority of plant species, most soil nutrients are available when the pH range is 5.5 to 6.5. Nutrient imbalances are common for soils with pH greater than 8.5 or less than 5.5 and can lead to toxicity problems. The surface of most soils in the Victoria catchment are in the pH range 5.5 to 8.5 (Figure 2-11a) and thus would not limit crop growth in most instances. In terms of physiographic units (Figure 2-4), Marine plains, Limestone hills and Basalt gentle plains (i.e. clayier soils like SGGs 2, 3, 7 and 9) typically show values in the pH range 7.0 to 8.5, that is, neutral to alkaline. The remaining SGGs and physiographic units coincide with soils in the acid to neutral range (pH 5.5 to 7.0). The highly calcareous soils (SGG 10) developed on dolomite and limestone have consistently high surface pH (>8.0). Mapping reliability is highest in areas of the Tertiary sedimentary plains and some areas of Limestone gentle plains, which are the more homogeneous landscapes, and consistently lowest for the Marine plains physiographic unit where lack of data produces less reliable results (Figure 2-11b). Figure 2-11 (a) Surface soil pH (top 10 cm) of the Victoria catchment as predicted by digital soil mapping and (b) reliability of the prediction Map of soil surface pH DSM attribute \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\2_Victoria\1_GIS\1_Map_docs\LL-V-513_DSM_1x2_v2_Surf_pH.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Soil thickness Soil thickness is a measure of the potential root space and the depth of soil from which plants obtain their water and nutrients. Deeper soils (e.g. SGGs 2, 3, 4.1, 4.2, 6.1 and 9) are strongly associated with the Marine plains, Alluvial plains and Tertiary sedimentary plains physiographic units (Figure 2-12a). Shallower soils (which coincide with SGG 7 in particular) dominate the physiographic units with high relief, including Sandstone hills, Limestone hills and Basalt hills units. Moderately deep soils (especially SGGs 2 and 9) dominate the physiographic units with moderate relief, including the Basalt gentle plains and Limestone gentle plains. Shallower soils (e.g. SGG 7) are consistent with erosional landscapes where the rate of removal of weathering material exceeds the rate of accumulation. Mapping reliability is moderate to high overall and strongest where soils are moderately deep to deep, reflecting a data collection bias towards deeper soils. The less reliable areas are the higher-relief physiographic units having less data (Figure 2-12b). Figure 2-12 (a) Soil thickness of the Victoria catchment as predicted by digital soil mapping and (b) reliability of the prediction Map of soil thickness DSM attribute \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\2_Victoria\1_GIS\1_Map_docs\LL-V-516_DSM_1x2_v2_Soil_Thickness.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Soil surface texture Soil texture refers to the proportion of sand, silt and clay 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. Surface textures in the Victoria catchment are dominated by sandy soils, which coincide with Tertiary sedimentary plains, Sandstone hills, Limestone hills and Limestone gentle plains physiographic units (Figure 2-13a). These areas are dominated by SGGs 2, 4.1, 4.2, 6.1 and 7. The presence of these light-textured soils in the low-relief plains of the Tertiary sedimentary plains unit is explained by sandstone geology and in some places the influence of the Tanami dunefields and sands blown in, mantling the Tertiary landscapes. There are also extensive areas of clayey surface soils on basalt parent material (i.e. physiographic units Basalt hills and Basalt gentle plains; SGGs 2, 7 and 9) and on alluvial areas including the Marine plains and Alluvial plains physiographic units, which are generally composed of SGGs 3 and 9. Areas of loamy soils are less common in the catchment; they are generally associated with some Tertiary sedimentary plains (SGG 4.1) and zones within the Alluvial plains, Limestone gentle plains, Basalt hills and Basalt gentle plains (SGG 2) physiographic units. Areas of highest prediction reliability (Figure 2-13b) are found around the physiographic units of the Tertiary sedimentary plains, areas of Basalt gentle plains and much of the Sandstone hills, reflecting consistent textures within the units. Reliability tends to be lower around physiographic units of Marine plains, Basalt hills and Limestone gentle plains, reflecting a lack of data. Figure 2-13 (a) Soil surface texture of the Victoria catchment as predicted by digital soil mapping and (b) reliability of the prediction Map of surface texture DSM attribute \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\2_Victoria\1_GIS\1_Map_docs\LL-V-515_DSM_1x2_v2_Texture.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au 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, which reduces root zone drainage (i.e. water passing 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. The lowest soil permeabilities are found in the clay-rich soils, especially those coinciding with Marine plains, Alluvial plains and Basalt gentle plains physiographic units, hence dominated by SGGs 3 and 9 (Figure 2-14a). Most of the Assessment area is covered by moderate- to high-permeability soils. The highest permeabilities are experienced in the sandier soils that dominate physiographic units including Sandstone hills and Tertiary sedimentary plains, where SGGs 6.1 and 7 predominate. Mapping reliability (Figure 2-14b) is generally low to moderate throughout with little correlation to physiographic units or SGGs due to the complexity in landscape positions and their related permeability within each unit. Figure 2-14 (a) Soil permeability of the Victoria catchment as predicted by digital soil mapping and (b) reliability of the prediction Map of soil permeability DSM attribute \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\2_Victoria\1_GIS\1_Map_docs\LL-V-511_DSM_1x2_v2_Perm.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Availability water capacity to 100 cm 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 the volume of water required to wet up the soil profile. Soils with low AWC 100 require more frequent watering and lower volumes of water per irrigation. For rainfed agriculture, AWC 100 determines the capacity of crops to grow and prosper during dry spells. The largest AWC values are found where soils are deep and are clay-rich (Figure 2-15a), especially the physiographic units (Figure 2-4) of Marine plains, Alluvial plains and Basalt gentle plains (SGGs 3 and 9). Moderate AWCs are found in Tertiary sedimentary plains and Limestone gentle plains physiographic units. These moderate AWC soils tend to coincide with SGGs 2 and 4.1. The other physiographic units have low AWCs, reflecting the combination of shallowness and coarser textures. Mapping reliability (Figure 2-15b) is generally moderate to high, reflecting confidence in the data collected. It is notably lower for the Basalt gentle plains and Marine plains physiographic units and some areas of the Alluvial plains where there is more variation in the limited data. Figure 2-15 (a) Available water capacity in the upper 100 cm of the soil profile (AWC 100) as predicted by digital soil mapping in the Victoria catchment and (b) reliability of the prediction Map of AWC to 100 cm DSM attribute \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\2_Victoria\1_GIS\1_Map_docs\LL-V-512_DSM_1x2_v2_AWC.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Rockiness The rockiness of the soil affects agricultural management and the growth of some crops, particularly root crops. Coarse fragments (e.g. pebbles, gravel, cobbles, stones and boulders), hard segregations and rock outcrops 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 alluvial physiographic units (Marine plains and Alluvial plains) and the Tertiary sedimentary plains physiographic unit are generally free of surface rocks (Figure 2-4; Figure 2-16a). These non-rocky soils are dominated by SGGs 2, 3 and 9 on the alluvial plains and 4.1 and 6.1 on Tertiary sedimentary plains. All other units tend to be rocky at the surface, consistent with their shallow status (e.g. SGG 7) or with high-relief conditions associated with hilly physiographic units. The moderately deep to deep cracking clay soils on the Basalt gentle plains have surface rock due to the shrink−swell properties of the soil pushing rocks to the surface. The reliability of mapping is variable throughout, although generally most reliable in the Alluvial plain physiographic unit and in areas of the Tertiary sedimentary plains where soils are consistently rock free, and in the Sandstone hills and Limestone hills physiographic units (Figure 2-16b), where soils are consistently rocky. The less reliable areas have variable levels of rock and thus poor correlation. Figure 2-16 (a) Surface rockiness in soils of the Victoria catchment represented by presence or absence as predicted by digital soil mapping and (b) reliability of the prediction Map of soil rockiness DSM attribute \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\2_Victoria\1_GIS\1_Map_docs\LL-V-514_DSM_1x2_v2_Rockiness.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au 2.4 Climate of the Victoria catchment 2.4.1 Introduction Weather, which is defined as short-term atmospheric conditions, is the key source of uncertainty affecting hydrology and crop yield. It influences the rate and vigour of crop growth, and catastrophic weather events can cause extensive crop losses. Climate is defined as weather of a specific region averaged over a long period of time. 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 (Scientific Information for Land Owners) climate data surfaces (Jeffrey et al., 2001) unless stated otherwise. Very few climate observations are available in the region before 1890, therefore the 132-year period from 1 September 1890 to 31 August 2022 is used in the analysis presented below. 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 Victoria catchment The Victoria catchment is characterised by distinctive wet and dry seasons (Figure 2-17) due to its location in the Australian summer monsoon belt. Rainfall is highest in the northern parts of the catchment which are more frequently affected by monsoon westerly winds. The monsoon trough, a primary trigger for diurnal thunderstorm activity over the catchment, separates moist maritime winds to its north and much drier continental air to its south. The mean annual rainfall, averaged over the Victoria catchment for the 132-year historical period, is 681 mm. All climate results in the Assessment will be reported over the water year, defined as the period 1 September to 31 August, unless specified otherwise. Annual rainfall is highest in the northern part of the catchment and lowest in the most southerly part the catchment (Figure 2-17). This is because the more northerly regions of the catchment receive more wet-season rainfall as a result of active monsoon episodes. The Victoria catchment is relatively flat, so there is no noticeable topographic influence on climate parameters such as rainfall or temperature. Approximately 95% of the annual rainfall total in the Victoria catchment falls 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 2-17. Median wet-season rainfall exhibits a very similar spatial pattern to median annual rainfall, while median dry-season rainfall exhibits no strong spatial patterns. The highest monthly rainfall totals typically occur during January, February and March (Figure 2-18). Figure 2-17 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 Victoria 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 in which 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 For more information on this figure please contact CSIRO on enquiries@csiro.au on climate (McJannet et al., 2023)) since tropical cyclones do not affect the Victoria catchment in nearly three out of four years. For the 53 tropical cyclone seasons from 1969–70 to 2021–22, 72% of seasons registered no tropical cyclones tracking over the Victoria catchment, 21% experienced one tropical cyclone and 6% 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. Evaporation primarily affects the potential for irrigation by influencing: • runoff and deep drainage and, hence, the ability to fill water storages (Section 2.5) • crop water requirements (Section 4.3) • 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 Victoria catchment has a mean annual PE of 1900 mm (over the period 1890 to 2022), but unlike rainfall, has no strong north–south gradient across the catchment (Figure 2-17d). Preliminary estimates of mean annual (or seasonal) irrigation demand and net evaporation from water storages are sometimes calculated by subtracting the mean annual (or seasonal) PE from the mean annual (or seasonal) rainfall. This is commonly referred to as the mean annual (or seasonal) rainfall deficit (Figure 2-17g-i). The mean annual rainfall deficit, or mean annual net evaporative water loss, in the Victoria catchment is about 1250 mm, and the deficit increases with distance from the coast. 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 Victoria catchment as mainly ‘Semi-arid’, and the Köppen–Geiger classification classifies it as ‘Arid hot steppe’ (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 or 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 mean rainfall. Separating climate variability from climate change is difficult, especially when comparing climate on a year-to-year basis. In the Victoria catchment, 95% of the rain falls during the wet season (November to April). The highest monthly rainfall in the Victoria catchment typically falls in January or February (Figure 2-18). The months with the lowest rainfall are June to September. In Figure 2-18, the blue shading (labelled ‘A range’) represents the range under Scenario A (i.e. the historical climate from 1 September 1890 to 31 August 2022). The upper limit of the A range is the value at which monthly rainfall (or PE in Figure 2-19) is exceeded during only 10% of years (10% exceedance). The lower limit of the A range is the value at which monthly rainfall (or PE) is exceeded during 90% of years (90% 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. PE also exhibits a seasonal pattern: mean PE is at its highest during October (~205 mm) and its lowest during June (~110 mm) (Figure 2-19). Months where PE is high correspond to those months in which the demand for water by plants is also high. Mean wet-season and dry-season PEs in the Victoria catchment are shown in Figure 2-17. Compared to rainfall, the variation in monthly PE from one year to the next is small (Figure 2-19). The variation in rainfall from one year to the next is moderate compared to elsewhere in northern Australia but is high compared to other parts of the world with similar mean annual rainfall. Under Scenario A, rainfall for the Victoria catchment still exhibits considerable variation from one year to the next (Figure 2-18). Using Kalkarindji as an example, the highest annual rainfall (1204 mm, which occurred in the 2000–01 wet season) is nearly eight times the lowest annual rainfall (159 mm in 1953–54) and more than twice the median annual rainfall value (507 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 mean of the past 10 years of data including the current year. At Kalkarindji, for example, the 10-year running mean varied from 369 to 741 mm. This figure illustrates that the period between 2000 and 2010 was particularly 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)). The coefficient of variation (CV) provides a measure of the variability of rainfall from one year to the next. CV is calculated as the standard deviation of mean annual rainfall divided by the mean annual rainfall, and the larger the CV value, the larger the variation in annual rainfall relative to a location’s mean annual rainfall. Figure 2-20a shows the CV of annual rainfall for rainfall stations with a long- term record around Australia. Figure 2-20b shows that the inter-annual variation in rainfall in the Victoria catchment is about average for northern Australia catchments but is larger than stations in southern Australia with similar mean annual rainfall. 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 Victoria catchment and other parts of the world with a similar climate. Several factors are 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 Interdecadal Pacific Oscillation. Figure 2-18 Historical monthly rainfall (left) and time series of annual rainfall (right) in the Victoria catchment at Auvergne, Yarralin, Kalkarindji and Top Springs ‘A range’ is the 10th to 90th exceedance values for monthly rainfall. Note: the ‘A mean’ line is directly under the ‘A median’ line in some months in these figures. The solid blue line in the right column is the 10-year running mean. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-19 Historical monthly potential evaporation (PE) (left) and time series of annual PE (right) in the Victoria catchment at Auvergne, Yarralin, Kalkarindji and Top Springs ‘A range’ is the 10th to 90th exceedance values for monthly PE. Note: the ‘A mean’ line is directly under the ‘A median’ line in some months in these figures. The solid blue line in the right column is the 10-year running mean. For more information on this figure please contact CSIRO on enquiries@csiro.au (a) (b) \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\1_Climate\2_Victoria\1_GIS\1_Map_Docs\1_Exports\Cl-V-516_Cv_map_of_selected_stations_v1_1031.png Figure 2-20 (a) Coefficient of variation (CV) of annual rainfall and (b) the CV of annual rainfall plotted against mean annual rainfall for 99 rainfall stations around Australia (a) The grey polygon indicates the extent of the Victoria catchment. (b) The rainfall station in the Victoria catchment is indicated by a red symbol. The light blue diamonds are rainfall stations from the rest of northern Australia (RoNA), and hollow squares are rainfall stations from southern Australia (SA). Of these influences, the ENSO phenomenon is considered to be the primary source of global climate variability over the 2- to 6-year timescale (Rasmusson and Arkin, 1993), and it is reported to be 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 1890 and 2022, the mean rainfall onset date (defined as being the date on which 50 mm of rain has accumulated after the dry season) for the Victoria catchment is the last 10 days of December (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 determine whether 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, respectively, the median rainfall onset dates for the Victoria catchment are late December, mid-December and early December. 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. CLA-001 \\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\1_Climate\1_All\2_Reporting\NAWRA2-TR-Cl-A-WB1-v11.xlsm 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 Victoria catchment is likely to experience dry periods of similar severity to many areas in the Murray– Darling Basin and on the east coast of Australia. Victoria catchment is characterised by irregular periods of consistently low rainfall when successive wet seasons fail, in addition to the typical annual dry season. Runs of wet years 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 Auvergne, Yarralin, Kalkarindji and Top Springs 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 Victoria catchment indicates equally long runs of wet and dry years and nothing unusual about the length of the runs of dry years. Palaeoclimate records for northern Australia The instrumental record of climate data 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. the Pacific circulation responsible for ENSO) have been in place since about 3 to 2.5 Ma (Bowman et al., 2010). This suggests 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 instrumental 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). Furthermore, 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. Figure 2-21 Runs of wet and dry years at Auvergne, Yarralin, Kalkarindji and Top Springs (1890 to 2022) Wet years are shown by the blue columns and dry years by the red columns. For more information on this figure please contact CSIRO on enquiries@csiro.au 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 global climate models (GCMs) to represent a world where the global mean surface air temperatures are 1.6 °C higher than approximate 1990 global temperatures. This emission scenario is referred to as SSP2-4.5 (IPCC, 2022) and in this report as Scenario C. SSP2- 4.5 is the most likely future climate scenario according to 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, pattern scaled) and are 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 Victoria catchment, and the GCM-PSs are ranked in order of increasing mean annual rainfall. This figure shows that four (13%) of the projections for GCM-PSs indicate an increase in mean annual rainfall by more than 5%, two (6%) of the projections indicate a decrease in mean annual rainfall by more than 5%, and about 81% of the projections indicate a change in future mean annual rainfall of less than 5% under a 1.6 °C warming scenario. Hence, it can be argued that, based on the selected 32 GCM-PSs, the consensus result is that mean annual rainfall in the Victoria catchment is not likely to change under Scenario C. 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. 10% exceedance or Scenario Cwet), the middle (or 11th- wettest) GCM-PS (i.e. 50% exceedance or Scenario Cmid), and the third-driest GCM-PS (i.e. 90% exceedance or Scenario Cdry) are shown. Figure 2-24a shows mean monthly rainfall under scenarios A and C. The data suggest that mean monthly rainfall under Scenario Cmid will be similar to 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. Figure 2-22 Percentage change in rainfall and potential evaporation per degree of global warming for the 32 Scenario C simulations relative to Scenario A values for the Victoria catchment GCM-PSs are ranked by increasing rainfall for SSP2-4.5. "\\fs1-cbr\{lw-rowra}\work\1_Climate\2_Victoria\2_Reporting\plots\future_climate\mean.annual.percentage.change.per.degree.SSP245.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 Victoria catchment under scenarios (a) Cwet, (b) Cmid and (c) Cdry Figure 2-24 (a) Monthly rainfall and (b) potential evaporation for the Victoria catchment under scenarios A and C ‘C range’ is based on the computation of the 10% and 90% monthly exceedance values, so the lower and upper limits in ‘C range’ are not the same as scenarios Cdry and Cwet. Note: the ‘A mean’ line is directly under the ‘Cmid’ line in (a). Potential evaporation The mean annual change in GCM-PS PE shows projected PE increases of about 2% 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 as to how PE may change under a warmer climate. See Petheram et al. (2012) and Petheram and Yang (2013) for more detailed discussions. 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 Victoria catchment are summarised in Table 2-4. This information may be considered in coastal aquaculture developments and flood inundation of coastal areas. Mean annual rainfall scenarios \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\1_Climate\2_Victoria\1_GIS\1_Map_Docs\Cl-V-514-annuralRainfal-Cwet-Cmid-Cdry-v2.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au "\\fs1-cbr\{lw-rowra}\work\1_Climate\2_Victoria\3_future_climate\summary\ssp245\mean_month_rain_pet.png" Table 2-4 Projected sea-level rise for the coast of the Victoria catchment Values are the median of Coupled Model Intercomparison Project (CMIP) Phase 5 global climate models (GCMs). Numbers in parentheses are the 5% to 95% range of the 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 increases around Australia are projected with very high confidence for all emissions scenarios, which show 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 sea-surface temperature 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 the Victoria catchment, the corresponding projected sea-surface temperature increases are 0.8 °C (range across climate models is 0.6 to 1.1 °C) in 2030 under RCP 4.5 and 3.0 °C (2.5 to 3.9 °C) in 2090 under RCP 8.5. These changes are relative to a 1986 to 2005 baseline (CSIRO and Bureau of Meteorology, 2015). Note that the data in Table 2-4 use the CMIP5 dataset to provide estimates of sea-level rise. An updated product that uses CMIP6 was not available at the time of writing. 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 be based on scientific evidence and take a genuine long-term view. A hydroclimate baseline from 1890 to 2022 (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 developing a water resource plan, or in assessing 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. First, the transformation of rainfall to runoff and rainfall to groundwater recharge is non-linear. For example, averaged across the catchment of the Flinders River in northern Australia (Figure 1-1), 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, the median annual rainfall between 1895 and 1945 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 when 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 (e.g. market demand, interest rates) being equal, consecutive dry years are usually also the most limiting time periods for new water resource developments and/or 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, 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, in some circumstances 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 there has been a marked reduction in runoff in south-western Australia since the mid-1970s, 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 the instrumental record does not appear to be anomalous when compared to the longer-term instrumental record. In deciding upon an appropriate time period over which to analyse the water resources of the Victoria catchment, consideration was given to the above arguments, as well as to palaeoclimate records, observed trends in the historical instrumental rainfall data and future climate projections. For the Victoria catchment, although there is evidence of an increasing trend in rainfall in the recent instrumental record, 81% of the GCM-PSs project no change in mean annual rainfall for a 1.6 °C warming scenario. Furthermore, palaeoclimate records indicate that 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)). Very few climate data are available in the Victoria catchment before 1890, so the baseline adopted for this Assessment was 1890 to 2022. Note, however, that as climate is changing on a variety of timescales, detailed scenario modelling and planning (e.g. 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 Victoria catchment 2.5.1 Introduction The timing and event-driven nature of rainfall events and high PE rates across the Victoria catchment have important consequences for the catchment’s hydrology. The spatial and temporal patterns of rainfall and PE across the Victoria catchment are discussed in Section 2.4. Rainfall can be broadly broken into evaporated and non-evaporated components (the latter is 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. Figure 2-25 Simplified schematic diagram of terrestrial water balance in the Victoria 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. For more information on this figure please contact CSIRO on enquiries@csiro.au Section 2.5 covers the remaining terms of the terrestrial water balance (accounting for water inputs and outputs) of the Victoria catchment, with particular reference to the processes and terms relevant to irrigation at the catchment scale. Information is provided on groundwater, groundwater recharge and surface water – groundwater connectivity. Runoff, streamflow, flooding and persistent waterholes in the Victoria catchment are then discussed. Figure 2-25 is a schematic diagram of the water balance of the Victoria 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 groundwater characterisation (Taylor et al., 2024). Similarly, the material in Section 2.5.5 draws on the findings of the companion technical report on river modelling (Hughes et al., 2024), unless stated otherwise. 2.5.2 Groundwater Within the Victoria catchment, the distribution, availability and quality of groundwater resources are heavily influenced by the physical characteristics of the sediments and rocks of the major hydrogeological basins and provinces (see Section 2.2). Aquifers are the rocks and sediments in the subsurface that store and transmit groundwater. Figure 2-26 summarises the spatial distribution of the rocks of the major geological groups and units hosted in each hydrogeological basin and province. The physical properties of the different rocks and sediments that comprise each geological unit influence the nature of the different aquifer types and the groundwater systems they host. Essentially, the catchment has three main types of aquifers: • fractured and/or karstic dolostones and limestones and fractured and weathered or porous sandstones hosting productive aquifers • fractured and weathered basalt or sandstones hosting variably productive aquifers • fractured and weathered shale, siltstone and mudstone rocks hosting only partial aquifers. In addition, minor aquifers occur across parts of the catchment hosted in: (i) surficial sediments that predominantly include undifferentiated sandstone, siltstone and claystone, and (ii) in alluvium (clay, silt, sand and gravel) associated with the major rivers and their tributaries. These minor aquifers are limited in extent and host variably productive aquifers. The limestone and dolostone rocks of the Montejinni Limestone in the Wiso Basin are generally flat lying to gently dipping and occur along the eastern margin of the Victoria catchment. The Montejinni Limestone hosts one of the most productive groundwater systems beneath the catchment – the Cambrian Limestone Aquifer (CLA). The CLA is a key water resource in the NT and extends for about 1000 kilometres to the south-east and a couple of hundred kilometres to the north and south of the catchment, occupying an area of approximately 460,000 km2 across parts of the NT (see Figure 2-30). In the Victoria catchment, the CLA underlies approximately 12,000 km2 of its eastern margin. These carbonate rocks are fractured and fissured and weathered by dissolution in places, forming a complex, interconnected and highly productive regional-scale groundwater system. That is, the distance between the recharge areas (where there is inflow of water through the soil, past the root zone and into an aquifer) and discharge areas (where there is outflow of water from an aquifer into a water body or as evaporation from the soil or vegetation) across parts of the aquifer can be hundreds of kilometres, and the time taken for groundwater to discharge following recharge can potentially be in the order of hundreds to thousands of years or even longer. Hence, the surface water catchment boundary is not the groundwater flow boundary (or groundwater divide). Groundwater in the CLA flows from areas inside the catchment with higher groundwater levels to areas outside the catchment with lower groundwater levels. Dolostone and sandstone rocks of the Bullita and Limbunya groups of the Birrindudu Basin occur in the centre and south of the Victoria catchment and exhibit structural complexity in places where they can dip steeply in the subsurface (Figure 2-26). The combined groups have a subsurface extent within the catchment of approximately 37,000 km2, of which about 7000 km2 either outcrops or subcrops beneath overlying Cenozoic sediments. These carbonate rocks are fractured and fissured and weathered by dissolution in places, and the sandstones are fractured and/or porous, creating productive aquifers that host intermediate-scale groundwater systems. 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 potentially be in the order of tens to hundreds of years and possibly up to a few thousand years. As the Limbunya Group extends for about 100 km to the south of the Victoria catchment, the surface water catchment boundary is not the groundwater flow boundary for aquifers hosted in this geological group (Figure 2-26). Groundwater in the dolostones and sandstones flows from areas outside the Victoria catchment with higher groundwater levels to areas inside the catchment with lower groundwater levels. Basaltic rocks of the Antrim Plateau Volcanics in the KIP, have a subsurface extent of approximately 40,000 km2 beneath the eastern, southern and western parts of the Victoria catchment (Figure 2-26). These basalt rocks either outcrop or subcrop beneath surficial Cenozoic cover across an area of approximately 28,000 km2 in the catchment. They are almost entirely flat lying but are faulted, fractured and weathered. In addition, the basalt rocks can co-occur with sandstone and chert interbeds or basal agglomerate in places, and they host localised and isolated groundwater systems of varying productivity. Sandstone and siltstone rocks of the Auvergne and Wattie groups of the Victoria and Birrindudu basins, respectively, are flat lying to gently dipping and faulted in places, and they host localised and isolated groundwater systems of varying productivity. These weathered and fractured rocks occur across large areas of the north, centre and west of the Victoria catchment and host local-scale groundwater systems (Figure 2-26). The sandstone, siltstone and shale rocks of the Duerdin, Tijunna and Weaber groups occur in places across the north of the Victoria catchment with the siltstone and shale rocks only hosting partial aquifers containing little groundwater (Figure 2-26) (Dunster et al., 2000). Where sandstone rocks occur and they are fractured and/or porous, they host localised groundwater systems of variable productivity. Figure 2-26 Simplified regional geology of the Victoria catchment This map does not represent outcropping areas of all geological units: the blanket of surficial Cretaceous to Cenozoic rocks and sediments has been removed to highlight the spatial extent of various regional geological units in the subsurface. 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) Simplified regional geology \\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\2_Victoria\1_GIS\1_Map_docs\Gr-V-504_simplified_regional_geology_v4_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Hydrogeological units The major hydrogeological units of the Victoria catchment are shown in Figure 2-27. The rocks and sediments of these hydrogeological units host a diverse range of aquifers that vary in extent, storage and productivity. Figure 2-27 Simplified regional hydrogeology of the Victoria catchment This map does not represent outcropping areas of all hydrogeological units; the blanket of surficial Cretaceous to Cenozoic rocks and sediments has been removed to highlight the spatial extent of various regional hydrogeological units in the subsurface. Geology data sources adapted from: Department of Industry, Tourism and Trade (2014) and Department of Environment, Parks and Water Security (2008); Springs data source: Department of Environment, Parks and Water Security (2013); Sinkholes data source: Department of Environment, Parks and Water Security (2014) Simplified regional hydrogeology map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\2_Victoria\1_GIS\1_Map_docs\Gr-V-503_simplified_regional_hydrogeology_v05_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Major aquifers in the Victoria catchment contain intermediate- to regional-scale groundwater systems and are found in the fractured and karstic Cambrian limestone and Proterozoic dolostone, respectively (Figure 2-27 and Figure 2-29). For this Assessment, major aquifer systems are considered to be aquifers that occur across large areas and contain regional- and intermediate-scale groundwater systems with adequate storage volumes (i.e. gigalitres to teralitres) that could potentially yield water at a sufficient rate (>10 L/second) and be of a sufficient water quality (<1000 mg/L TDS) for a range of irrigated cropping. These larger groundwater systems provide greater opportunities for groundwater development because they often: (i) store and transmit larger amounts of water, (ii) provide opportunities for development away from existing groundwater users and groundwater-dependent ecosystems, and (iii) have greater potential to coincide with larger areas of soils that may have potential for agricultural intensification. Minor aquifers in the Victoria catchment are found in the Proterozoic sandstone and shale, and Cambrian basalt, which contain local-scale groundwater systems across smaller areas with lower storage (i.e. megalitres to a few gigalitres) (Figure 2-27 and Figure 2-29). The yields from minor aquifers can vary significantly but are often low (<5 L/second), and minor aquifers have highly variable water quality ranging from fresh (~500 mg/L TDS) to saline (~20,000 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 (Taylor et al., 2024). Only the major aquifers relevant to potential opportunities for future groundwater resource development are discussed in detail. Figure 2-28 Groundwater dependent ecosystems at Kidman Springs Photo: CSIRO – Nathan Dyer Figure 2-29 Major types of aquifers occurring beneath the Victoria catchment Localised surficial aquifers hosted in Quaternary alluvium, and consolidated Cretaceous rocks and sediments, are not shown. Aquifer type data source: Department of Environment Parks and Water Security (2008) Major aquifer types map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\2_Victoria\1_GIS\1_Map_docs\Gr-V-502_aquifer_type_v02_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Limestone aquifers Limestone aquifers are hosted in the Cambrian limestone along the eastern margin of the Victoria catchment (Figure 2-27). The Cambrian limestone comprised almost entirely of limestone and dolostone rocks of the Montejinni Limestone (Figure 2-26), hosts the fractured and karstic regional- scale CLA. The CLA consists of three equivalent hydrogeological units (Montejinni Limestone, Tindall Limestone and Gum Ridge Formation) occupying an area of about 460,000 km2 across the adjoining Wiso, Daly and Georgina basins of the NT, extending to the far north-east and south-east of the catchment (Figure 2-30). However, only a small portion of the Montejinni Limestone part of the CLA in the Wiso Basin occurs beneath the Victoria catchment; about 12,000 km2 of it along the eastern margin of the catchment, equivalent to about 15% of the total catchment area (Figure 2-27 and Figure 2-30). The CLA is complex regional-scale fractured and karstic (containing sinkholes, caves, caverns and springs) aquifer exhibiting a high degree of variability. It can be highly productive in places and is one of the largest and most productive groundwater resources in and beyond the Victoria catchment. The complexity of the system arises from the variability and interconnectivity between fractures, fissures and karsts across the spatial extent of the aquifer. Groundwater resources from the CLA in the catchment have mostly been developed for stock and domestic use and for the community water supply at Top Springs. Elsewhere to the north-east and south-east of the catchment, groundwater resources from highly productive parts of the CLA have been developed for groundwater-based irrigation. For more information on current groundwater use, see Section 3.3.4. Recharge to the CLA occurs either directly in the aquifer outcrop or where it is unconfined (connected with the atmosphere via open pore spaces of the overlying soil or rocks) beneath overlying Cretaceous sandstone, siltstone and claystone and/or Cenozoic sediments. In the Victoria catchment, the CLA outcrops around and to the north and south of Top Springs (see Figure 2-3) but remains unconfined beneath the veneer of overlying Cretaceous and Cenozoic rocks and sediments along the eastern margin of the catchment. Recharge to the CLA in the Victoria catchment occurs via a combination of: (i) localised preferential infiltration of rainfall and streamflow via sinkholes directly in the aquifer outcrop, and (ii) broad diffuse infiltration of rainfall through the overlying Cretaceous and Cenozoic rocks and sediments which then vertically leak to the underlying CLA. Low recharge rates to the CLA (see Section 2.5.3), high permeabilities of the karstic features and structural highs of the underlying Antrim Plateau Volcanics, influence the thickness of the CLA in the Victoria catchment. In some places, the CLA can be either unsaturated or have a thin saturated thickness (<20 m) (see Section 5). The aquifer discharges via a combination of: (i) intermittent lateral outflow to streams (Armstrong River and Bullock, Cattle and Montejinni creeks) where they are incised into the aquifer outcrop, (ii) perennial localised spring discharge (Old Top, Lonely, Palm and Illawarra springs) (see Figure 2-27 and Figure 2-31), (iii) vertical outflow to underlying basalt aquifers, (iv) evapotranspiration via riparian and spring-fed vegetation, and (v) groundwater extraction for stock and domestic use, including community water supply. The sources of intermittent groundwater discharge to ephemeral streams and perennial groundwater discharge to springs is from localised discharge from the aquifer outcrop around Top Springs. Figure 2-30 Simplified regional hydrogeology of the Victoria catchment relative to the entire spatial extent of the Cambrian limestone across large parts of the Northern Territory Cambrian Limestone Aquifer full extent map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\2_Victoria\1_GIS\1_Map_docs\Gr-V-503_simplified_regional_hydrogeology_ExtRegion_v05_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-31 Lonely Spring surrounded by dense spring-fed vegetation Photo: CSIRO Groundwater flow in the aquifer is complex due to the variability in the frequency, distribution and connectivity of karstic features across the aquifer and the spatial variability in seasonal recharge and discharge across large areas. At a local scale, groundwater flow can occur 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 medium (i.e. one with sufficient spaces between rocks for groundwater flow to occur across large areas). Along the eastern margin of the Victoria catchment, subtle topographic gradients on the edge of the Sturt Plateau create a groundwater flow divide inside the catchment margin. Regional groundwater flow and discharge occurs to the north-east outside the catchment further into the Wiso and Daly basins. Whereas, local to intermediate scale flow occurs to the west within the aquifer outcrop discharging along the western aquifer margin at localised spring complexes around Top Springs (see Section 5). Bore yields are variable due to the complex nature of the karstic aquifer. In the Victoria catchment, few properly constructed production bores have been installed and only limited pumping tests have been conducted. However, bore yields from stock and domestic bores in the CLA often range between 2 and 10 L/second, indicating that higher yields may be achievable from larger appropriately constructed production bores (Figure 2-32). Elsewhere in the CLA, east of the catchment, it has been found that where appropriately constructed production bores have been installed, bore yields can commonly be more than 10 L/second. In some cases where the aquifer is highly karstic across large areas to the east, bore yields can be as high as 100 L/second. Groundwater quality in terms of salinity ranges from fresh (<500 mg/L TDS) to slightly brackish (<2500 mg/L TDS) (Figure 2-33). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-32 Groundwater bore yields for the major aquifers across the Victoria catchment Symbol shapes indicate the aquifer within which the bore is sited; colours indicate bore yield class. Cambrian basalt data shown despite hosting minor aquifers due to their large spatial extent. Bore yield data source: Department of Environment, Parks and Water Security (2019) Yield major aquifer map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\2_Victoria\1_GIS\1_Map_docs\Gr-V-535_Yield_Major_Aquifers_v04_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Dolostone aquifers Dolostones aquifers are hosted in the Proterozoic dolostone rocks of the Bullita and Limbunya groups of the Birrindudu Basin across the centre and south of the Victoria catchment (Figure 2-26 and Figure 2-27). The dolostones host productive karstic intermediate- to local-scale aquifers (Figure 2-29). However, information for them is sparse. Proterozoic dolostone aquifers (PDAs) of the Bullita Group mostly outcrop in the centre of the catchment around Timber Creek and Yarralin. PDAs of the Limbunya Group mostly outcrop in the south of the catchment, west of Daguragu and Kalkarindji, and near Limbunya, which sits just outside and to the south of the catchment boundary (Figure 2-26 and Figure 2-27). Outcropping and subcropping (which occurs immediately beneath the overlying Cenozoic cover) parts of the Bullita and Limbunya groups occur across a combined total area of about 7000 km2. The most significant PDAs are hosted in the Skull Creek and Timber Creek formations of the Buillta Group between Timber Creek and Yarralin. For the Limbunya Group, it is the Campbell Springs and Pear Tree dolostones around Daguragu and Limbunya. Similar to the CLA, the PDAs are complex due to the variability and interconnectivity between fractures, fissures and karsts across their spatial extent. Groundwater resources in the aquifers have to date only been developed for stock and domestic water supplies and for the community water supply at Timber Creek. For more information on current groundwater use, see Section 3.3.4. The nature of and interconnectivity between karstic features influence the physical properties of the PDAs and groundwater flow processes across their spatial extent. Where the PDAs are unconfined in either outcropping or subcropping areas beneath overlying Cenozoic sediments, recharge is spatially variable and is inferred to occur via a combination of: (i) localised preferential infiltration of rainfall or streamflow where streams traverse the outcrop via sinkholes, fractures and faults, and (ii) broad diffuse infiltration of rainfall through the overlying Cenozoic sediments which vertically leaks to the underlying aquifers. Elsewhere, dolostone aquifers are confined (sealed off from the atmosphere by overlying rock and the groundwater is pressurised) by overlying Proterozoic sandstones and shales of the Auvergne and Tijunna groups, respectively, or the Antrim Plateau Volcanics (Figure 2-26). These overlying units influence the spatial variability in recharge to, and discharge from, the aquifers. The PDAs discharge via a combination of: (i) intermittent lateral outflow to streams (East Baines River and Crawford, Giles and Middle creeks) where they are incised into the aquifer outcrop (Figure 2-26 and Figure 2-27), (ii) perennial localised spring discharge at Bulls Head, Kidman and Crawford springs across the Buillta Group, and Depot, Farquharson and Wickham springs across the Limbunya Group (Figure 2-27 and Figure 2-34), (iii) evapotranspiration via riparian and spring-fed vegetation, and (iv) groundwater extraction for stock and domestic use, including community water supply at Timber Creek (see Section 3.3.4). Information on the directions and scale of groundwater flow in the aquifers are sparse, and groundwater flow is anticipated to be complex due to the variability in the amount and connectivity of karstic features across the aquifer and the spatial and temporal variability in annual recharge and discharge. Groundwater flow is inferred to generally occur from the elevated parts of the outcropping areas radially towards the outcrop margins where spring complexes occur. Bore yields are variable due to the complex nature of the karstic aquifer but yields often range from 5 to 15 L/second (Figure 2-32). However, where appropriately constructed production bores have been installed and pumping tests carried out for community water supply, yields have been as high as about 40 L/second. Groundwater quality expressed as salinity is generally fresh (<500 mg/L TDS) but can be subtly brackish in places (<2000 mg/L TDS) (Figure 2-33). Figure 2-33 Groundwater salinity for the major aquifers in the Victoria catchment Symbol shapes indicate the aquifer within which the bore is sited; colours indicate the level of total dissolved solids (TDS). Cambrian basalt data shown despite hosting minor aquifers due to their large spatial extent. Groundwater salinity data source: Department of Environment, Parks and Water Security (2019) TDS 10% Major Aquifers map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\2_Victoria\1_GIS\1_Map_docs\Gr-V-537_TDS_10%_Major_Aquifers_v04_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-34 Bulls Head Spring surrounded by dense spring-fed vegetation Photo: CSIRO Basalt aquifers Basalt aquifers are hosted in the Cambrian basalt, particularly the Antrim Plateau Volcanics of the Kalkarindji Igneous Province, and occur across large parts of the east, south and to a lesser extent the west of the Victoria catchment (Figure 2-26 and Figure 2-27). These basalt rocks are highly heterogenous and occur in association with sandstone and chert interbeds or basal agglomerate. They 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 contain local- scale flow systems (Figure 2-29). Most groundwater storage and flow results from the size and connectivity of secondary porosity features such as joints, fractures or faults, except where porous sandstone, chert or agglomerate occur. Recharge occurs as localised infiltration of rainfall and some streamflow (where streams traverse these geological units) through the weathered, fractured and jointed basalt. Recharge also occurs as broad diffuse infiltration of rainfall through the overlying Cenozoic strata in the south of the catchment which then vertically leaks to the underlying basalt aquifers, which are unconfined in these areas. Where basalt underlies limestone in the east of the catchment, the basalt aquifers are recharged in places from vertical leakage from the overlying limestone aquifers. The main discharge mechanisms are: (i) bores extracting groundwater for stock and domestic use, (ii) evaporation from shallow watertables, (iii) lateral discharge to streams, and (iv) localised discharge at discrete springs. Individual bore yields are variable but often low (<2 L/second, Figure 2-32), and water quality is variable, ranging from fresh (~500 mg/L TDS) to brackish (~3000 mg/L TDS, Figure 2-33). These aquifers offer little potential for future groundwater resource development beyond stock and For more information on this figure please contact CSIRO on enquiries@csiro.au domestic purposes. The exception to this may be where they occur in conjunction with, and are connected to, limestone aquifers hosted in the overlying Montejinni Limestone. Sandstone aquifers Sandstone aquifers are hosted in the Proterozoic sandstone of the Auvergne Group in the Victoria Basin, particularly the Jasper Gorge Sandstone. The Jasper Gorge Sandstone outcrops extensively across the north and west of the Victoria catchment and occupies most of the 16,000 km2 of outcropping and subcropping rocks of the Victoria Basin (Figure 2-26 and Figure 2-27). The sandstone is flat lying to gently dipping and faulted in places, and it hosts aquifers with variable productivity containing local-scale groundwater flow systems. These sandstones localised aquifers provide an important source of groundwater for stock and domestic use (Figure 2-29). Little information exists for these aquifers other than sparse information from stock and domestic bores. The most productive parts of the sandstone aquifers occur where the sandstone outcrop has undergone prolonged weathering and has been heavily fractured in and around fault zones. Groundwater storage and flow occurs via the secondary porosity features such as fractures, faults and jointing. Recharge occurs as: (i)localised infiltration of rainfall and some streamflow (where streams traverse the sandstone) intovertical fractures and joints, or (ii) broad diffuse infiltration of rainfall through the overlying Cenozoicstrata which then vertically leaks to the underlying sandstone aquifers, which are unconfined in theseareas. The main discharge mechanisms are bores extracting groundwater for stock and domestic use, evaporation (through the soil or plants) from shallow watertables (the start of the saturated zone ofan aquifer) and discharge to streams. Bore yields are variable depending on the degree and interconnectivity of fractures and joints around the bore casing. Bore yields can often be low (<2 L/second) where secondary porosity features are infrequent. However, where fracturing and jointing are common, yields of between 10 and 20 L/second can be achieved (Figure 2-35). Water quality for these aquifers is variable, ranging between fresh (~500 mg/L TDS) to brackish (~9000 mg/L TDS, Figure 2-37). These aquifers offer little potential for future groundwater resource development beyond stock and domestic purposes. Even though individual bore yields can be reasonable where fracturing is prominent, groundwater storage is still limited to these secondary porosity features, which means the aquifer can be vulnerable to depletion with prolonged (hours to days) groundwater extraction. Figure 2-35 Groundwater bore yields for minor aquifers across the Victoria catchment Symbol shapes indicate the aquifer within which the bore is sited; colours indicate bore yield class. Unknown sites could not be attributed to an aquifer or classified as a major or minor aquifer. Bore yield data source: Department of Environment, Parks and Water Security (2019) Yield minor aquifer map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\2_Victoria\1_GIS\1_Map_docs\Gr-V-534_Yield_Minor_Aquifers_v06_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Siltstone and shale aquifers Proterozoic siltstones and shales occur in the Auvergne Group of the Victoria Basin and the Tijunna Group of the Birrindudu Basin (Figure 2-5 and Figure 2-26). The most prominent siltstone and shale hydrogeological units across the Victoria catchment include the Angalarri Siltstone and Saddle Creek Formation of the Auvergne Group and the Stubb Formation of the Tijunna Group. These units host only partial aquifers that are highly localised and contain minor and very low yielding local-scale groundwater flow systems. These units outcrop over large areas in the centre and north of the catchment (Figure 2-26 and Figure 2-27). Very little information is available for these units other than from sparse stock and domestic bores. Recharge is inferred to occur via broad diffuse infiltration of rainfall and streamflow where streams traverse the outcropping areas of these units into the upper highly weathered parts of the siltstones and shales. Where these units subcrop beneath overlying Cenozoic strata, recharge occurs via diffuse vertical leakage from Cenozoic strata to the underlying aquifers, which are unconfined in these areas. The main discharge mechanisms are: (i) bores extracting groundwater for stock and domestic use, (ii) evaporation from shallow watertables, (iii) lateral discharge to streams, and (iv) localised discharge at discrete springs. These aquifers are highly variable in composition and are very low yielding (often <2 L/second, Figure 2-35). They contain highly variable water quality, and salinity ranges from fresh (<500 mg/L TDS) to brackish (i.e. ~9000 mg/L TDS, Figure 2-37). These partial aquifers host only minor groundwater resources and offer little to no potential for future groundwater resource development beyond stock and domestic purposes. Even developing them for stock and domestic purposes can be challenging due to poor bore yields and highly variable water quality. Figure 2-36 Jasper Gorge a spectacular sandstone gorge dissecting extensive plateau of low open woodlands and spinifex on shallow and rocky soils Photo: CSIRO – Nathan Dyer Figure 2-37 Groundwater salinity for the minor aquifers in the Victoria catchment Symbol shapes indicate the aquifer within which the bore is sited; colours indicate the level of total dissolved solids (TDS). Unknown sites could not be attributed to an aquifer or classified as a major or minor aquifer. Groundwater salinity data source: Department of Environment, Parks and Water Security (2019) TDS 10% Minor Aquifers map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\2_Victoria\1_GIS\1_Map_docs\Gr-V-536_TDS_10%_Minor_Aquifers_v04_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Surficial aquifers Surficial sediments and rocks include Cretaceous sandstone, siltstone and claystone of the geological Carpentaria Basin and unconsolidated Cenozoic alluvial and colluvial deposits of clay, silt, sand and gravel. Cretaceous rocks and sediments host basal sandstone aquifers, and Cenozoic alluvium hosts surficial aquifers that occasionally occur in association with minor parts of the rivers, creeks and their floodplains and channels throughout the catchment. However, these aquifers have limited extent and are poorly characterised, so there is very little information. Aquifers hosted in the Cretaceous rocks are mostly of sandstone. Recharge to these aquifers occurs via diffuse rainfall infiltration through overlying regolith. The main discharge mechanisms are: (i) bores extracting groundwater for stock and domestic use, (ii) evaporation from shallow watertables, and (iii) discharge to rivers, creeks and underlying hydrogeological units. Individual bore yields are highly variable, ranging from less than 1 L/second to approximately 10 L/second (Figure 2-35), and water quality as salinity is also highly variable, ranging from fresh (~500 mg/L TDS) to brackish (~13,000 mg/L, Figure 2-37). 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. This is particularly important for aquifers with low storage or aquifers 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 Victoria catchment. Figure 2-38 provides an example of 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 groundwater characterisation (Taylor et al., 2024). Figure 2-38 Annual recharge estimates for the Victoria catchment Estimates based on upscaled chloride mass balance (CMB) method for the (a) 50th, (b) 5th and (c) 95th percent exceedance. Figure 2-39 provides a summary of the range in recharge estimates for the outcropping area of seven key hydrogeological units across the Victoria catchment. Recharge estimates are based on the mean of the 5th and 95th percent exceedance and range from approximately: •8 to 28 mm/year for the Quaternary alluvium •3 to 13 mm/year for the Cambrian limestone •6 to 24 mm/year for the Cambrian basalt •8 to 63 mm/year for the Devonian–Carboniferous sandstone •10 to 38 mm/year for the Proterozoic sandstone •14 to 48 mm/year for the Proterozoic shale •15 to 49 mm/year for the Proterozoic dolostone. 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 accounts for aquifer storage (available space in the aquifer), so it is unclear whether the aquifers can accept these rates of Recharge percent exceedance map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\2_Victoria\1_GIS\1_Map_docs\Gr-V-515_CMB_R_percentiles_Vic_v1_cr.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au 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 Victoria 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. Figure 2-39 Summary of recharge statistics to outcropping areas of key hydrogeological units across the Victoria catchment Recharge rates are based on upscaled chloride mass balance (CMB) method and calculated as the 5th, 50th and 95th percent exceedance. Error bars represent the standard deviation from the mean. 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 Victoria catchment. Areas of groundwater discharge are important for sustaining both aquatic and terrestrial groundwater-dependent ecosystems. These groundwater discharge areas have been mapped in Figure 2-40 as three categories: perennial, 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 tens 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 Victoria River. These areas may have a component of coastal submarine groundwater discharge but also have mangroves that use fresh to saline water within the freshwater–saltwater interface. For more information on this figure please contact CSIRO on enquiries@csiro.au 020406080100120Mean annual recharge (mm/y) Hydrogeological unit95th percent exceedance50th percent exceedance5th percent exceedance Figure 2-40 Spatial distribution of groundwater discharge classes including surface water – groundwater connectivity across the Victoria catchment Groundwater discharge classes are inferred from remotely sensed estimates of evapotranspiration and open water persistence. Note: the size of polygons has been greatly exaggerated to allow them to be seen at this scale. Spring data source: Department of Environment, Parks and Water Security (2013) Groundwater discharge map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\2_Victoria\1_GIS\1_Map_docs\Gr-V-516_GW_discharge_v4_hydro_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au The largest area of groundwater discharge in the Victoria catchment is in the seasonally varying class associated with the alluvium along the major rivers, including the Victoria, Wickham and West Baines rivers (Figure 2-40). This groundwater discharge helps to maintain perennial waterholes in the rivers and dry-season flows. Discharge from the CLA in the Victoria catchment occurs at small springs that do not support perennial streams. The CLA in the Victoria catchment only has a small contributing area, and some of the groundwater recharged to the CLA within the Victoria catchment flows out of the catchment to the north-east. The springs sourced from the CLA in the Victoria catchment are mostly located to the south of Top Springs and occur near the boundary of the CLA and Antrim Plateau Volcanics; these include Old Top Springs, Lonely Spring, Palm Spring and Horse Spring. There are also groundwater discharges associated with the springs of the Proterozoic dolostones. These springs generally have small discharges but provide a permanent water supply through the dry season in an otherwise arid area. These include Kidman, Crawford and Dead springs sourced from the Bullita Group and Wickham and Depot springs sourced from the Limbunya Group. The local flow systems of the Cambrian basalt, Proterozoic sandstone and Proterozoic shale also support localised discharge via small discrete springs. 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 2-41). To place the Victoria 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. Figure 2-41 shows the magnitude of median annual streamflow of major rivers across Australia prior to water resource development. The Victoria River and its tributaries, the most substantial of which are the Baines, Wickham, Armstrong, Camfield and Angalarri rivers, define a catchment area of 82,400 km2 (Figure 2-42). The Victoria River itself spans approximately 500 km from Entrance Island at its mouth to Kalkarindji in the far south of the catchment. Tidal variation at the mouth of the Victoria River is up to 8 m, and these tides propagate upstream to just downstream of Timber Creek (Power and Water Authority, 1987). As discussed in Section 2.4, the catchment has a north−south rainfall gradient which influences the local hydrological response. The Camfield River in the drier far south of the catchment has an estimated mean runoff coefficient of 5%, while the Angalarri River in the north-east of the catchment has an estimated mean runoff coefficient of 17%. Mean annual flow at the catchment outlet of the Victoria River is estimated at 6990 GL, while median annual flow is 5730 GL. Annual variation is high, and annual flow is estimated to range between 800 and 23,000 GL. Flow is highly seasonal, and 93% of all flow occurs in the months December to March, inclusive. Flow statistics for a selection of streamflow gauging stations are shown in Table 2-5. "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\1_GIS\1_Map_docs\1_Export\Hy-V-501_Aust_accumulated_AnnualMedian_flow_AWRA_Victoria_rescaled.png" Figure 2-41 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 or extractions) assuming the historical climate (i.e. 1890 to 2015). Source: Petheram et al. (2017) Streamflow gauge observation data locations map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\1_GIS\1_Map_docs\Hy-V-502_Victoria_stream_gauges_v3_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-42 Streamflow observation data availability in the Victoria catchment Points labelled with letters refer to Figure 2-54. Table 2-5 Streamflow metrics at gauging stations in the Victoria catchment under Scenario A The 20th, 50th and 80th refer to 20%, 50% and 80% exceedance, respectively. These data are shown schematically in Figure 2-43 and Figure 2-44. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 2-43 shows that median annual streamflow increases towards the coast in the Victoria catchment. As an indication of variability, Figure 2-44 shows the 20% and 80% exceedance of annual streamflow in the Victoria catchment. Median annual streamflow map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\1_GIS\1_Map_docs\Hy-V-000_Victoria_accumulated_AnnualMedian_flow_(E50)_v05_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-43 Median annual streamflow (50% exceedance) in the Victoria catchment under Scenario A Exceedance of annual streamflow map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\1_GIS\1_Map_docs\Hy-V-003_2x1Victoria_accumulated_E20_E80_flow_v04.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-44 (a) 20% and (b) 80% exceedance of annual streamflow in the Victoria catchment under Scenario A Figure 2-45 illustrates the increase in catchment area and decrease in elevation along the Victoria River from a headwater catchment upstream of Kalkarindji to its mouth. The large ‘step’ changes in catchment area are where major tributaries join the river. "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\0_Working\2_Justin\6_catch_report\river_area_elevation_Victoria_v2.png" Figure 2-45 Catchment area and elevation profile along the Victoria River from upstream of Kalkarindji to its mouth Catchment runoff The simulated mean annual runoff averaged over the Victoria catchment under Scenario A is 87 mm. Figure 2-46 shows the spatial distribution of mean annual rainfall and runoff under Scenario A (1890 to 2022) across the Victoria catchment. Mean annual runoff broadly follows the same spatial patterns as mean annual rainfall: highest in the north of the study area and lowest in the south. Monthly and annual runoff data in the Victoria catchment exhibit less variation from one year to the next than other parts of northern Australia. The annual runoff volumes at 20%, 50% (median) and 80% exceedance averaged across the Victoria catchment are 125, 71 and 38 mm, respectively. That is, runoff spatially averaged across the Victoria catchment will exceed 125 mm 1 year in 5, 71 mm half the time and 38 mm 4 years in 5. Figure 2-47 shows the spatial distribution of annual runoff at 20%, 50% and 80% exceedance under Scenario A. "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\1_GIS\1_Map_docs\1_Export\Hy-V-506_Rain_Runoff_1x2.png" Figure 2-46 Mean annual (a) rainfall and (b) runoff across the Victoria 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\101_Victoria\1_GIS\1_Map_docs\1_Export\Hy-V-507_20_50_80_runoff_1x3.png" Figure 2-47 Annual runoff at (a) 20%, (b) 50% and (c) 80% exceedance across the Victoria catchment under Scenario A Pixel scale variation in mean annual runoff is due to modelled variation in soil type. Intra- and inter-annual variability in runoff Rainfall, runoff and streamflow in the Victoria catchment are variable between and within years. Approximately 82% of all runoff in the Victoria catchment occurs in the 3 months from January to March, which is a very high concentration of runoff relative to rivers in southern Australia (Petheram et al., 2008). A feature of streamflow data in the Victoria catchment is the almost total absence of dry-season data, which is due to the emphasis on flood information in this area. In some locations, such as gauge 8110007 (Coolibah Homestead) and 8110013 (Dashwood Crossing), there is some evidence of near-perennial flow. Perennial flow is also likely at gauge 8110074 on Montejinni Creek (where monitoring has been discontinued). In most other cases, flow is ephemeral. Figure 2-48b illustrates that during the wet season there is a high variation in monthly runoff from one year to the next. For example, during February, the spatial mean runoff exceeded 49 mm in 20% of years and was less than 5 mm in 20% of years. The largest catchment mean annual runoff under Scenario A was 287 mm in 1973–74, and the smallest was 10 mm in 1951–52 (Figure 2-48a). The CV of annual runoff in the Victoria catchment varies from 1.4 in the drier south to 0.7 in the north. Based on data from Petheram et al. (2008), the variability in annual runoff in the Victoria catchment is low compared to the annual variability in runoff of other rivers in northern and southern Australia with a comparable mean annual runoff. A close-up of a graph "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\0_Working\6_Ang\plots_for_catchment_report\hist\catch_month_annual_runoff.png" Figure 2-48 Runoff in the Victoria catchment under Scenario A showing (a) time series of annual runoff and (b) monthly runoff averaged across the catchment The solid blue line in (a) is the 10-year running mean. In (b) ‘A range’ represents the 80% to 20% exceedance totals for that month. Flooding Intense seasonal rains from monsoonal bursts and tropical cyclones from December to March create flooding in parts of Victoria catchment and inundate large areas of floodplains on both sides of Victoria River and its two major tributaries, the Baines and Angalarri rivers (Figure 2-49). This is an unregulated catchment, and its overbank flow is generally governed by the topography of the floodplain. Flooding is widespread at the junction of Victoria and Baines rivers, downstream of Timber Creek. Since 1980, there have been 37 floods greater than an annual exceedance probability (AEP) of 1 in 1 in the catchment. While floods can occur in any month from November to April, about 92% of historical floods have occurred between January and March, inclusive. Characterising these flood events is important for a range of reasons. Flooding can be catastrophic to agricultural production in terms of loss of stock, pasture and topsoil, and damage to crops and infrastructure. It can also isolate properties and disrupt vehicle traffic providing goods and services to people in the catchment. However, flood events also provide opportunities for offstream wetlands to connect to the main river channel. The high biodiversity found in many unregulated floodplain systems in northern Australia is thought to largely depend on seasonal flood pulses, which allow biophysical exchanges to occur between rivers and offstream wetlands. Flood inundation map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\1_GIS\1_Map_docs\Hy-V-503_MODIS_flood_v02_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-49 Flood inundation map of the Victoria 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 2023. Further observations of flood characteristics in the Victoria catchment are as follows: •Flood peaks typically take about 2 to 3 days to travel from Dashwood Crossing to Timber Creek at amean speed of 3.4 km/hour. •For flood events of AEP 1 in 2, 1 in 5 and 1 in 10, the peak discharges at the Coolibah Homestead onthe Victoria River are 2760, 4050 and 5800 m3/second, respectively. •Between 1953 and 2023 (70 years), events with a discharge greater than or equal to AEP 1 in 1occurred in all months from December to April, and about 91% of these events occurred betweenJanuary and March. Of the ten largest flood peak discharges at Coolibah Homestead, six occurred inMarch, three in February and one in December. •The maximum area inundated by a flood event of AEP 1 in 18 that occurred in March 2023 was1355 km2 (Figure 2-50). "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\1_GIS\1_Map_docs\1_Export\HFV-203_Inundation_HD_model.png" Figure 2-50 Flood inundation across the Victoria catchment for a flood event of 1 in 18 annual exceedance probability (AEP) in March 2023 Flood frequency in the Victoria floodplain Flood frequency analysis was performed in the Victoria catchment to establish streamflow thresholds above which a flood event would occur. Flood frequencies were estimated for the two major rivers in this catchment (Victoria and West Baines). For the Victoria River, flood frequencies were estimated using streamflow observations from gauging station 8110007 (Victoria River at Coolibah Homestead) as this gauge has a long historical record (>50 years) and has reasonable-quality data. Similarly, flood frequencies were estimated for the West Baines River using streamflow observations from gauging station 8110006 (West Baines River at Victoria Highway). Traditionally, flood frequencies are estimated based on maximum discharge for an individual event. However, in the Assessment, to help determine the magnitude of the events, the flood frequency analysis accounted for total flow volume as well as peak discharge for each event. This is motivated by the knowledge that the duration of an event, and not only its maximum discharge, can have a great impact on the inundated area. Figure 2-51 displays the relationship between peak flow and AEP for the two gauges: one on West BainesRiver (81100070) and the other on Victoria River (8110007). While flow volume is higher for largerfloods, duration of flood is a key factor for volume of flood flow. "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\0_Working\2_Justin\6_catch_report\AEP_2panelPlot.png" Figure 2-51 Peak flood discharge and annual exceedance probability (AEP) at (a) gauge 8110006 (West Baines River at Victoria Highway) and (b) gauge 8110007 (Victoria River at Coolibah Homestead) Colours indicate the total event volume of flood water in gigalitres (GL) for different events. Instream waterholes during the dry season The rivers in the Victoria catchment are largely ephemeral in the majority of reaches. 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 be sustaining ecosystems in the Victoria 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. Stream gauge data indicate that there is very little to no late dry-season flow for where gauge data are available (Figure 2-53). However, note that some late dry-season flow was recorded at gauge 8110113 (Dashwood Crossing) in response to relatively high rainfall in the 2000 to 2010 period. This was likely to be baseflow given the concurrent low dry-season rainfall. Minimum monthly flows increase for most locations from October to December; however, these increases are likely in response to early wet season storms. These data confirm that baseflow is very low and generally absent in the late dry season across most of the Victoria catchment. Minimum simulated October flows across many locations for the 132-year time series are shown in Figure 2-54. 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., 2024). The formation of waterholes following a cease-to-flow event can be captured using satellite imagery. Figure 2-55 shows an example of this for a reach of the Flinders River in northern Queensland. Figure 2-57 maps 1 km river reaches (or segments) in the Victoria catchment in which water is recorded in greater than 90% of dry-season satellite imagery. This is denoted the water index threshold and provides an indication of the river reaches that contain permanent water. Figure 2-52 Riparian vegetation along the West Baines River in the Victoria catchment. These areas are subject to regular flooding and the riparian vegetation plays an important role in regulating stream water quality. Photo: CSIRO – Nathan Dyer "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\0_Working\2_Justin\14_minimum_flows\minimum_drySeason_flow_observed_v3.png" Figure 2-53 Minimum dry-season flow observed at gauging stations 8110006, 8110007 and 8110113 Dry-season rainfall (July–September) and annual catchment rainfall are included below for context. "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\0_Working\2_Justin\6_catch_report\minFlowOctNovDec_v3.png" Figure 2-54 Minimum monthly flow over 132 years of simulation for October, November and December Assessed at either stream gauge locations or river model node locations indicated by labels ‘A’ and ‘B’ in Figure 2-42. Locations are listed in order from upstream (on the left) to downstream (on the right). The dashed blue horizontal lines equate to 200 ML/day, and the red horizontal dotted lines equate to 400 ML/day. Maps of instream waterhole evolution. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-55 Instream waterhole evolution in a reach of the Flinders River This figure shows the area of waterholes in the reach of the river a given time after flow ceased and the ability of the water index threshold to track the change in waterhole area and distribution. A dirt road with trees and a tower Description automatically generated Figure 2-56 Streamflow gauging station in the Victoria catchment Photo: CSIRO – Nathan Dyer Permanent waterholes map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\1_GIS\1_Map_docs\Hy-V-504_Victoria_permanent_waterholes_v1_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-57 Location of river reaches containing permanent water in the Victoria 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 Victoria catchment. Surface water quality A literature search on water quality in the Victoria catchment revealed only one significant investigation into water quality, which was conducted by the Power and Water Authority in 1982 and 1984 (Power and Water Authority, 1987). The investigation was conducted during baseflow conditions and measured major cations, anions, electrical conductivity, turbidity, dissolved oxygen and pH. Summaries of the spatial distribution of selected parameters are shown in Figure 2-58. "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\101_Victoria\0_Working\2_Justin\6_catch_report\geochem_maps_v2.png" Figure 2-58 Baseflow water quality in the Victoria catchment for parameters (a) electrical conductivity (EC), (b) chloride concentration, (c) total alkalinity, (d) calcium to sodium ratio, (e) silica concentration and (f) turbidity Data source: Power and Water Authority (1987) The most obvious features of Figure 2-58 are the elevated EC, chloride and turbidity values from the mouth of the river upstream to approximately Timber Creek. These high values are associated with the tidal movement of sea water. In the case of turbidity, river velocities remain relatively high in the tidal zone even in periods of low freshwater flow, such as those experienced when these samples were taken. No analysis of heavy metal concentration in stream water has been conducted in the catchment. Presumably, this is partly because no mining has taken place within the catchment. 2.6 References Ahmad M and Munson TJ (2013) Chapter 36: Bonaparte Basin. In: Ahmad M and Munson TJ (compilers) Geology and mineral resources of the Northern Territory. Northern Territory Geological Survey, Special Publication 5. Viewed September 2023: https://geoscience.nt.gov.au/gemis/ntgsjspui/bitstream/1/81516/1/GNT_Ch36_Bon.pdf. BOM (2023) Tropical cyclone databases. Bureau of Meteorology, Canberra. Viewed 6 February 2023, Hyperlink to: Tropical cyclone 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. Hyperlink to: Biogeography of the Australian monsoon tropics . 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