Water resource assessment for the Southern Gulf catchments Australia’s National Science Agency A report from the CSIRO Southern Gulf Water Resource Assessment for the National Water Grid Editors: Ian Watson, Caroline Bruce, Seonaid Philip, Cuan Petheram and Chris Chilcott ISBN 978-1-4863-2081-3 (print) ISBN 978-1-4863-2082-0 (online) Citation Watson I, Bruce C, Philip S, Petheram C and Chilcott C (eds) (2024) Water resource assessment for the Southern Gulf catchments. A report from the CSIRO Southern Gulf Water Resource Assessment for the National Water Grid. CSIRO, Australia. Chapters should be cited in the format of the following example: Philip S, Watson I, Petheram C and Bruce C (2024) Chapter 1: Preamble. In: Watson I, Bruce C, Philip S, Petheram C, and Chilcott C (eds) (2024) Water resource assessment for the Southern Gulf catchments. A report from the CSIRO Southern Gulf 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 Southern Gulf 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) and Queensland governments. The Assessment was guided by two committees: i. The 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 Southern Gulf catchments Steering Committee: Amateur Fishermen’s Association of the NT; Austral Fisheries; Burketown Shire; Carpentaria Land Council Aboriginal Corporation; Health and Wellbeing Queensland; National Water Grid (Department of Climate Change, Energy, the Environment and Water); Northern Prawn Fisheries; Queensland Department of Agriculture and Fisheries; NT Department of Environment, Parks and Water Security; NT Department of Industry, Tourism and Trade; Office of Northern Australia; Queensland Department of Regional Development, Manufacturing and Water; Southern Gulf NRM 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 Mr Mike Grundy (Independent consultant). Individual chapters were reviewed by Dr Peter Wilson, CSIRO (Chapter 2); Dr Andrew Hoskins, CSIRO (Chapter 3); Dr Brendan Malone, CSIRO (Chapter 4); Dr James Bennett, CSIRO (Chapter 5); Dr Nikki Dumbrell, CSIRO (Chapter 6); Mr Darran King, 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 xxviii. 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 Saltwater Arm, a tributary of the Albert River. This view typifies the tidal rivers and estuaries along the southern coast of the Gulf of Carpentaria. Source: Shutterstock 2 Physical environment of the Southern Gulf catchments Authors: Matthias Raiber, Matt Gibbs, Peter Zund, Andrew R Taylor, Seonaid Philip, Steve Marvanek, David McJannet, Fazlul Karim, Bill Wang, Cuan Petheram, Russell Crosbie, Justin Hughes Chapter 2 examines the physical environment of the catchments of the Southern Gulf rivers – that is, Settlement Creek, Gregory–Nicholson River and Leichhardt River, the Morning Inlet catchments and the Wellesley island groupsFigure 2-1. 1 – 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 1 Only those islands greater than 1000 ha are mapped. 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" Numbers in blue refer to sections in this report. 2.1 Summary This chapter provides a resource assessment of the geology, soil, climate, groundwater and surface water resources of the Southern Gulf catchments. 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 The soils with potential for agriculture in the Southern Gulf catchments are dominated by cracking clay soils (23% of the catchment), which are principally found on floodplains and alluvial plains of the Armraynald Plain and Barkly Tableland physiographic units. They typically have a moderate to high agricultural potential, are deep with a high soil available water capacity (AWC) and are suited to a wide variety of irrigated dry-season crops. Flooding, access and trafficability in the wet season are common constraints across the lower parts of the Armraynald Plain, and crop tolerance to poor soil drainage conditions restricts wet-season cropping in these areas. High salt levels within the soil profile in areas of the Armraynald Plain need management consideration. Friable, non-cracking clay soils (3% of the catchments) and loamy soils (~3% of the catchments) make up substantial areas. Both of these soils are well drained, and the friable, non-cracking clay soils have a generally high agricultural potential while the loamy soils have moderate to high agricultural potential. Sandy soils (10% of the catchment) have moderate agricultural potential with spray or trickle irrigation, although low to moderate water-holding capacity and hardsetting surface soils are common constraints. Shallow and/or rocky soils make up just over 56% of the catchment and have been assessed as having limited to no potential for agricultural development. These areas do, however, have substantial habitat value and broader biodiversity and cultural values. Climate The Southern Gulf catchments have a hot and arid climate that is highly seasonal with an extended dry season. They receive a mean rainfall of 602 mm/year, 94% 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 Southern Gulf catchments generally suits a wide range of crops, though in most years rainfall would need to be supplemented with irrigation. The variation in rainfall from one year to the next is moderately high compared to elsewhere in northern Australia and high compared to other parts of the world with similar mean annual rainfall. The Southern Gulf catchments do not experience unusually long sequences of dry years, and the magnitude of dry spells is similar to many areas in the Murray–Darling Basin and the east coast of Australia. Since the 1969–70 water year (1 September to 31 August), the Southern Gulf catchments have experienced one tropical cyclone in 36% of cyclone seasons and two tropical cyclones in 4% of seasons. Approximately 10% of the global climate models (GCMs) project an increase in mean annual rainfall by more than 5%, 22% project a decrease in mean annual rainfall by more than 5% and about 73% indicate ‘little change’. Surface water and groundwater The timing and event-driven nature of rainfall events and high potential evaporation rates across the Southern Gulf catchments have important consequences for the catchments’ hydrology. Approximately 97% of runoff occurs during the wet season (November to April, inclusive), and 87% of all runoff occurs during the 3-month period from January to March, which is very different to southern Australia where rainfall and runoff are less seasonal. This means that, in the absence of groundwater, water storages are essential for dry-season irrigation. The major aquifers in the Southern Gulf catchments occur within dissolution features in the dolostones and limestones of the Cambrian (541 to 485 million years ago (Ma)) Georgina Basin in the west of the Southern Gulf catchments, the Late Jurassic to Early Cretaceous Gilbert River Formation and, to a lesser extent, the Normanton Formation within the Carpentaria Sub-basin of the Great Artesian Basin (GAB). The carbonate rocks of the Georgina Basin, in particular the Camooweal Dolostone and Thorntonia Limestone, form part of a complex, interconnected and highly productive regional-scale groundwater system (about 460,000 km2) that extends for hundreds of thousands of square kilometres west, south and north of the south-western boundaries of the Southern Gulf catchments. Mean annual volumetric recharge over the aquifers of the Georgina Basin within the Southern Gulf catchments is estimated to be 122 GL/year. Bore yields are variable due to the complex nature of the karstic aquifer, but often range from less than 1 to 20 L/second. The carbonate rocks of the Georgina Basin are a complex regional-scale groundwater system due to the variability and interconnectivity between fractures, fissures and karsts. Currently about 1.8 GL/year of groundwater is licensed to be extracted from the Camooweal Dolostone and Thorntonia Limestone within the Southern Gulf catchments (see Section 3.3.4). The Gilbert River Formation and Normanton Formation host the most regionally extensive aquifer systems within the Southern Gulf catchments, where they extend over the eastern part of the catchment in Queensland and are entirely covered by the Cenozoic (66 Ma to present) Karumba Basin. Bore yields of the Gilbert River Formation are variable, ranging from less than 1 to 46 L/second. The aquifers of the geological Carpentaria Basin form part of the 1.7 million km2 large GAB, which reaches far to the south and east of the Southern Gulf catchments and includes the Eromanga and Surat basins and parts of the Clarence–Moreton Basin. Other local- to intermediate-scale aquifer systems are likely to exist in other geological formations within the Southern Gulf catchments. For example, the Cenozoic alluvium may offer potential in terms of water quality (median total dissolved solids (TDS) of approximately 600 mg/L). However, only limited data are available to assess lithological variability, hydraulic properties and the spatial extent of potential productive aquifers. The Southern Gulf catchments consist of various rivers and streams that discharge into the southern Gulf of Carpentaria. The most substantial of these are the Leichhardt, Gregory and Nicholson rivers. The median and mean annual discharges from the Southern Gulf catchments into the Gulf of Carpentaria are 4961 and 6759 GL/year, respectively. The majority of this volume is relatively evenly split between the Leichhardt and Gregory-Nicholson River catchments. Most rivers cease to flow over the dry season (27% to 79% of the time), however the Gregory River is a notable exception with perennial flow resulting from discharge from the Thorntonia Limestone hydrogeological unit. Current surface water licences across the study area total about 114 GL (i.e. 2.3% of the median annual discharge from the study area), with the majority of these licences in the Leichhardt River catchment, including those supplied from Lake Moondarra and Lake Julius (Section 3.34). Many rivers in the catchments, particularly those in the southern parts of the Southern Gulf catchments, are ephemeral and are reduced to a few scarce and vulnerable waterholes during the dry season. Some waterholes and river reaches, particularly those in the upper reaches of the Gregory River, are permanent and are replenished by groundwater (see Section 2.5.4). 2.1.2 Introduction This chapter seeks to address the question: What soil and water resources are available for irrigated agriculture in the Southern Gulf catchments? The chapter is structured as follows: • Section 2.2 examines the geology of the Southern Gulf catchments, 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 Southern Gulf catchments and discusses management considerations. • Section 2.4 examines the climate of the Southern Gulf catchments, including historical data and future projections of patterns in rainfall. • Section 2.5 examines the groundwater and surface water hydrology of the Southern Gulf catchments, including groundwater recharge, streamflow and flooding. 2.2 Geology and physical geography of the Southern Gulf catchments 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 Southern Gulf catchments are late Proterozoic (1780 to 1400 million years old). They consist of repeated thick sequences of sedimentary and metamorphic rocks and volcanics that include numerous prominent beds of sandstone (Figure 2-2). These sequences were deposited in a series of basins (e.g. the McArthur and South Nicholson basins and the Isa Superbasin) extending across the area and then folded, faulted and intruded (i.e. broken through) by igneous rocks to form mountain chains. Towards the end of the Proterozoic, the mountain chains had been eroded down to a level not far above that of the current topography. Surface geology map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\4_Water_storage\3_S_Gulf\1_GIS\1_Map_docs\WS510-S_Catchment_1M_Geology_overview_v02_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-2 Surface geology of the Southern Gulf catchments Source: adapted from Raymond (2012) During the Neoproterozoic to late Palaeozoic (850 to 350 Ma), the limestones, dolomites and minor sandstones of the Georgina Basin were deposited on a tectonically inactive platform. The Cambrian strata provide an important regional groundwater source in the west and south-west of the Southern Gulf catchments and are largely concealed by overlying Cretaceous sediments. The Jurassic to Cretaceous (125 to 100 Ma) geological Carpentaria Basin, a sub-basin of the GAB, hosts sequences of interbedded sandstones, mudstones and siltstones. These sequences underlie most of the eastern part of the Southern Gulf catchments and extend and thicken offshore. They include deposits of the Gilbert River Formation (GRF), which is composed of fluvial quartzose sandstones and forms the major GAB aquifer and is an important groundwater resource within the Southern Gulf catchments. Following deposition of the GRF, widespread transgression and then major regression in the late Middle Cretaceous led to deposition of the thick mudstone successions of the Wallumbilla Formation and erosion and deposition of a thin succession of Cretaceous shallow marine sandstones, conglomerates and mudstones. Extensive Cenozoic alluvial plains deposits unconformably overlie the geological Carpentaria Basin and mostly correspond to the Cenozoic Karumba Basin, which is persistent across much of northern Queensland and extends into eastern parts of the NT. Overlying the Karumba Basin are the youngest sediments in the catchments: the alluvial sands, silts and gravels associated with the beds, channels and floodplains of the catchments’ rivers and creeks and their tributaries. 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 era about 25 Ma. During this time, stable crustal conditions and subaerial exposure led to patchy erosion of the Cretaceous rocks and prolonged subaerial weathering of the remaining Cretaceous and Proterozoic rocks, resulting in the formation of deep weathering profiles and associated iron-cemented capping. Between the mid-Cenozoic and the present day, there has been gentle uplift and warping of the various surfaces and their weathered cappings. Continued erosion has led to the emergence of the present-day landscape, and extensive floodplains and coastal deposits were built up on the margins of modern drainage systems and the coastline, respectively, in the Southern Gulf catchments. 2.2.2 Physiography of the Southern Gulf catchments Ten physiographic units have been identified based on the geological controls outlined in Section 2.2.1. They are described in the companion technical report on digital soil mapping and land suitability (Thomas et al., 2024) and shown in Figure 2-3. Physiographic map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\4_S_Gulf\1_GIS\1_Map_docs\LL-S-501_location_v2_v11_Arc10_8.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-3 Physiographic units of the Southern Gulf catchments Physiographic units as described in the companion technical report on digital soil mapping and land suitability (Thomas et al., 2024) and based on Grimes (1974). Significant settlements and roads are overlaid on hillshaded terrain relief. The mainland Assessment areas can be split into the uplands and the Carpentaria Plains (Grimes, 1974). The upland area in the south and west reaches 620 metres above sea level (mASL) and is the headwaters for Assessment area catchments. The uplands can be divided into four physiographic units: Isa Highland, Barkly Tableland, Dissected Barkly Tableland and Gulf Fall (Figure 2-3). The oldest and most elevated and rugged unit is the Isa Highland (Twidale, 1956). It consists of Precambrian (>545 Ma) volcanic and sedimentary rocks that have been metamorphosed, weathered and eroded. Soil parent materials within the Isa Highland from west to east include rhyolite, basalt, dolomitic sediments, siltstone, meta basalt, granite, quartzite and metasediments. Land surface relief is moderate (200 to 230 m) and generally in a south–north alignment. The next most elevated upland physiographic unit is a small part of Barkly Tableland to the west of the Isa Highland. The tableland started out as a sedimentary basin in Precambrian times and then was uplifted, folded and eroded. During the Cambrian period, seas transgressed the area and deposited carbonate sediments in the depressions. Later the Cambrian-period sediments were exposed and eroded. During the Mesozoic, isolated lakes and swamps developed (Randal, 1966), and subsequently, during the Cenozoic period, the upland areas experienced deep weathering and laterisation. However, areas covered by lakes and swamps did not undergo strong leaching, and as the landscape dried, the current cracking clay soils formed on relatively fresh sediments (Christian et al., 1954). The clay soils overlie dolomitic rocks. Relief is very low (9 to 30 m), and Mitchell grasslands dominate. Since the Cambrian period, the drainage network that flows towards the Gulf of Carpentaria has dissected the tableland, leaving remnant land features defined by deep narrow gorges. This area is mapped as the Dissected Barkly Tableland physiographic unit in Figure 2-3. Dissection has been amplified because the underlying rocks formed from dolomitic sediments are relatively soluble compared to surrounding rocks. These gorges have intersected the groundwater systems of the tableland, resulting in spring-fed permanent creeks and rivers such as the O’Shannassy, Gregory and Lawn Hill Creek subcatchments. The remaining parts of the uplands, comprising mainly Mesozoic sedimentary formations (sandstones), have been eroded into a complex pattern of easterly flowing streams and valleys separated by ranges and outcrops of sedimentary formations (Mullera Formation, Constance Sandstone and Fickling Beds; Smith and Roberts (1972)). This physiographic unit is known as the Gulf Fall, and the Nicholson and South Nicholson rivers are the primary systems draining this area. Musselbrook, Lagoon, Settlement, Gold and Running creeks also drain this area. To the east of the uplands are the Carpentaria Plains comprising a series of plains, pediments and remanent plateaux that can be divided into six physiographic units: Cloncurry Plain, Doomadgee Plain, Armraynald Plain, Karumba Plain, Donors Plateau and Mornington Plateau (Figure 2-3). The most elevated sedimentary plain (30 to 150 mASL) is the Cloncurry Plain physiographic unit immediately east of the uplands. It consists of gently sloping colluvial and fluvial sedimentary plains and pediments with isolated low hills of Precambrian rock. Streams are few and incised into the pediments with narrow alluvial plains (Grimes, 1974). The Cloncurry Plain unit extends from the middle reach of the Leichhardt River to Lawn Hill Creek. In the northern Assessment area, the Doomadgee Plain physiographic unit lies below and adjacent to the Cloncurry Plain. It is predominantly a sandy, gently undulating plain overlying a deeply weathered Cenozoic land surface. Low eucalypt and paperbark scrub cover the lands. Widely spaced creeks drain the plains, currently in a radial north-westerly direction towards the coast. This suggests that the underlying old land surface could have been a large sedimentary fan. Prior streams of sandier soils, shallow swampy and water-filled depressions (particularly between Lilly and Moonlight creeks) and small pits caused by ferricrete subsidence occur throughout the plains (Grimes, 1974). In the southern half of the Assessment area, the Armraynald Plain physiographic unit lies below and adjacent to the Cloncurry Plain unit. It consists of argillaceous Cenozoic (Quaternary period) sediments (Armraynald Beds) that form black soils covered in grasslands. Stream channels are few, widely spaced and deeply incised due to sea-level changes. The plains extend up the Lawn Hill Creek, and the Gregory and Leichhardt valleys. Lawn Hill Creek and Gregory River are spring-fed permanent streams. The Gregory River splits into a giant braid (20 km at its widest) of permanent streams consisting of the Gregory River, Beames Brook, Barkly River and Running Creek downstream of the Gregory Crossing. Monsoonal rainforest grows immediately adjacent to these permanent streams that cross the otherwise grassland plains (Grimes, 1974). Downslope of both the Doomadgee Plain and Armraynald Plain lies the coastal Karumba Plain physiographic unit. This coastal unit extends 10 to 35 km inland from the Gulf of Carpentaria coast and is widest near the Albert River mouth (upstream called the Nicholson and Gregory rivers). This plain consists of Holocene beach ridges and tidal and extratidal flats and plains. Some of the inland plains only flood when the rivers are in spate or when the north-westerly winds cause exceptionally high tides during the monsoon. Because the plain is generally flat and wide and the tidal range is moderate (about 3.5 m), tidal waters can rapidly inundate the land. Mangroves and tidal flats dominate the coastline; beaches are few and consist of white shelly sand. Small crescent dunes have formed in places from wind action (Grimes, 1974). Strong north-easterly winds across the bare plains, especially in November, may cause a fog-like effect in Burketown from suspended particles. Due to the flatness of the plain, streams meander in complex patterns. To the east of the Armraynald Plain lies the Donors Plateau physiographic unit. This slightly elevated unit (10 to 80 mASL) forms a watershed between the catchments of the Leichhardt and Flinders rivers and forms the eastern boundary of the Assessment area. The plateau consists of siliceous sediments laid down in the Early Cretaceous epoch from upland sediment sources of the Normanton Formation. The plain, which was once more extensive, was deeply weathered and lateralised in the highest elevation parts in the Tertiary period and has subsequently been stripped away in parts, leaving today’s Donors Plateau and exposed older Cretaceous sediments. In parts, the older sediments have been re-covered by sediments laid down during the Pleistocene, forming the Armraynald Plain (Ingram, 1972). Much of the Wellesley Islands in the Gulf of Carpentaria represent remnants of a mainland laterised Cretaceous period plain called the Mornington Plateau physiographic unit. This unit is 5 to 20 mASL. Dissection of the unit is not as extensive as in the Donors Plateau unit. The Mornington Plateau unit is generally fringed with marine plains consisting of coastal sediments or dune fields lower in the landscape that support small mangroves; higher up are sea cliffs and wavecut platforms. The cliff faces exhibit well-developed laterite profiles. Some of the coastal features in the unit are 5 m above current high-tide levels, which is explained by changes in sea level and upwarping of the islands (Grimes, 1974). 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 limestone or dolomitic rocks that may contain solution features that could cause leakage, 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. 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. 2.2.3 Major hydrogeological provinces of the Southern Gulf catchments In terms of groundwater, five major hydrogeological provinces exist in the Southern Gulf catchments: (i) the McArthur Basin, which underlies the north-west of the catchment, (ii) the Georgina Basin, which overlies the McArthur Basin in the south to south-west of the catchment, (iii) the Isa Highland, (iv) the geological Carpentaria Basin, which rests on an erosional surface of deformed Proterozoic rocks of the Isa Superbasin and South Nicholson Basin, and (v) the Cenozoic Karumba Basin, which unconformably overlies the geological Carpentaria Basin (Figure 2-4) and alluvial plains. The broad major rock types associated with each geological province include igneous and sedimentary rocks (McArthur, South Nicholson, Georgina and geological Carpentaria basins and Isa Superbasin) and unconsolidated (surficial regolith) to consolidated sediments (Karumba Basin) (Figure 2-4). The McArthur Basin is a geological province underlain by an approximately 10 km thick sequence of sedimentary rocks that in places are intruded by minor igneous rocks of Precambrian age (Paleoproterozoic to Mesoproterozoic). The McArthur Basin extends well beyond the Southern Gulf catchments. In the Southern Gulf catchments, the McArthur Basin is undulating with isolated ranges of quartzite and igneous rocks dissected by river valleys. The rocks of the McArthur Basin have been intruded with dolerite, folded, faulted and uplifted, and subjected to long periods of erosion (both physical and chemical weathering) since they were formed. Most of the sedimentary and igneous rocks of the McArthur Basin have very low primary porosity (<2%), and their pores are very small and not interconnected. Consequently, they do not hold or yield much groundwater and can be impermeable across large areas. Where the upper parts of the sedimentary and igneous rocks are weathered and fractured, they can contain volumes of water that, while not large, can have local importance for stock and domestic use and community water supplies. Geological basins and provinces map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\4_S_Gulf\1_GIS\1_Map_docs\Gr-S-513_Geological_provinces_v04_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-4 Major geological provinces of the Southern Gulf catchments Source: adapted from Raymond (2018) The most well-known and significant intermediate- to regional-scale groundwater systems within the Southern Gulf catchments are hosted by the Georgina and Carpentaria basins, which underlie approximately 12,400 km2 of the Southern Gulf catchments. There may also be other hydrogeological units that are currently poorly characterised but host local- to intermediate-scale groundwater systems with potential to support small-scale localised irrigated agriculture opportunities (i.e. the Proterozoic rocks and the Bulimba Formation). However, data and information for these systems are very limited. The major hydrostratigraphic units hosted by the Georgina Basin within the Southern Gulf catchments are the Camooweal Dolostone, Wonarah Formation and Thorntonia Limestone. Recent stratigraphic drilling conducted as part of the National Drilling Initiative (Exploring for the future, n.d.) identified a thickness of more than 600 m of Georgina Basin sedimentary rocks (mostly limestone and dolostone) within the Southern Gulf catchments. The Camooweal Dolostone is the shallowest of the Georgina Basin’s more prospective hydrogeological units in the Southern Gulf catchments. It is composed of dolostone and dolomitic limestone. The Wonarah Formation of the Georgina Basin also occurs in the south-west portion of the Southern Gulf catchments and merges laterally into the Anthony Lagoon Formation (the Wiso Basin stratigraphic equivalent). It is predominantly composed of silty dolostone with interbedded (minor) dolomitic and calcareous mudstone and siliciclastic mudstone. Very little information exists for the Wonarah Formation in the Southern Gulf catchments, and it therefore remains unclear if it exhibits the same hydraulic characteristics as its stratigraphic equivalents in adjacent areas. The Thorntonia Limestone is composed mostly of dolostone and dolomitic limestone. The limestone aquifers occur mostly in the NT, but within the Southern Gulf catchments they also outcrop or subcrop in far western Queensland and host a significant regional groundwater system. The sedimentary sequences of the geological Carpentaria Basin are mostly sandstone, siltstone, mudstone and claystone. They have a combined thickness of up to approximately 600 m in the Southern Gulf catchments and thicken significantly offshore. Within the Southern Gulf catchments, the geological Carpentaria Basin rocks overlie the formations of the McArthur Basin in the north-east and the Isa Superbasin and South Nicholson Basin in the east and south-east (Figure 2-4). Most of the rocks of the geological Carpentaria Basin have very low primary porosity and are considered aquitards. However, the extensive and porous sandstones of the GRF are considered a productive aquifer and the Normanton Formation is considered an aquifer or partial aquifer. Groundwater within the GRF is under artesian pressure in some parts of the Southern Gulf catchments (e.g. at the Burketown bore). Consisting of lacustrine, fluvial and, to a lesser extent, shallow-shelf marine sediments, the widespread Cenozoic sediments of the Karumba Basin unconformably overlie the Carpentaria Sub-basin of the GAB. Within the Southern Gulf catchments, Cenozoic sediments are expected to reach a maximum thickness of 40 m (north of Doomadgee), and the most productive aquifer is expected to correspond to the basal section of the Bulimba Formation, which is composed mostly of fine-grained quartzose sediments. Overlying the Karumba Basin are the youngest sediments in the catchments: the alluvial sands, silts and gravels associated with the beds, channels and floodplains of the Leichhardt, Nicholson and Gregory rivers, and Settlement Creek and their tributaries. Only a few groundwater bores intersect the youngest alluvial aquifers; these existing bores suggest that the alluvial sediments are generally relatively shallow (drilled to depths of up to approximately 25 m). 30 | Water resource assessment for the Southern Gulf catchments 2.3 Soils of the Southern Gulf catchments 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 these soils and their attributes closely reflects the geology and landforms 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 generic groups (Section 2.3.2) and soil attributes (Section 2.3.3) in the Southern Gulf catchments. The management considerations for irrigated agriculture are also summarised in Table 2-1. 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 Southern Gulf catchments are presented in a soil generic group (SGG) classification (Figure 2-5; Table 2-1; Table 2-2). 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. The soil groups and soil characteristics presented below can be viewed in the context of their relationship to physiographic units within the catchment (Figure 2-3; Table 2-1). These physiographic units serve as a useful framework to understand the extent of different SGGs because 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. Soil generic group map and locations \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\10_Reporting\4_S_Gulf\1_GIS\1_Map_Docs\CR-S-512-Ch4_SGG_v1_Arc10_8.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-5 The soil generic groups (SGGs) of the Southern Gulf catchments 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). Table 2-1 Soil generic groups (SGGs), descriptions, management considerations and correlations to Australian Soil Classification (ASC) for the Southern Gulf catchments For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au The Southern Gulf catchments contain soils from nine of the ten SGGs (Figure 2-5); peaty soils (SGG 5) are not found. Only two of the nine SGGs occupy more than 10% of the area, and together these soils represent 78% of the study area (Table 2-2). They are the shallow and/or rocky soils, which are associated with uplands and plateaux (SGG 7, 55.9%), and the cracking clay soils found on the Barkly Tableland and the Armraynald Plain (SGG 9, 22.5%). Brown, yellow and grey sandy soils (SGG 6.2) make up 7.6% of the catchment and seasonally or permanently wet soils (SGG 3) another 6.0%, both being common on the Doomadgee Plain. 34 | Water resource assessment for the Southern Gulf catchments Table 2-2 Area and proportions covered by each soil generic group (SGG) in the Southern Gulf catchments SGG SOIL DESCRIPTION AREA (ha) % OF ASSESSMENT AREA (ROUNDED) 1.1 Sand or loam over relatively friable red clay subsoils 49,000 <1 1.2 Sand or loam over relatively friable brown, yellow and grey clay subsoils 59,000 <1 2 Friable non-cracking clay or clay loam soils 378,000 3 3 Seasonally or permanently wet soils 675,500 6 4.1 Red loamy soils 58,300 <1 4.2 Brown, yellow and grey loamy soils 202,100 2 5 Peaty soils 0 0 6.1 Red sandy soils 168,000 2 6.2 Brown, yellow and grey sandy soils 830,300 8 7 Shallow and/or rocky soils 6,057,700 56 8 Sand or loam over sodic clay subsoils 25,200 <1 9 Cracking clay soils 2,434,800 23 10 Highly calcareous soils 1,700 <1 Total 10,820,000 The soils with some of the greatest agricultural potential in the Assessment area are the cracking clays or Vertosols (SGG 9), which cover 2,434,800 ha. Both the Armraynald Plain (C1 in Figure 2-5) and Barkly Tableland (C2) are extensive natural grasslands with few trees, reflecting the cracking nature of the soils (Figure 2-6). These soils are medium to heavy clays that crack when dry and swell when wet, reducing water permeability. They have a self-mulching clay surface, deep to very deep (1.2 to 1.5 m) effective rooting depth, and the clay texture means the soils have a very high (>220 mm) soil available water capacity (AWC). The clays soils of the Armraynald Plain (C1) are imperfectly drained and can have high salt levels within the profile, whereas the Barkly Tableland soils on the uplands (C2) are moderately well drained and gravel is common. On the Armraynald Plain, soils are suited to a variety of vegetables (but not root crops), rice (Oryza spp.), sugarcane (Saccharum officinarum) and dry-season grain, forage, pulse crops, sweetcorn (Zea mays convar. saccharata var. rugosa) and cotton (Gossypium spp.). On the Barkly Tableland, soils are suited to trickle-irrigated mangoes (Mangifera indica) and vegetables as well as wet-season cotton, grain and forage crops. Along the middle reaches of the Leichhardt River (F1 in Figure 2-5), a moderately well-drained, friable non-cracking clay or clay loam soil (SGG 2; 295,460 ha on the mainland) has formed on the floodplains (Figure 2-7). The soils are moderately well to well drained and have a weakly structured, fine sandy to loam surface soil over a structured sandy clay loam or silty clay subsoil. Effective rooting depth is very deep (>1.5 m) and AWC is moderate (>160 mm). On Mornington Island (F2), this soil is shallower with varying amounts of ironstone gravel reducing the AWC. Soil or landscape photo \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\4_S_Gulf\2_Reporting\SGG_Photos For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-6 Cracking clay (brown Vertosol; SGG 9) Mitchell grass (Astrebla spp.) downs with whitewood (Elaeocarpus sp.) and gutta percha (Palaquium spp.) on the Armraynald Plain physiographic unit, east of the Leichhardt River Photo: CSIRO Soil or landscape photo \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\4_S_Gulf\2_Reporting\SGG_Photos For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-7 Brown Dermosol (SGG 2) buffel grass (Cenchrus ciliaris) open woodland with silver leaf box (Eucalyptus pruinosa) on the Armraynald Plain physiographic unit in the middle reaches of the Leichhardt River Photo: CSIRO On the floodplains of the upper lowlands of the Leichhardt River (U1 in Figure 2-5), a well-drained, sandy loam over a structured red clay subsoil (SGG 1.1; 49,100 ha) has formed. This soil also has a very deep (>1.5 m) effective rooting depth and a high (>180 mm) AWC. Sandy soils (SGG 6) are found on the Doomadgee Plain (S1 in Figure 2-5), on Donors Plateau (S2) (Figure 2-8), in the Gulf Fall (S3) and on the elevated terraces north of the Nicholson River (S4) in the Armraynald Plain, covering 996,660 ha on the mainland. The Doomadgee Plain soils (S1) are brown sands (SGG 6.2) that are well drained and commonly limited by ferricrete rock within 0.6 to 1 m of the soil surface. The soil has a very low AWC (25 to 60 mm), depending on soil depth. On Donors Plateau (S2), the yellow sandy soils (SGG 6.2) are deeper and have a high ironstone gravel content throughout the profile reducing AWC. In the Gulf Fall (S3), the brown (SGG 6.2) and red (SGG 6.1) deep sands occur in the subcatchments of Buddycurrawa, Breakfast, Running and Sandy creeks, and the upper parts of the catchment of the Nicholson River. Near the Doomadgee township (S4), soils are predominantly red sands (SGG 6.1) that are well drained but have a low AWC (60 to 100 mm). Soil or landscape photo \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\4_S_Gulf\2_Reporting\SGG_Photos For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-8 Red sandy soil (SGG 6.1) open woodland of Darwin box (Eucalyptus tectifica), bauhinia (Bauhinia spp.) and Cooktown ironwood (Erythrophleum chlorostachys) near Doomadgee on the Armraynald Plain physiographic unit north of the Nicholson River Photo: CSIRO Loamy soils (SGG 4) have formed along the Nicholson River (K1 in Figure 2-5) and on the Doomadgee (K2) and Cloncurry (K3) plains, and in other isolated areas. They are either red (SGG 4.1; 58,300 ha) well drained or brown (SGG 4.2; 202,100 ha) moderately well drained soils, and they have a high AWC (>170 mm). On the Doomadgee Plain along Westmoreland Road (K2) are brown, yellow and grey sandy clay loams over sandy clays (SGG 4.2) that are poorly to moderately well drained. Soils vary in depth depending on the depth of the underlying rock and have a low to moderate AWC (70 to 150 mm). The red loamy soils (SGG 4.1) on the Cloncurry Plain (K3) are shallower and sandier and commonly have gravel and ironstone throughout the profile; accordingly, these soils have a lower AWC. Irrigation potential is limited to spray- and trickle-irrigated crops on the moderately deep to deep (>1 m) soils. Seasonally or permanently wet soils (SGG 3; 648,510 ha on the mainland) occur on local alluvia along creeks and in swamps, particularly between Lilly and Moonlight creeks on the Doomadgee Plain (W1 in Figure 2-5), and the tidal flats and wetlands of the Karumba Plain (W2). The soils are very poorly drained. Texture contrast soils that have sodic clay subsoils (SGG 8; 25,200 ha on the mainland) are minor areas occurring on Nineteen Mile Creek (D1 in Figure 2-5), a tributary of the Leichhardt River, and on a meander plain just upstream of the Gregory–Nicholson River junction (D2). Shallow soils (<0.25 m depth) or rocky soils (>50% rock) (SGG 7; 6,049,320 ha on the mainland) occur extensively in more than half of the Assessment area (Table 2-2), particularly in the mountainous Isa Highland, Gulf Fall, Dissected Barkly Tableland and Donors Plateau physiographic units. More detail on the soils and their relationship to the physiographic units shown in Figure 2-3 can be found in the companion technical report on digital soil mapping and land suitability (Thomas et al., 2024). Figure 2-9 Level to gently undulating cracking clay soils of the Armraynald Plain suitable for broadacre irrigation Photo: CSIRO – Nathan Dyer 2.3.3 Soil attribute mapping Using a combination of field sampling (Figure 2-10) and digital soil mapping techniques, the Assessment mapped 18 attributes affecting the agricultural and aquaculture suitability of soil for the Southern Gulf catchments, 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 in the upper 100 cm of the soil profile (referred to as AWC 100) •surface rockiness. An important feature of each predicted attributes map is the companion reliability map showing the relative confidence in the accuracy of the attribute predictions. Note that mapping is only provided here for regional-scale assessment. Areas of high reliability allow users to be more confident in the accuracy of mapping, whereas areas of low reliability show where users should be cautious. Soil or landscape photo \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\4_S_Gulf\2_Reporting\SGG_Photos For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-10 Soil profile of the brown Vertosol (SGG 9) sampled on the Armraynald Plain physiographic unit east of the Leichhardt River Topsoil is on the left-hand side of the lower 1 m of core tray and the subsoil in the top core tray. The sample core was 1.5 m deep. Photo: CSIRO Surface soil pH The pH value of a soil reflects the extent 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. However, almost all of the study area is in the pH range 5.5 to 8.5 (Figure 2-11), which is within the acceptable agronomic threshold (Peverill et al., 1999). Areas in the acid-to-neutral range (pH 5.5 to 7.0) are more associated with the sandier surface soils (Figure 2-13) dominated by shallow and/or rocky soils (SGG 7), red sandy soils (SGG 6.1) and brown, yellow and grey sandy soils (SGG 6.2). Sandier soils tend to be more acidic because of increased soil permeability (Figure 2-14) and have higher leaching rates of neutralising soil components. Coarse-textured soils also tend to have lower buffering capacity by virtue of lower cation exchange capacity supplied by clay minerals and organic matter. The more alkaline soils (pH 7.0 to 8.5) are associated with soils with higher clay content, especially the cracking clay soils (SGG 9) and some areas of seasonally or permanently wet soils (SGG 3). Mapping reliability is generally low to moderate across the Assessment area, being lowest in the Dissected Barkly Tableland and the Karumba Plain physiographic units and in areas of the Gulf Fall unit where lack of data produces less reliable results. Mapping reliability is stronger for the Doomadgee Plain physiographic unit, indicating good data correlation. Soil surface pH map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\4_S_Gulf\1_GIS\1_Map_docs\LL-S-511-516_DSM_2x1_v2_ArcGIS10_8.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-11 (a) Surface soil pH (top 10 cm) of the Southern Gulf catchments as predicted by digital soil mapping and (b) reliability of the prediction Soil thickness Soil thickness defines the potential root space and the amount of soil from which plants obtain their water and nutrients. The deeper soils dominate the lowlands and Wellesley Islands and the western margins of the uplands (Figure 2-12). The lowland deeper soils are associated with cracking clay soils (SGG 9) of the Cloncurry Plain and Armraynald Plain, and on the Doomadgee Plain with significant areas of brown, yellow and grey sandy soils (SGG 6.2), and brown, yellow and grey loamy soils (SGG 4.2). Additionally, the Karumba Plain is dominated by seasonally or permanently wet soils (SGG 3) that are also generally deep. In upland areas, there are extensive areas of cracking clay soils (SGG 9) on the Barkly Tableland physiographic unit, and the Gulf Fall unit has large areas of deep red sandy soils (SGG 6.1) and deep brown, yellow and grey sandy soils (SGG 6.2) on the western margins of the Dissected Barkly Tableland and Gulf Fall physiographic units. Deep friable non-cracking clay or clay loam soils (SGG 2) dominate Mornington Island. Shallow and/or rocky soils (SGG 7) dominate the Isa Highland physiographic unit and the eastern areas of the Dissected Barkly Tableland and Gulf Fall units. Soil thickness mapping is least reliable along the coastal fringes and the Wellesley Islands, as these areas have few data points, and most reliable in the uplands, especially the Isa Highland physiographic unit where there is less variation in soil thickness across the landscape. Soil thickness map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\4_S_Gulf\1_GIS\1_Map_docs\LL-S-511-516_DSM_2x1_v2_ArcGIS10_8.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-12 (a) Soil thickness of the Southern Gulf catchments as predicted by digital soil mapping and (b) reliability of the prediction Soil surface texture Soil texture refers to the proportion of sand, silt and clay 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-textured soils are generally those high in sand, and heavy-textured soils are dominated by clay. The clayey surface soils represent a large proportion of the Assessment area and dominate the Barkly Tableland, the Dissected Barkly Tableland, the Armraynald Plain, the Karumba Plain and the Donors Plateau physiographic units (Figure 2-13). These soils are mainly cracking clay soils (SGG 9) and seasonally or permanently wet soils (SGG 3). Sandy surface soils co-dominate the area and are mainly represented by shallow and/or rocky soils (SGG 7) and the brown, yellow and grey sandy soils (SGG 6.2) of the Isa Highland and Doomadgee Plain physiographic units. Sandy-textured surface soils like SGG 7 are also strongly represented in the Donors Plateau physiographic unit. Loamy soil surfaces are associated mainly with SGG 7 soils located in the Gulf Fall and Isa Highland physiographic units, although their contribution across the Assessment area is minor. Mapping reliability of soil surface texture tends to be lower in the areas of higher relief than in the plains. The upland Isa Highland and Dissected Barkly Tableland physiographic units have particularly low reliability, reflecting the variability of surface textures across the landscape. Reliability is highest in the Doomadgee Plain and Armraynald Plain physiographic units in the lowlands and the Barkly Tableland physiographic unit in the uplands, reflecting more consistent surface textures within these units. Soil surface texture map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\4_S_Gulf\1_GIS\1_Map_docs\LL-S-511-516_DSM_2x1_v2_ArcGIS10_8.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-13 (a) Soil surface texture of the Southern Gulf catchments as predicted by digital soil mapping and (b) reliability of the prediction Permeability The permeability of the profile is a measure of how easily water moves through a soil. Flood and furrow irrigation is most successful on soils with low and very low permeability, 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 Assessment area is dominated by moderately permeable soils and, to a lesser extent, slowly permeable soils (Figure 2-14). The latter correlate with patterns of cracking clay soils (SGG 9) of the Armraynald Plain and Barkly Tableland physiographic units. Highly permeable soils are associated with brown, yellow and grey loamy soils (SGG 6.2) of the Doomadgee Plain unit, areas of the Gulf Fall unit, and a part of the Cloncurry Plain unit on the eastern margin of the Assessment area dominated by shallow and/or rocky soils (SGG 7). Generally, mapping reliability in the Assessment area is low where areas have limited data and on the Doomadgee Plain where permeability is quite variable across the landscape. Higher map reliability occurs on the Armraynald Plain physiographic unit where soils have a more consistent permeability. Soil permeability map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\4_S_Gulf\1_GIS\1_Map_docs\LL-S-511_Perm_2x1_v2_ArcGIS10_8.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-14 (a) Soil permeability of the Southern Gulf catchments as predicted by digital soil mapping and (b) reliability of the prediction Available 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 dryland agriculture, AWC 100 determines the capacity of crops to grow and prosper during dry spells. Patterns of AWC 100 (Figure 2-15) closely correlate with soil thickness patterns (Figure 2-12) and surface soil textures (Figure 2-13). While these soil attributes do not definitively indicate subsurface soil textures and consequently water-holding capacity, the correlation shows the largest AWC values are found where soils are deep and are clay rich, especially the Armraynald Plain, Cloncurry Plain and Barkly Tableland physiographic units. Mapping reliability for AWC is generally highest in upland areas of the Gulf Fall and northern areas of Isa Highland physiographic units. It is least reliable in the Barkly Tableland physiographic unit having fewer measurements. Soil available water capacity to 1m map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\4_S_Gulf\1_GIS\1_Map_docs\LL-S-511-516_DSM_2x1_v2_ArcGIS10_8.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-15 (a) Available water capacity in the upper 100 cm of the soil profile (AWC 100) of the Southern Gulf catchments as predicted by digital soil mapping and (b) reliability of the prediction 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 rocky soil surfaces (Figure 2-16) closely coincide with the shallow and/or rocky soils (SGG 7). Overall, the reliability of mapping is high (good correlation of data as areas are either consistently rocky or consistently rock-free), although relatively localised areas of lower reliability are found in SGG 7 soil areas, reflecting a lack of data. Soil rockiness map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\3_Land_suitability\4_S_Gulf\1_GIS\1_Map_docs\LL-S-511-516_DSM_2x1_v2_ArcGIS10_8.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-16 (a) Surface rockiness in soils of the Southern Gulf catchments represented by presence or absence as predicted by digital soil mapping and (b) reliability of the prediction 2.4 Climate of the Southern Gulf catchments 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 result in 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 data 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 Southern Gulf catchments The Southern Gulf catchments are characterised by distinctive wet and dry seasons (Figure 2-17) due to their location in the Australian summer monsoon belt. During the build-up months (typically September to December), the Southern Gulf catchments typically experience low-level easterly winds, which can carry pockets of dry or humid air and result in short-lived thunderstorm activity under favourable conditions. Over inland areas of the Southern Gulf catchments, storms form more frequently during the afternoon because of increased air temperature, which enhances instability and leads to convective cloud formation. Storms also form more readily near the heat trough that is a semi-permanent feature over inland Queensland (predominantly located south of the catchment) during late spring and summer months. There is also a high incidence of thunderstorms in the Southern Gulf catchments where sea breeze convergence and/or boundaries act as a trigger. Thunderstorms show a strong diurnal variation, with most occurring during the afternoon and early evening. Dynamic forcing can cause thunderstorms to develop or persist well beyond the normal diurnal cycle, and if the dynamics are strong enough, thunderstorms can occur at any time. In the Southern Gulf catchments, convection and rain from sea breeze convergence typically occur just inland from the coast from early afternoon in the warmer months. Rain can continue overnight and into the following morning if conditions are favourable, particularly if there is a moisture feed from a strong easterly nocturnal jet stream. During the day, the air over land warms and rises, resulting in lower pressure at the surface. Air flows from the Gulf of Carpentaria waters towards the land to fill this area of lower pressure, resulting in the sea breeze, which can converge with the usually south-easterly synoptic winds. During the wet season, the convergence of persistent easterly winds and broader synoptic north-westerly winds over the Gulf of Carpentaria may trigger storms over the coast after sunset. This phenomenon is most pronounced in the near-coastal areas of the catchment. Historical climate, map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\1_Climate\4_S_Gulf\1_GIS\1_Map_docs\1_Exports\Cl-SG-507-Hist-MedAnnRF-ET-RFdeficit.png For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au 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 Southern Gulf catchments. Rainfall deficit is rainfall minus potential evaporation. The mean annual rainfall, averaged over the Southern Gulf catchments for the 132-year historical period (1 September 1890 to 31 August 2022), is 602 mm. Rainfall totals are highest near the coast and decline in a southerly direction (Figure 2-17). This is because the more northerly regions of the catchments receive more wet-season rainfall as a result of active monsoon episodes. The Southern Gulf catchments are relatively flat, so there is no noticeable topographic influence on climate parameters such as rainfall or temperature. Approximately 94% of the rain falls in the Southern Gulf catchments during the wet-season months (1 November to 30 April). Figure 2-17 shows the spatial distribution of rainfall over the year and during the wet and dry seasons. Median wet-season rainfall exhibits a very similar spatial pattern to median annual rainfall. Median dry-season rainfall is highest in the most southern part of the Leichhardt catchment and lowest near the coast. The highest monthly rainfall totals typically occur during January and February (Figure 2-18). Tropical cyclones and tropical lows contribute a considerable proportion of total annual rainfall, but the actual amount is highly variable from one year to the next (see the companion technical report on climate (McJannet et al., 2023)) since tropical cyclones do not affect the Southern Gulf catchments in more than half of years. For the 53 tropical cyclone seasons from 1969–70 to 2021–22, 60% of seasons registered no tropical cyclones tracking over the Southern Gulf catchments, 36% experienced one tropical cyclone and 4% 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 Southern Gulf catchments have a mean annual PE of 1900 mm (over the period 1890 to 2022) (Figure 2-17). Evaporation is high all year round, but exhibits a strong seasonal pattern, ranging from about 200 mm/month during November and December to about 100 mm/month during the middle of the dry season (June) (Figure 2-19). 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 2-17). The mean annual rainfall deficit, or mean annual net evaporative water loss, in the Southern Gulf catchments is about 1305 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 Southern Gulf catchments 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 Southern Gulf catchments, 94% of the rain falls during the wet season (November to April). The highest monthly rainfall in the Southern Gulf catchments typically occurs 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 (Figure 2-19): mean PE is about 210 mm during December and it is at its lowest during June (100 mm). Months in which PE is high correspond to those months where the demand for water by plants is also high. Mean wet-season and dry-season PE in the Southern Gulf catchments 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). Relative to other catchments in southern and northern Australia, the Southern Gulf catchments have high variability in rainfall from one year to the next. This variability is demonstrated for four locations in the Southern Gulf catchments in Figure 2-18. The highest annual rainfall at Mount Isa (948 mm) occurred in the 1894–95 wet season; it was more than seven times the lowest annual rainfall (127 mm in 1925–26) and more than twice the median annual rainfall value (396 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. Using Mount Isa as an example, the 10-year running mean varied from 291 to 562 mm. Figure 2-18 illustrates that the period between 2000 and 2010 was wetter than average. Under Scenario A, PE exhibits much less inter-annual variability than rainfall at the four demonstration locations (Figure 2-19). Rainfall, graph \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\1_Climate\4_S_Gulf\2_Reporting\plots\climate_report\annual_monthly_rainfall_range_4_station_short2cm.png Figure 2-18 Historical monthly rainfall (left) and time series of annual rainfall (right) in the Southern Gulf catchments at Mount Isa, Doomadgee, Gregory and Burketown ‘A range’ is the 10% to 90% exceedance values of 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. Potential evaporation, graph \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\1_Climate\4_S_Gulf\2_Reporting\plots\climate_report\annual_monthly_pet_range_4_station_short2cm.png Figure 2-19 Historical monthly potential evaporation (PE) (left) and time series of annual PE (right) in the Southern Gulf catchments at Mount Isa, Doomadgee, Gregory and Burketown ‘A range’ is the 10% to 90% exceedance values of monthly rainfall. Note: the ‘A mean’ line is directly under the ‘A median’ line in these figures. The solid blue line in the right column is the 10-year running mean 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 Southern Gulf catchments is moderately high for northern Australia catchments and high compared to stations in southern Australia with similar mean annual rainfall. (a) (b) \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\1_Climate\4_S_Gulf\1_GIS\1_Map_docs\1_Exports\Cl-SG-516_Cv_map_of_selected_stations_v1_1031.png For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au \\fs1-cbr\{lw-rowra}\work\1_Climate\1_All\2_Reporting\NAWRA2-TR-Cl-A-WB1-v11.xlsm For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au 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 Southern Gulf catchments. (b) The rainfall station in the Southern Gulf catchments is indicated by a red symbol. The light blue diamonds indicate rainfall stations from the rest of northern Australia (RoNA), and hollow squares indicate rainfall stations from southern Australia (SA). Furthermore, Petheram et al. (2008) observed that the inter-annual variability of rainfall in northern Australia is about 30% higher than that observed at rainfall stations from the rest of the world for the same type of climate as northern Australia. Hence, caution should be exercised before drawing comparisons between the agricultural potential of the Southern Gulf catchments 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. 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). 52 | Water resource assessment for the Southern Gulf catchments 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 Southern Gulf catchments is the last 10 days of October (see the companion technical report on climate (McJannet et al., 2023)). The mean SOI for the September to December period in each year was used to 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 Southern Gulf catchments are the middle of January, early January and early December. Trends Previously, CSIRO (2009a) found that rainfall in northern Australia between 1997 and 2007 was statistically different to that between 1930 and 1997. In other work, Evans et al. (2014) found a strong relationship between monsoon active periods and the Madden–Julian Oscillation, and that the increasing rainfall trend observed at Darwin Airport was related to increased frequency of active monsoon days rather than increased intensity during active periods. Runs of wet and dry years The rainfall-generating systems in northern Australia and their modes of variability combine to produce irregular runs of wet and dry years. In particular, length and magnitude (intensity) of dry spells strongly influence the scale, profitability and risk of water-resource-related investments. The Southern Gulf catchments are likely to experience dry periods of similar severity to many centres in the Murray–Darling Basin and on the east coast of Australia. The Southern Gulf catchments are 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 Mount Isa, Doomadgee, Gregory and Burketown 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 Southern Gulf catchments indicates equally long runs of wet and dry years and nothing unusual about the length of the runs of dry years. A graph of numbers and lines Description automatically generated with medium confidence Figure 2-21 Runs of wet and dry years at Mount Isa, Doomadgee, Gregory and Burketown Wet years are shown by the blue columns and dry years by the red columns. 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. 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 Southern Gulf catchments, and the GCM-PSs are ranked in order of increasing mean annual rainfall. This figure shows that three (10%) of the projections for GCM-PSs indicate an increase in mean annual rainfall by more than 5%, seven (22%) of the projections indicate a decrease in mean annual rainfall by more than 5%, and about two-thirds 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 Southern Gulf catchments is not likely to change significantly 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. 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. Change in rainfall and evaporation, graph \\fs1-cbr\{lw-rowra}\work\1_Climate\4_S_Gulf\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 2-22 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 Southern Gulf catchments GCM-PS ranked by increasing rainfall for SSP2-4.5. Scenario rainfall, map \\fs1-cbr\{lw-rowra}\work\1_Climate\4_S_Gulf\1_GIS\1_Map_docs\1_Exports\Cl-SG-514-annualRain-Cwet-Cmid-Cdry.png For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-23 Spatial distribution of mean annual rainfall across the Southern Gulf catchments under scenarios (a) Cwet, (b)Cmid and (c) Cdry "\\fs1-cbr\{lw-rowra}\work\1_Climate\4_S_Gulf\3_future_climate\summary\ssp245\mean_month_rain_pet.png" For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 2-24 (a) Monthly rainfall and (b) potential evaporation for the Southern Gulf catchments 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 9% (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 Southern Gulf catchments are summarised in Table 2-3. This information may be considered in coastal aquaculture developments and flood inundation of coastal areas. Table 2-3 Projected sea-level rise for the coast of the Southern Gulf catchments 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 Southern Gulf catchments, the corresponding projected sea-surface temperature increases are 0.7 °C (range across climate models is 0.5 to 1.0 °C) in 2030 under RCP 4.5 and 2.9 °C (2.4 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 2-3 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, 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 Southern Gulf catchments, 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 Southern Gulf catchments, although there is evidence of an increasing trend in rainfall in the recent instrumental record, two-thirds of the GCM-PSs project no change in mean annual rainfall for a 1.6 °C warming scenario. Furthermore, palaeoclimate records indicate multiple wetter and drier periods have occurred in the recent geological past (Northern Australia Water Resource Assessment technical report on climate (Charles et al., 2017)). Very few climate data are available in the Southern Gulf catchments 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 Southern Gulf catchments 2.5.1 Introduction The timing and event-driven nature of rainfall events and high PE rates across the Southern Gulf catchments have important consequences for the catchments’ hydrology. The spatial and temporal patterns of rainfall and PE across the Southern Gulf catchments 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. Water balance, diagram \\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\10_Reporting\1_All\9_Graphics_artist\2_SoG\C Petheram Southern Gulf 1_7_2024 Waterbalance Chpter 2.jpg For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-25 Simplified schematic diagram of terrestrial water balance in the Southern Gulf catchments 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. Section 2.5 covers the remaining terms of the terrestrial water balance (accounting for water inputs and outputs) of the Southern Gulf catchments, 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 Southern Gulf catchments are then discussed. Figure 2-25 shows a schematic diagram of the water balance of the Southern Gulf catchments, along with estimates of the mean annual value spatially averaged across the catchment and an estimate of the uncertainty for each term. The ‘water balance’ comprises all the water inflows and outflows to and from a particular catchment over a given time period. Unless stated otherwise, the material in sections 2.5.2 to 2.5.4 is based on findings described in the companion technical report on hydrogeological assessment (Raiber et al., 2024). Similarly, the material in Section 2.5.5 draws on the findings of the companion technical report on river modelling (Gibbs et al., 2024), unless stated otherwise. 2.5.2 Groundwater Within the Southern Gulf catchments, the distribution, availability and quality of groundwater resources are heavily influenced by the physical characteristics of the sediments and rocks of the major geological divisions (see Section 2.2). Aquifers are the rocks and sediments in the subsurface that store and transmit groundwater. The catchments have several types of aquifer: • fractured and weathered rocks associated with Proterozoic basins (associated with the Proterozoic McArthur and South Nicholson basins and Isa Superbasin) (mostly covered by younger strata) • fractured, fissured and karstic carbonate rocks of the Georgina Basin (Cambrian limestone and Cambrian dolostone in Figure 2-26) • extensive porous sedimentary sandstones of the Great Artesian Basin (not present at the surface in the Southern Gulf catchments; for extent, see Figure 2-4 and Figure 2-31) • porous sandstones of the Karumba Basin (Cenozoic sediments, Figure 2-4) • surficial unconsolidated to consolidated alluvial sands and gravels (Cenozoic alluvium in Figure 2-26). The sedimentary limestones of the Georgina Basin – in particular, the Camooweal Dolostone and Thorntonia Limestone – and the aquifers of the Carpentaria Sub-basin of the GAB host the largest, most regionally extensive groundwater resource in the Southern Gulf catchments. The upper fractured, fissured and karstic parts of the Cambrian carbonate rocks of the Georgina Basin (Figure 2-4 and Figure 2-31) in the Southern Gulf catchments may be regionally connected to the limestones of the adjacent Daly and Wiso basins, which combined are often referred to as the Cambrian Limestone Aquifer. The Cambrian Limestone Aquifer extends for several hundred thousand square kilometres to the north, west and south of the Southern Gulf catchments. It hosts limestones that form 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) can be tens of kilometres to hundreds of kilometres, and the time taken for groundwater to discharge following recharge can be in the order of thousands to hundreds of thousands of years. Simplified regional hydrogeology map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\4_S_Gulf\1_GIS\1_Map_docs\Gr-S-515_simplified_regional_hydrogeology_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-26 Simplified regional hydrogeology of the Southern Gulf catchments This map does not represent outcropping areas of all hydrogeological units: the blanket of surficial Cretaceous to Quaternary regolith 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) and Raymond et al. (2012) Spring data sources: Department of Environment, Parks and Water Security (2013); the Groundwater Dependent Ecosystems Atlas (Bureau of Meteorology, 2017) and Queensland Government (2021) The porous sandstones of the geological Carpentaria Basin (Figure 2-4) – in particular the Gilbert River Formation (GRF) and to a lesser extent the Normanton Formation – host the most regionally extensive aquifer systems within the Southern Gulf catchments. They extend over the eastern part of the catchment in Queensland and are entirely covered by the Cenozoic Karumba Basin. The aquifers of the geological Carpentaria Basin form part of the 1.7 million km2 large GAB that reaches very far to the south and east of the Southern Gulf catchments (Figure 2-31) and also includes the Eromanga and Surat basins and parts of the Clarence–Moreton Basin. Within the Southern Gulf catchments, the geological Carpentaria Basin is entirely covered by the Cenozoic Karumba Basin. Recharge and discharge processes within the geological Carpentaria Basin are relatively poorly characterised due to the relatively limited number of groundwater monitoring bores. The near-absence of environmental tracer data in the geological Carpentaria Basin was identified as a key data gap within the GAB in a recent GAB-wide environmental tracer study (Raiber et al., 2022). Other more local- to intermediate-scale aquifer systems may exist in other geological formations within the Southern Gulf catchments. Previous studies described the Proterozoic rocks trending north-west to south-east across the central part of the Southern Gulf catchments (Figure 2-2) as not a feasible groundwater resource. However, other studies suggested that the Mount Isa town water supply was sourced from fractured shales until the construction of the Lake Moondarra dam. These shales occur at depths of 60 to 80 metres below ground level (mBGL), and bores yielded 5 to 10 L/second. In addition, approximately 50% of the bores with stratigraphic information in the Southern Gulf catchments correspond to Proterozoic units composed of unassigned granites or are attributed to one of the geological units of the South Nicholson Basin and Isa Superbasin. Furthermore, the variability of groundwater chemistry found among the Proterozoic aquifers, with dominant ions varying from magnesium, calcium and bicarbonate (Mg–Ca–HCO3) to sodium and sulfate (Na–SO4), may be an indication of localised recharge and interactions of groundwater with variable mineral-rich rocks. The surficial fluvial and, to a lesser extent, shallow-shelf marine Cenozoic sediments of the Karumba Basin and younger alluvial aquifer systems in the Southern Gulf catchments may form local- to intermediate-scale aquifer systems. The basal Bulimba Formation has been described as the most productive aquifer of the Karumba Basin. Its lithological properties have been described as highly variable, from shale to sandy ferricrete, resulting in variable hydraulic properties and groundwater yields. Generally, the sedimentary sequences of the Karumba Basin and younger alluvial aquifer systems within the Southern Gulf catchments remain poorly characterised. Hydrogeological units Hydrogeological units of the Southern Gulf catchments are shown in Figure 2-26. The rocks and sediments of these geological units host a diverse range of aquifers that vary in extent, storage and productivity. The major and most extensive aquifers in the Southern Gulf catchments are found in the Cambrian limestone of the Georgina Basin and the sandstones of the Jurassic to Cretaceous Carpentaria Sub-basin of the GAB. For this Assessment, major aquifer systems are defined as aquifers that contain regional and intermediate-scale groundwater systems with adequate storage volumes (i.e. gigalitres) that could potentially yield water at a sufficient rate (>10 L/second) and be of a sufficient water quality (<1000 mg/L total dissolved solids (TDS)) for a range of irrigated cropping. Minor aquifers are defined as aquifers that contain local-scale groundwater systems with lower storage (i.e. megalitres) (Figure 2-27). The yields from minor aquifers are variable and often low (<5 L/second), and minor aquifers have variable water quality ranging from fresh (~500 mg/L TDS) to brackish (~3000 mg/L TDS). The distribution and characteristics of these rocks is covered in Section Unless otherwise stated, the material in Section 2.5.2 is based on findings described in the companion technical report on hydrogeological assessment by Raiber et al. (2024). Only the major aquifers relevant to potential opportunities for future groundwater resource development are discussed in detail. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 2-27 Two-dimensional conceptual schematic of the interconnected aquifer system and its variability Bore yields vary significantly depending on where (which geological unit) a bore is installed and at what depth. Adapted from DENR (2016) Cambrian limestone aquifers The Cambrian limestone aquifers of the Georgina Basin occur in the western part of the Southern Gulf catchments (Figure 2-26). Due to the limited number of groundwater bores within the Southern Gulf catchments, data from groundwater bores outside the boundary of the catchment were also used to provide a more robust overview of the hydrogeological properties of the different formations. Camooweal Dolostone and Thorntonia Limestone – Georgina Basin Based on existing hydrogeological data, the fractured and karstic carbonated rocks hosted in the Georgina Basin also offer potential to support productive bores (i.e. yields greater than 10 L/s), but they remain poorly characterised. The Camooweal Dolostone, which occurs in the south-west of the catchments (Cambrian dolostone in Figure 2-26), is mostly composed of dolostone and dolomitic limestone (Geoscience Australia and Australian Stratigraphy Commission, 2021; Matthews, 1992). It is the shallowest of the Georgina Basin’s more prospective hydrogeological units in the Southern Gulf catchments and has an approximate spatial outcrop extent of approximately 2972 km2 within the Southern Gulf catchments. At a new stratigraphic well (Carrara 1) drilled as part of the National Drilling Initiative within the Southern Gulf catchments, the thickness of the Camooweal Dolostone is more than 200 m. The Thorntonia Limestone occurs in the south-west of the Southern Gulf catchments (Cambrian limestone in Figure 2-26) and is composed mostly of dolostone and dolomitic limestone (Geoscience Australia and Australian Stratigraphy Commission, 2021). It is overlain by the Camooweal Dolostone and is confined in parts by the Wonarah Formation. The limestone aquifers occur mostly in the NT, but also outcrop and subcrop in far western Queensland, and host a significant regional groundwater system (Figure 2-26). The Thorntonia Limestone has an outcropping zone of approximately 2515 km2 within the Southern Gulf catchments and is up to approximately 100 m thick (CSIRO, 2009b; Taylor et al., 2021). For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-28 Gregory River at Riversleigh Road Photo: CSIRO – Russell Crosbie The median depth of groundwater bores screened within the Cambrian limestones (Camooweal Dolostone and Thorntonia Limestone, undifferentiated) in the Southern Gulf catchments is 101 m (with a range from 24 to 537 m). Groundwater within these formations is mostly fresh to brackish with a median salinity of approximately 1300 mg/L TDS (ranging from <100 to approximately 6,000 mg/L TDS). The ionic composition of groundwater within the Cambrian limestones is variable, with a predominance of calcium, magnesium and bicarbonate (Ca–Mg–HCO3) or sodium, bicarbonate and chloride (Na–HCO3–Cl For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-29 Lawn Hill Gorge Photo: CSIRO – Russell Crosbie Wonarah Formation – Georgina Basin The Wonarah Formation of the Georgina Basin occurs in the south-west portion of the Southern Gulf catchments (Cambrian siltstone in Figure 2-26) and merges laterally into the Anthony Lagoon Formation (the Wiso Basin stratigraphic equivalent) (Tickell and Bruwer, 2017). The Wonarah Formation is predominantly composed of silty dolostone with interbedded (minor) dolomitic and calcareous mudstone and siliciclastic mudstone. It has a maximum thickness of 244 m in holes drilled in its equivalent Anthony Lagoon Formation (Kruse and Dunster, 2013). Very little information exists for the Wonarah Formation in the Southern Gulf catchments. It therefore remains unclear if it exhibits the same characteristics as the Anthony Lagoon Formation in the Wiso Basin much further to the west in the NT. These formations are fractured, highly heterogeneous and partially karstic, with estimated transmissivities of 13 to 8200 m2/day (Kruse and Dunster, 2013). Indicative hydrogeological data and anecdotal evidence for aquifers in the Wonarah Formation and equivalent come from Tickell (2003). Tickell (2003) described the hydrogeological unit in the Barkly Tableland as fractured and cavernous rocks assigned to both the Georgina and Wiso basins. Aquifers range in depth from 50 to 125 mBGL, and standing water levels vary from 30 to 100 mBGL. Indicative bore yield data based on airlifting at the time of drilling varied from 0.5 to 5 L/second. However, mud circulation losses during drilling were commonly reported, suggesting the presence of cavernous limestones and potential for high-yielding aquifers. Groundwater quality was also described as fresh to brackish (500 to 1500 mg/L TDS) across most of the area investigated (Tickell, 2003). Gilbert River Formation – Carpentaria Sub-basin of the Great Artesian Basin The Gilbert River Formation (GRF), or Gilbert River Aquifer (GRA), is the major aquifer within the Jurassic to Cretaceous sedimentary rocks of the geological Carpentaria Basin in the north-east of the catchments (Figure 2-31). The extensive sandstone aquifers are intersected by bores ranging in depth from approximately 100 to 750 mBGL. The GRF in the Southern Gulf catchments and adjacent areas has significant potential as an aquifer for groundwater-based irrigation. Hydraulic conductivity values for the GRF in the northern geological Carpentaria Basin range from 0.1 to 10 m/day, and transmissivities range from 4 to 570 m2/day (Horn et al., 1995; Klohn Crippen Berger, 2016). Reported bore yields range from less than 0.5 L/second up to 46 L/second (Figure 2-32). It is anticipated that yields would likely exceed 10 L/second with appropriately constructed large-diameter production bores. The GRF is confined by the thick and laterally continuous aquitard of the Rolling Downs Group. In some places this results in artesian conditions as, for example, reported for a bore in Burketown. Groundwater salinity in the GRF ranges from fresh (<500 mg/L TDS) to brackish (>3000 mg/L TDS) with a median TDS of 500 mg/L (Figure 2-33), which is still suitable for irrigation use. The ionic composition of the groundwater of the GRF is dominated by Na–HCO3–Cl and Na–Cl–HCO3, which is indicative of mature groundwaters evolved along very long flow paths. Low calcium and magnesium and high concentrations of fluoride have also been observed. An assessment of recharge processes and flow dynamics in the GAB by Raiber et al. (2022) identified the lack of environmental tracer data in the geological Carpentaria Basin as a key data gap within the GAB. Figure 2-30 The Gregory River receives groundwater discharge from the Cambrian Limestone Aquifer Photo: CSIRO – Nathan Dyer Full extent of Basins and GAB, map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\4_S_Gulf\1_GIS\1_Map_docs\Gr-S-559_Georgina_GAB_v03_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-31 Full extent of the Georgina Basin and Carpentaria Sub-basin of the Great Artesian Basin. Inset map shows full extent of Great Artesian Basin Geology data sources adapted from: Department of Industry, Tourism and Trade (2014) and Department of Environment, Parks and Water Security (2008) and Raymond et al. (2012) Major aquifer bore yield, map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\4_S_Gulf\1_GIS\1_Map_docs\Gr-S-525_Yield_by_aquifer_Major_v06.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-32 Groundwater bore yields for major aquifers across the Southern Gulf catchments Symbol shapes indicate the aquifer within which the bore is sited; colours indicate bore yield class. Bore yield data source: Department of Environment, Parks and Water Security (2014); DNRME (2023) Major aquifer groundwater salinity, map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\4_S_Gulf\1_GIS\1_Map_docs\Gr-S-527_TDS_by_aquifer_Major_v06.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-33 Groundwater salinity for major aquifers in the Southern Gulf catchments Symbol shapes indicate the aquifer within which the bore is sited; colours indicate total dissolved solids (TDS). Salinity data source: Department of Environment, Parks and Water Security (2014); DNRME (2023) Bulimba Formation and alluvial aquifer systems Consisting of lacustrine, fluvial and, to a lesser extent, shallow-shelf marine sediments, the widespread Cenozoic sediments of the Karumba Basin unconformably overlie the Carpentaria Sub- basin of the GAB (Bradshaw et al., 2009). Within the Southern Gulf catchments, Cenozoic sediments are expected to reach a maximum thickness of 40 m (north of Doomadgee). The most productive aquifer is expected to correspond to the basal section of the Bulimba Formation, which is mostly composed of fine-grained quartzose sediments (Herbert, 2000; Radke et al., 2012). Lithological properties of the Bulimba Formation are highly variable, from shale to sandy ferricrete, resulting in variable hydraulic properties and groundwater yields (i.e. 0.25 to 4.5 L/second) (DNRME, 2021). Nevertheless, at the central area of Nicholson catchment, where the alluvial deposits are wider (downstream of Doomadgee) (Figure 2-26), the Bulimba Formation is the most accessed aquifer from the Karumba Basin (Buchanan et al., 2020). Its hydraulic conductivity ranges from 150 to 300 m/day with specific yield of 0.1 (Smerdon et al., 2012). The Cenozoic alluvial aquifers in the middle to lower reaches of the Nicholson, Gregory and Leichhardt rivers, and Settlement Creek and its tributaries, host local-scale groundwater systems. However, data are very sparse, and these water resources remain poorly understood. The median TDS of shallow alluvial groundwaters in the Southern Gulf catchments is approximately 600 mg/L, and the reported yields are relatively low (median of 2 L/second). However, this is based on very few data points, and additional data on the extent, thickness, internal architecture and hydraulic properties of alluvial aquifers are required to assess their suitability as productive and reliable water supplies. Fractured rock aquifers CSIRO (2009c) described the Proterozoic rocks trending north-west to south-east across the central part of the Southern Gulf catchments (Figure 2-2) as not a feasible groundwater resource. However, McEniery (1980) pointed out that the Mount Isa town water supply was sourced from fractured shales until the construction of the Lake Moondarra dam. These shales occur at depths of 60 to 80 mBGL and yield from 5 to 10 L/second. Approximately 50% of the bores with stratigraphic information in the Southern Gulf catchments correspond to Proterozoic units composed of unassigned granites or are attributed to one of the following geological units: Lawn Hill, Corella, Paradise Creek, Surprise Creek, Toole Creek Volcanics, Lady Loretta and Gunpowder Creek. These geological units are considered to potentially host local-scale fractured and weathered rock aquifers (Raiber et al., 2024). According to the data compiled by Bardwell and Grey (2016), a significant spatial variability in groundwater chemistry is found among the Proterozoic aquifers, with dominant ions varying from Mg–Ca–HCO3 to Na–SO4. Such characteristics may be an indication of localised recharge and interactions of groundwater with variable mineral-rich rocks. 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 Southern Gulf catchments. Figure 2-34 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 hydrogeological assessment (Raiber et al., 2024). Annual recharge, map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\4_S_Gulf\1_GIS\1_Map_docs\Gr-S-509_CMB_R_percentiles_constrained_SG_v2_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-34 Annual recharge estimates for the Southern Gulf catchments Estimates based on upscaled chloride mass balance (CMB) method for the (a) 50th, (b) 5th and (c) 95th percentiles. Figure 2-35 provides a summary of the range in recharge estimates for the outcropping area of seven key hydrogeological units across the Southern Gulf catchments (Figure 2-26). Recharge estimates are based on the mean of the 5th and 95th percentiles and range from approximately: •3 to 28 mm/year for the Camooweal Dolostone •14 to 46 mm/year for the Cretaceous sediments •14 to 55 mm/year for the fractured rocks • 22 to 89 mm/year for the Proterozoic carbonates • 3 to 30 mm/year for the Thorntonia Limestone (or equivalent) • 3 to 20 mm/year for the Wonarah Formation • 9 to 29 mm/year for the alluvium. 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 on an annual basis. The methods also do not account for potential preferential recharge from streamflow or overbank flooding or through karst features across parts of the Southern Gulf catchments. Therefore, the key features of an aquifer must be carefully conceptualised before simply deriving a recharge volume based on the surface area of an aquifer outcrop and an estimated recharge rate. For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-35 Summary of recharge statistics to outcropping areas of key hydrogeological units across the Southern Gulf catchments Recharge rates are based on upscaled chloride mass balance (CMB) method and calculated as the mean of the 5th and 95th percentiles. 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 Southern Gulf catchments. Areas of groundwater discharge are important for sustaining both aquatic and terrestrial groundwater-dependent ecosystems and often have cultural significance. These groundwater discharge areas have been mapped in Figure 2-36 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 hundred kilometres. Areas of seasonally varying groundwater discharge generally exhibit localised alluvial and fractured and weathered rock aquifer systems, and surface water recharges waterholes and alluvial and fractured rock aquifers adjacent to streams 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 recede during the dry season. Coastal discharge 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. Groundwater discharge at spring vents connected to stream channels likely occurs in some parts of the Southern Gulf catchments. For example, permanently saturated springs of the Boodjamulla spring complex (Figure 2-36) are mapped along Lawn Hill Creek in the headwaters of the Nicholson River. The occurrence of springs in this area has been linked to groundwater discharge from underlying Proterozoic sedimentary rock aquifers (Buchanan et al., 2020). The drivers of potential groundwater discharge from deeper units in this area are not currently well understood due to a lack of hydrochemistry and environmental tracer data. However, multiple possible mechanisms have been described. This includes, for example, groundwater discharge driven by gas overpressure from the shale-rich sequences in the Isa Superbasin. Surface faults and regional structural features occur near the springs, and the location of these springs also corresponds with a distinct change in elevation of approximately 100 m from west to east, which could indicate groundwater discharge at a break in slope (Buchanan et al., 2020). A recent assessment by Geoscience Australia (Dixon-Jain et al., 2024) used geological and historical and newly acquired geophysical (seismic and airborne electromagnetic) data to study the influence of geological structures on the occurrence of groundwater-dependent ecosystems. The study indicated that some of the springs north of Lawn Hill Creek of the Boodjamulla spring complex (Figure 2-36) appear to be associated with the Constance Sandstone, which is part of the South Nicholson Basin, with faults and fractures potentially forming pathways for groundwater discharge. The study also highlighted that other potential spring-source aquifers may exist, including a possible indirect groundwater source originating from the Thorntonia Limestone (part of the Georgina Basin) and flowing into the Constance Sandstone (with alternative hydrogeological conceptualisations discussed in more detail in the companion hydrogeological technical report by Raiber et al., 2024). Other mapped, permanently active springs in the south-western part of the Southern Gulf catchments in the headwaters of the Gregory River (Figure 2-28) are spatially associated with and likely drain the Thorntonia Limestone. Dixon-Jain et al. (2024) highlighted that further hydrochemical sampling, including environmental tracers, is required to understand the hydrogeology of the springs within the Southern Gulf catchments. Shallow groundwater from the Karumba Basin sediments likely supports terrestrial and aquatic groundwater-dependent ecosystems on the alluvial floodplains. Groundwater discharge, map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\11_Groundwater\4_S_Gulf\1_GIS\1_Map_docs\Gr-S-512_GW_discharge_v3_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-36 Spatial distribution of groundwater discharge classes including surface water – groundwater connectivity across the Southern Gulf catchments Groundwater discharge classes are inferred from remotely sensed estimates of evapotranspiration and open water persistence (based on findings described in the companion technical report on hydrogeological assessment by Raiber et al., 2024). Note: the size of polygons has been greatly exaggerated to allow them to be seen at this scale. Geology data sources: adapted from Department of Industry, Tourism and Trade (2014) and Department of Environment, Parks and Water Security (2008) and Raymond et al. (2012) Spring data sources: Department of Environment, Parks and Water Security (2013); the Groundwater Dependent Ecosystems Atlas (Bureau of Meteorology, 2017) and Queensland Government (2021) 2.5.5 Surface water Streamflow Approximately 60% of Australia’s runoff is generated in northern Australia (Petheram et al., 2010, 2014). Unlike the large internally draining Murray–Darling Basin, however, northern Australia’s runoff is distributed across many hundreds of smaller externally draining catchments (Figure 2-37). To place the Southern Gulf catchments 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 2-37 shows the magnitude of median annual streamflow of major rivers across Australia prior to water resource development. Streamflow Australia, map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\201_SG\1_GIS\1_Map_Documents\Hy-S-501_Aust_accumulated_AnnualMedian_flow_AWRA_SouGulf_rescaled.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-37 Modelled streamflow under natural conditions Streamflow under natural conditions is indicative of median annual streamflow prior to European settlement (i.e. without any large-scale water resource development or extractions) assuming the historical climate (i.e. 1890 to 2015). Source: Petheram et al. (2017) The Southern Gulf catchments consist of the contributing area of various rivers and streams that discharge into the southern Gulf of Carpentaria. The most substantial of these are the Leichhardt, Gregory and Nicholson rivers. The catchments of these rivers, plus those on the Wellesley Islands in the Gulf of Carpentaria, Settlement Creek, Morning Inlet, and numerous small coastal creeks, have a total area of 108,200 km2. The Leichhardt catchment has an area of 33,400 km2, and the river itself extends approximately 550 km from the river mouth to Mount Isa in the south of the catchment. Tidal influence on streamflow is detectable approximately 15 km upstream of Burketown on the Albert River before the water becomes brackish. On the Leichhardt River, brackish salinity is likely to extend as far up as Leichhardt Falls, 113 km from the mouth. The falls are just below station 913007B on Leichhardt River at Floraville Station (Figure 2-38), the only operating streamflow station in the lower plains in the Southern Gulf catchments. The median and mean annual discharges from all Southern Gulf catchments are 4961 and 6759 GL/year, respectively. The pronounced difference between the mean and median is due to the mean being biased by a number of very high flow years (Table 2-4). Current surface water licences across the study area total about 114 GL (2.3% of median annual flow). See Section 3.3 for more information. The cease-to-flow column in Table 2-4 gives the percentage of time that no streamflow was observed at each of the streamflow gauging stations in the catchments. Gauges on the Gregory River indicate perennial flow, with dry-season baseflow resulting from discharge from the Thorntonia Limestone hydrogeological unit. Other rivers ceased to flow for 27% to 79% of the time, driven by the seasonal rainfall that has on average 94% of the rainfall falling during the wet season (Section 2.4). The influence of storage at Lake Julius results in a higher cease-to-flow period, with spills occurring 13% of the time. Lake Julius and Lake Moondarra, two large storages (approximately 107 GL each) in the upper Leichhardt River, are used for town and industrial water supply near Mount Isa. There are 75.1 GL of licences allocated from the storages; however, only a portion of this volume is currently used each year (typically ~40%). Table 2-4 Streamflow metrics at selected gauging stations in the Southern Gulf catchments Annual streamflow data are calculated under Scenario A. The 20th, 50th and 80th refer to 20%, 50% and 80% percentile exceedance, respectively. Cease-to-flow percentage (the percentage of all observation days where no streamflow was recorded) is determined using observed data, where streamflow less than 0.1 ML/day was assumed to be equal to zero. The annual streamflow data are shown schematically in Figure 2-39 and Figure 2-40. STATION ID STATION NAME CATCHMENT AREA (HA) ANNUAL STREAMFLOW (GL) CEASE- TO-FLOW % RUNOFF COEFFICIENT MEAN 80TH 50TH 20TH 913014A Leichhardt River at Doughboy Creek 3,520 115 27 72 153 63 0.07 913015A Leichhardt River at Julius Dam 4,748 122 0 41 163 87 0.06 913004A Leichhardt River at Miranda Creek 5,959 213 27 92 277 62 0.08 913006A Gunpowder Creek at Gunpowder 2,412 128 42 85 171 72 0.11 913010A Fiery Creek at 16 Mile Waterhole 721 41 12 27 55 79 0.11 913007B Leichhardt River at Floraville Homestead 23,679 1157 308 693 1670 49 0.10 912108A O Shannassy River at 17.7 Km 5,591 238 103 179 334 0 0.09 912105A Gregory River at Riversleigh No.2 11,382 475 213 343 681 0 0.09 912101A Gregory River at Gregory Downs 12,567 506 226 363 708 0 0.09 912103A Lawn Hill Creek at Lawn Hill No 2 4,003 164 59 116 219 27 0.09 912107A Nicholson River at Connolly’s Hole 13,875 649 82 268 1092 34 0.09 912116A Nicholson River at Doomadgee Mission 15,117 730 98 325 1186 32 0.09 streamflow gauges "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\201_SG\1_GIS\1_Map_Documents\Hy-S-508_streamflow_gauges.mxd" For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-38 Streamflow observation data availability in the Southern Gulf catchments Figure 2-39 shows how median annual streamflow increases towards the coast in the Southern Gulf catchment. As an indication of variability, Figure 2-40 shows the 20% and 80% exceedance of annual streamflow compared to the median (50% exceedance) in Figure 2-39. Streamflow exceedence, map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\201_SG\1_GIS\1_Map_Documents\Hy-S-000_SouGulf_accumulated_AnnualMedian_flow_(E50)_rescaled_v07.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-39 Median annual streamflow (50% exceedance) in the Southern Gulf catchments under Scenario A Commencement threshold for blue flow accumulation line is 5 GL/year. Labelled points refer to Figure 2-49 Figure 2-41 illustrates the increase in catchment area and decrease in elevation along the Gregory–Nicholson River (a) and Leichhardt River (b) from one of the most upstream source tributaries to their mouth. The large ‘step’ changes in catchment area are where major tributaries join the river. Scenario streamflow exceedence \\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\201_SG\1_GIS\1_Map_Documents\1_Export\Hy-S-503_2x1_SG_accumulated_E20_E80_flow_rescaled_v03.png For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-40 (a) 20% and (b) 80% exceedance of annual streamflow in the Southern Gulf catchments under Scenario A For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-41 Catchment area and elevation profile from the upper tributaries in each catchment to the mouth along the (a)Gregory–Nicholson River and (b) Leichhardt River Catchment runoff The simulated mean annual runoff averaged over the Southern Gulf catchments under Scenario A is 76 mm. Figure 2-42 shows the spatial distribution of mean annual rainfall and runoff under Scenario A (1890 to 2022) across the study area. 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 Southern Gulf catchments exhibit substantial variation from one year to the next. The annual runoff volumes at 20%, 50% (median) and 80% exceedance averaged across the Southern Gulf catchments are 114, 30 and 10 mm, respectively (Figure 2-43). That is, runoff spatially averaged across the catchments will on average exceed 114 mm 1 year in 5, 30 mm half the time and 10 mm 4 years in 5. Figure 2-43 shows the spatial distribution of annual runoff at 20%, 50% and 80% exceedance under Scenario A. Intra- and inter-annual variability in runoff Rainfall, runoff and streamflow in the Southern Gulf catchments are variable between and within years. Approximately 87% of all runoff in the catchments occurs in the 3 months from January to March, which is a very high concentration of runoff compared to rivers in southern Australia (Petheram et al., 2008). While streamflow is ephemeral at many gauge sites, some rivers in the Gregory catchment are perennial (Table 2-4). Figure 2-44b illustrates that there is a high variation in monthly wet-season runoff from one year to the next. For example, during March, spatial mean runoff exceeded 34 mm in 20% of years and was less than 1 mm in 20% of years. The largest catchment mean annual runoff under Scenario A was 491 mm in 1973–74, and the smallest was 3 mm in 1901–02 (Figure 2-44a). The CV of annual runoff aggregated across the Southern Gulf catchments is 1.2. Based on data from Petheram et al. (2008), this variability in annual runoff is slightly above the middle of the range of the annual variability in runoff of other rivers in northern Australia with a comparable mean annual runoff. \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\201_SG\1_GIS\1_Map_Documents\Hy-S-504_Rain_Runoff_1x2.mxd For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 2-42 Mean annual (a) rainfall and (b) runoff across the Southern Gulf catchments under Scenario A Pixel-scale variation in mean annual runoff is due to changes in climate and modelled variation based on physiographic units. \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\201_SG\1_GIS\1_Map_Documents\Hy-S-505_20_50_80_runoff_1x3.mxd For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 2-43 Annual runoff at (a) 20%, (b) 50% and (c) 80% exceedance across the Southern Gulf catchments under Scenario A Pixel-scale variation in mean annual runoff is due to changes in climate and modelled variation based on physiographic units. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 2-44 Total runoff across the Southern Gulf catchments under Scenario A showing (a) time series of annual runoff and (b) monthly runoff averaged across the catchments 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 November to March create flooding in parts of the Southern Gulf catchments and inundate large areas of floodplains, mostly in the downstream reaches between the Nicholson, Gregory and Leichhardt rivers. The floodplains along the Alexandra and Albert rivers are also heavily flooded (Figure 2-45). Floodplains along the Alexandra and Albert rivers (Figure 2-45) are also regularly flooded. Flooding is common in the Burketown area, which is located on a remnant of the main channel of the Albert River and represents the most eastward extent of a very flat ridgeline that provides the highest ground (~5 m above sea level) on the western bank of the river. Burketown is susceptible to flooding from the Albert River floodplain, as well as from overland flow paths within the town area. Flooding of the Albert River could also occur from floodwater breakout from the Nicholson or Gregory rivers. Rivers in the Southern Gulf catchments are largely unregulated. The notable exception is the Leichhardt River, which has two large dams: Moondarra Dam and Julius Dam. Overbank flow is generally governed by the topography of the floodplain. Since 1980, there have been 41 floods greater than or equal to 1 in 1 annual exceedance probability (AEP) in different parts of the Southern Gulf catchments. While floods occur in any month from November to April, historically the month with the most floods is January (42%). 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. Further observations of flood characteristics in the Southern Gulf catchments are as follows: • Flood peaks typically take about 2 days to travel from Gregory to Burketown at a mean speed of 3.2 km/hour. • For flood events of AEP 1 in 2, 1 in 5 and 1 in 10, the peak discharges at Riversleigh Road on the Gregory River gauge are 590, 1850 and 2280 m3/second, respectively. • Between 1980 and 2023 (43 years), 51 streamflow events broke the banks of the Gregory River at Riversleigh Road crossing. All events occurred between November and April (inclusive), and about 84% of these events occurred between January and March (inclusive). Of the ten largest flood peak discharges at Riversleigh on the Gregory River, six occurred in January, three in February and one in December. • The maximum area inundated by a flood event of AEP 1 in 38 that occurred in March 2023 was 5983 km2 (Figure 2-46). Flood inundation, map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\201_SG\1_GIS\1_Map_Documents\HC-S-243_Inundation_MODIS_Flood_CR.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-45 Flood inundation map of the Southern Gulf catchments 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. Lowland flood inundation, map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\201_SG\1_GIS\1_Map_Documents\HC-S-244_Flood_inundation.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-46 Flood inundation in the Southern Gulf catchments for a flood event of 1 in 38 annual exceedance probability (AEP) in March 2023 Flood frequency in the Southern Gulf catchments Flood frequency analysis was performed for the Southern Gulf catchments to establish streamflow thresholds above which a flood event would occur. Flood frequencies were estimated for the three major rivers (Nicholson, Gregory and Leichhardt). For the Nicholson River, flood frequencies were estimated using streamflow observations from gauging station 912107A (Nicholson River at Connolly’s Hole) as this gauge has reasonable quality data. Flood frequencies were estimated using streamflow observations from gauging station 912105A (Gregory River at Riversleigh) for the Gregory River. For the Leichhardt River, flood frequencies were estimated using streamflow observations from gauging station 913007B (Leichhardt River at Floraville). Traditionally, flood frequencies are estimated based on maximum discharge for individual events. However, in the Assessment, to help determine the true 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 2-47 displays the relationship between peak flow, flood volume and AEP for the three gauges: one on the Nicholson River (912107A) one on the Gregory River (912105A) and one on the Leichhardt River (913007B). While flow volume is higher for larger floods, duration of flood is a key factor for volume of flood flow. A diagram of different colors Description automatically generated with medium confidence Figure 2-47 Peak flood discharge and annual exceedance probability (AEP) at (a) gauge 912107A (Nicholson River at Connolly’s Hole), (b) gauge 912105A (Gregory River at Riversleigh) and (c) gauge 913007B (Leichhardt River at Floraville) 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 Southern Gulf catchments are largely ephemeral. However, perennial flow is associated with discharge from the dolostone aquifer underlying the Barkly Tableland to the Gregory River. In ephemeral reaches, such as most of the Leichhardt River, once streamflow has ceased the rivers break up into a series of waterholes during the dry season. Waterholes that persist from one year to the next are considered to be key aquatic refugia and are likely to be sustaining ecosystems in the catchments (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 most locations along the Leichhardt River and most of the Nicholson River, while baseflow is apparent in Gregory River observations (Figure 2-48). These properties are reflected in the river model simulations for the month of October when any streamflow is likely the result of groundwater discharge into streams (Figure 2-49). Substantial baseflow in October in the Gregory system is related to discharge from the dolostone aquifer in the upper Gregory catchment. The ecological importance and functioning of key aquatic refugia are discussed in more detail in the companion technical report on ecological modelling (Ponce Reyes et al., 2024). "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\201_SG\0_Working\1_justin\5_Catchment_report\min_sept_flow.png" Figure 2-48 Minimum observed September streamflow at two stream gauge locations on the Gregory River "\\fs1-cbr.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\201_SG\2_Reporting\1_Catch_report_river\03_minimumflow_boxplots\minOctoberFlow_v3.png" Figure 2-49 Minimum monthly flow over 132 years of simulation for the month of October Assessed at locations indicated by labels in Figure 2-39. Locations are listed in an upstream to downstream order (left to right). The dashed blue horizontal lines equate to 200 ML/day, and the dotted red horizontal lines equate to 400 ML/day As an example, the formation of waterholes following a cease-to-flow event were captured using satellite imagery for a reach of the Flinders River in northern Australia (Figure 2-50). Figure 2-52 maps 1 km river reaches (or segments) in the Southern Gulf catchments 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, typically waterholes that persist over the dry season. Some of these waterholes are maintained by perennial flow, for example, along the Gregory River. Below Gregory, the canopy along the Gregory River can be closed, obscuring the presence of water, and the river width becomes relatively small compared to the Landsat pixel size, which may influence the ability to detect permanent water using this technique. Maps of instream waterhole evolution. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 2-50 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. Figure 2-51 The Leichhardt River near Kajabbi looking south towards the Isa highlands. In the highly seasonal climate of the Southern Gulf catchments, springs and persistent waterholes provide important ecological refugia during the dry season Photo: CSIRO – Nathan Dyer Permanent waterholes, map \\FS1-CBR.nexus.csiro.au\{lw-rowra}\work\2_Hydrology\201_SG\1_GIS\1_Map_Documents\Hy-S-507_SouGulf_permanent_waterholes_v1.mxd For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-52 Location of river reaches containing permanent water in the Southern Gulf catchments 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 along the watercourses. Surface water quality Since the late 1960s, there has been systematic sampling of the water quality of the surface water system at the gauge network through the Surface Water Ambient Water Quality Network (SWAN) program in Queensland. Three sites remain open in the Southern Gulf Assessment area: Gregory River at Riversleigh (912105A), Gunpowder Creek at Gunpowder (913006A) and Leichhardt River at Floraville Homestead (913007B). The data available for regularly analysed water quality parameters of interest (of the over 60 samples at each site) are presented in Figure 2-54. Minimum, median and maximum statistics are also presented in Table 2-5. Environmental values define the suitable uses of the water by aquatic ecosystems and by humans (e.g. for drinking, irrigation, aquaculture, recreation), and water quality objectives define objectives for the physical, chemical and biological characteristics of the water (e.g. nitrogen content, dissolved oxygen, turbidity, toxicants, fish). Environmental values and water quality objectives are being progressively determined for Queensland waters but have not yet been developed for any Southern Gulf rivers. In the absence of local water quality objectives, the water quality data can be interpreted against national guidelines. Salinity concentrations were typically within safe drinking limits (<1000 μS/cm). The maximum value at the Gunpowder Creek site (Table 2-5) coincided with zero discharge, indicating these higher salinities are likely due to the evaporative concentration of water at the sampling location rather than representing water quality of the source water. Table 2-5 Summary of water quality data for the open Southern Gulf catchments sites, with values of minimum, median and maximum for each site and each water quality parameter For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au †NTU = nephelometric turbidity units. ‡ below detection limit represented by zero value. All recorded metal concentrations were below livestock drinking water trigger values from ANZECC and ARMCANZ (2000). Median constituent concentrations were well below long-term (100 year) trigger values for heavy metals and nutrients for irrigation water; however, a small number of samples exceeded the iron (0.2 mg/L) and phosphorus (0.05 mg/L) guidelines. All samples were above the national guideline values for total phosphorous for ecosystems in tropical lowland rivers (0.1 mg/L), and 34% of samples were above the national guideline values for total nitrogen (0.3 mg/L). This does not necessarily mean poor-quality water, as both Queensland’s Environmental Protection Policy (Water) and the national guidelines highlight the importance of using guideline values tailored to the local environment. A number of studies have investigated the water quality of the upper Leichhardt River, associated with mining activity, urban runoff and water supply to Mount Isa. Wilson et al. (2007) sampled disconnected dry-season remnant waterholes of the Leichhardt River upstream of Lake Moondarra. They found guideline values for healthy ecosystems for water soluble metals were exceeded, and a number of sites were found to be unfit for primary human contact. Mount Isa city’s sewerage infrastructure, associated waste water reuse scheme, and mining and smelting activities were identified as contributors to impaired water quality, highlighting the need for a catchment management approach to water quality improvement. The Lead Pathways Study investigated sources and pathways of heavy metals (primarily lead) to land, air and water at Mount Isa, including the Leichhardt River (Noller et al., 2012). Five sampling periods encompassing times before, during and after the wet season, were conducted from 2008 to 2010. A number of sites exceeded the ANZECC and ARMCANZ Water Quality Guidelines’ 90% and 95% trigger values for freshwater species for arsenic, cadmium, copper and lead. The water quality guideline process indicates further investigation should be undertaken to assess potential impacts on ecological health. The drinking water source for Mount Isa (Lake Moondarra) was found to meet drinking water guidelines, and sampled metal concentrations indicated a low risk to human health from recreational activities or eating fish caught in the lake or Leichhardt River. Figure 2-53 Accelerated erosion contributes sediment to streamflow Photo: CSIRO – Nathan Dyer For more information on this figure please contact CSIRO on enquiries@csiro.au Figure 2-54 Water quality samples for selected constituents on the Gregory River at Riversleigh (A912005A), Gunpowder Creek at Gunpowder (913006A) and Leichhardt River at Floraville Homestead (913007B) Values below detection limits are seen as constant values, for example, 0.01 mg/L for zinc and 0.05 mg/L for aluminium. Horizontal dashed line represents national guideline levels where relevant. 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