A review of water quality studies relevant to northern Australia Australia’s National Science Agency A technical report from the CSIRO Victoria and Southern Gulf Water Resource Assessments for the National Water Grid Katie Motson, Amrit Mishra, Nathan Waltham Centre for Tropical Water and Aquatic Ecosystem Research (TropWATER), James Cook University, Townsville, Australia A group of logos with a sun and waves Description automatically generated ISBN 978-1-4863-2111-7 (print) ISBN 978-1-4863-2112-4 (online) Citation Motson K, Mishra A and Waltham N (2024) A review of water quality studies relevant to northern Australia. A technical report from the CSIRO Victoria and Southern Gulf Water Resource Assessments for the National Water Grid. CSIRO, Australia. Copyright © Commonwealth Scientific and Industrial Research Organisation 2024. To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO. Important disclaimer CSIRO advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it. CSIRO is committed to providing web accessible content wherever possible. If you are having difficulties with accessing this document, please contact Email CSIRO Enquiries . CSIRO Victoria and Southern Gulf Water Resource Assessments 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 Assessments have been undertaken in conjunction with the Northern Territory and Queensland governments. The Assessments were guided by three 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; NTDepartment 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 joint Roper and Victoria River catchments Steering Committee: Amateur Fishermen’s Association of the NT; Austrade; Centrefarm; CSIRO; National Water Grid (Department of Climate Change, Energy, the Environment and Water); Northern Land Council; NT Cattlemen’s Association; NT Department of Environment, Parks and Water Security; NT Department of Industry, Tourism and Trade; NT Farmers; NT Seafood Council; Office of Northern Australia; Parks Australia; Regional Development Australia; Roper Gulf Regional CouncilShire; Watertrust iii.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 Assessments’ content lies with CSIRO. The Assessments’ committees did not have an opportunity to review the Assessments’ results or outputs prior to their release. 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 High sediment load, Alexander Falls, Southern Gulf. Source: CSIRO Director’s foreword Sustainable development and regional economic prosperity are priorities for the Australian, Queensland and Northern Territory (NT) governments. However, more comprehensive information on land and water resources across northern Australia is required to complement local information held by Indigenous Peoples and other landholders. Knowledge of the scale, nature, location and distribution of likely environmental, social, cultural and economic opportunities and the risks of any proposed developments is critical to sustainable development. Especially where resource use is contested, this knowledge informs the consultation and planning that underpin the resource security required to unlock investment, while at the same time protecting the environment and cultural values. In 2021, the Australian Government commissioned CSIRO to complete the Victoria River Water Resource Assessment and the Southern Gulf Water Resource Assessment. In response, CSIRO accessed expertise and collaborations from across Australia to generate data and provide insight to support consideration of the use of land and water resources in the Victoria and Southern Gulf catchments. The Assessments focus mainly on the potential for agricultural development, and the opportunities and constraints that development could experience. They also consider climate change impacts and a range of future development pathways without being prescriptive of what they might be. The detailed information provided on land and water resources, their potential uses and the consequences of those uses are carefully designed to be relevant to a wide range of regional-scale planning considerations by Indigenous Peoples, landholders, citizens, investors, local government, and the Australian, Queensland and NT governments. By fostering shared understanding of the opportunities and the risks among this wide array of stakeholders and decision makers, better informed conversations about future options will be possible. Importantly, the Assessments do not recommend one development over another, nor assume any particular development pathway, nor even assume that water resource development will occur. They provide a range of possibilities and the information required to interpret them (including risks that may attend any opportunities), consistent with regional values and aspirations. All data and reports produced by the Assessments will be publicly available. Chris Chilcott C:\Users\bru119\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.Word\C_Chilcott_high.jpg Project Director The Victoria and Southern Gulf Water Resource Assessment Team Project Director Chris Chilcott Project Leaders Cuan Petheram, Ian Watson Project Support Caroline Bruce, Seonaid Philip Communications Emily Brown, Chanel Koeleman, Jo Ashley, Nathan Dyer Activities Agriculture and socio- economics Tony Webster, Caroline Bruce, Kaylene Camuti1, Matt Curnock, Jenny Hayward, Simon Irvin, Shokhrukh Jalilov, Diane Jarvis1, Adam Liedloff, Stephen McFallan, Yvette Oliver, Di Prestwidge2, Tiemen Rhebergen, Robert Speed3, Chris Stokes, Thomas Vanderbyl3, John Virtue4 Climate David McJannet, Lynn Seo Ecology Danial Stratford, Rik Buckworth, Pascal Castellazzi, Bayley Costin, Roy Aijun Deng, Ruan Gannon, Steve Gao, Sophie Gilbey, Rob Kenyon, Shelly Lachish, Simon Linke, Heather McGinness, Linda Merrin, Katie Motson5, Rocio Ponce Reyes, Nathan Waltham5 Groundwater hydrology Andrew R. Taylor, Karen Barry, Russell Crosbie, Margaux Dupuy, Geoff Hodgson, Anthony Knapton6, Shane Mule, Stacey Priestley, Jodie Pritchard, Matthias Raiber, Steven Tickell7, Axel Suckow Indigenous water values, rights, interests and development goals Marcus Barber/Kirsty Wissing, Pethie Lyons, Peta Braedon, Kristina Fisher, Petina Pert Land suitability Ian Watson, Jenet Austin, Bart Edmeades7, Linda Gregory, Ben Harms10, Jason Hill7, Jeremy Manders10, Gordon McLachlan, Seonaid Philip, Ross Searle, Uta Stockmann, Evan Thomas10, Mark Thomas, Francis Wait7, Peter L. Wilson, Peter R. Wilson, Peter Zund Surface water hydrology Justin Hughes, Matt Gibbs, Fazlul Karim, Julien Lerat, Steve Marvanek, Cherry Mateo, Catherine Ticehurst, Biao Wang Surface water storage Cuan Petheram, Giulio Altamura8, Fred Baynes9, Jamie Campbell11, Lachlan Cherry11, Kev Devlin4, Nick Hombsch8, Peter Hyde8, Lee Rogers, Ang Yang Note: Assessment team as at September, 2024. All contributors are affiliated with CSIRO unless indicated otherwise. Activity Leaders are underlined. For the Indigenous water values, rights, interests and development goals activity (Victoria catchment), Marcus Barber was Activity Leader for the project duration except August 2022 – July 2023 when Kirsty Wissing (a CSIRO employee at the time) undertook this role. 1James Cook University; 2DBP Consulting; 3Badu Advisory Pty Ltd; 4Independent contractor; 5 Centre for Tropical Water and Aquatic Ecosystem Research. James Cook University; 6CloudGMS; 7NT Department of Environment, Parks and Water Security; 8Rider Levett Bucknall; 9Baynes Geologic; 10QG Department of Environment, Science and Innovation; 11Entura Shortened forms SHORT FORM FULL FORM AADTVTA ANZECC and ARMCANZ Default Trigger Value for Tropical Australia ANZECC Australian and New Zealand Environment and Conservation Council ARMCANZ Agriculture and Resource Management Council of Australia and New Zealand AWRIS Australian Water Resources Information System DO dissolved oxygen DOC dissolved organic carbon EC electrical conductivity NA not available NS not specified NT Northern Territory TDS total dissolved solids WA Western Australia N nitrogen P phosphorus NAWR Northern Australia Water Resources Units UNIT DESCRIPTION ha hectare L litre m metre mg milligram mm millimetre ms millisiemens NTU nephelometric turbidity unit Preface Sustainable development and regional economic prosperity are priorities for the Australian, NT and Queensland governments. In the Queensland Water Strategy, for example, the Queensland Government (2023) looks to enable regional economic prosperity through a vision which states ‘Sustainable and secure water resources are central to Queensland’s economic transformation and the legacy we pass on to future generations.’ Acknowledging the need for continued research, the NT Government (2023) announced a Territory Water Plan priority action to accelerate the existing water science program ‘to support best practice water resource management and sustainable development.’ Governments are actively seeking to diversify regional economies, considering a range of factors, including Australia’s energy transformation. The Queensland Government’s economic diversification strategy for north west Queensland (Department of State Development, Manufacturing, Infrastructure and Planning, 2019) includes mining and mineral processing; beef cattle production, cropping and commercial fishing; tourism with an outback focus; and small business, supply chains and emerging industry sectors. In its 2024–25 Budget, the Australian Government announced large investment in renewable hydrogen, low-carbon liquid fuels, critical minerals processing and clean energy processing (Budget Strategy and Outlook, 2024). This includes investing in regions that have ‘traditionally powered Australia’ – as the North West Minerals Province, situated mostly within the Southern Gulf catchments, has done. For very remote areas like the Victoria and Southern Gulf catchments, the land (Preface Figure 1-1), water and other environmental resources or assets will be key in determining how sustainable regional development might occur. Primary questions in any consideration of sustainable regional development relate to the nature and the scale of opportunities, and their risks. How people perceive those risks is critical, especially in the context of areas such as the Victoria and Southern Gulf catchments, where approximately 75% and 27% of the population (respectively) is Indigenous (compared to 3.2% for Australia as a whole) and where many Indigenous Peoples still live on the same lands they have inhabited for tens of thousands of years. About 31% of the Victoria catchment and 12% of the Southern Gulf catchments are owned by Indigenous Peoples as inalienable freehold. Access to reliable information about resources enables informed discussion and good decision making. Such information includes the amount and type of a resource or asset, where it is found (including in relation to complementary resources), what commercial uses it might have, how the resource changes within a year and across years, the underlying socio-economic context and the possible impacts of development. Most of northern Australia’s land and water resources have not been mapped in sufficient detail to provide the level of information required for reliable resource allocation, to mitigate investment or environmental risks, or to build policy settings that can support good judgments. The Victoria and Southern Gulf Water Resource Assessments aim to partly address this gap by providing data to better inform decisions on private investment and government expenditure, to account for intersections between existing and potential resource users, and to ensure that net development benefits are maximised. Preface Figure 1-1 Map of Australia showing Assessment areas (Victoria and Southern Gulf catchments) and other recent CSIRO Assessments FGARA = Flinders and Gilbert Agricultural Resource Assessment; NAWRA = Northern Australia Water Resource Assessment. The Assessments differ somewhat from many resource assessments in that they consider a wide range of resources or assets, rather than being single mapping exercises of, say, soils. They provide a lot of contextual information about the socio-economic profile of the catchments, and the economic possibilities and environmental impacts of development. Further, they consider many of the different resource and asset types in an integrated way, rather than separately. The Assessments have agricultural developments as their primary focus, but they also consider opportunities for and intersections between other types of water-dependent development. For example, the Assessments explore the nature, scale, location and impacts of developments relating to industrial, urban and aquaculture development, in relevant locations. The outcome of no change in land use or water resource development is also valid. The Assessments were designed to inform consideration of development, not to enable any particular development to occur. As such, the Assessments inform – but do not seek to replace – existing planning, regulatory or approval processes. Importantly, the Assessments do not assume a given policy or regulatory environment. Policy and regulations can change, so this flexibility enables the results to be applied to the widest range of uses for the longest possible time frame. It was not the intention of – and nor was it possible for – the Assessments to generate new information on all topics related to water and irrigation development in northern Australia. Topics For more information on this figure please contact CSIRO on enquiries@csiro.au not directly examined in the Assessments are discussed with reference to and in the context of the existing literature. CSIRO has strong organisational commitments to Indigenous reconciliation and to conducting ethical research with the free, prior and informed consent of human participants. The Assessments allocated significant time to consulting with Indigenous representative organisations and Traditional Owner groups from the catchments to aid their understanding and potential engagement with their requirements. The Assessments did not conduct significant fieldwork without the consent of Traditional Owners. Functionally, the Assessments adopted an activities-based approach (reflected in the content and structure of the outputs and products), comprising activity groups, each contributing its part to create a cohesive picture of regional development opportunities, costs and benefits, but also risks. Preface Figure 1-2 illustrates the high-level links between the activities and the general flow of information in the Assessments. Preface Figure 1-2 Schematic of the high-level linkages between the eight activity groups and the general flow of information in the Assessments Assessment reporting structure Development opportunities and their impacts are frequently highly interdependent and, consequently, so is the research undertaken through these Assessments. While each report may be read as a stand-alone document, the suite of reports for each Assessment most reliably informs discussion and decisions concerning regional development when read as a whole. For more information on this figure please contact CSIRO on enquiries@csiro.au The Assessments have produced a series of cascading reports and information products: • Technical reports present scientific work with sufficient detail for technical and scientific experts to reproduce the work. Each of the activities (Preface Figure 1-2) has one or more corresponding technical reports. • Catchment reports, one for each of the Victoria and Southern Gulf catchments, synthesise key material from the technical reports, providing well-informed (but not necessarily scientifically trained) users with the information required to inform decisions about the opportunities, costs and benefits associated with irrigated agriculture and other development options. • Summary reports, one for each of the Victoria and Southern Gulf catchments, provide a shorter summary and narrative for a general public audience in plain English. • Summary fact sheets, one for each of the Victoria and Southern Gulf catchments, provide key findings for a general public audience in the shortest possible format. The Assessments have also developed online information products to enable users to better access information that is not readily available in print format. All of these reports, information tools and data products are available online at https://www.csiro.au/victoriariver and https://www.csiro.au/southerngulf. The webpages give users access to a communications suite including fact sheets, multimedia content, FAQs, reports and links to related sites, particularly about other research in northern Australia. Executive summary This report reviews international and national water quality studies, focusing on the environmental impacts and irrigation factors that influence water quality in tropical wet-dry regions and, in particular, northern Australia. Key findings Environmental impacts Irrigation development can lead to significant environmental changes, including altered flow regimes and water quality degradation. Studies worldwide and from northern Australia highlight the risks posed by increased nutrient loading, pesticide application and salinity changes because of irrigated agriculture. Elevated levels of nitrogen, phosphorus and other contaminants may lead to eutrophication, affecting aquatic ecosystems and posing health risks. Groundwater and surface water concerns Irrigation practices in tropical wet-dry climates, such as those found in northern Australia, show a complex and context-specific relationship with water quality. Groundwater extraction for irrigation can lead to aquifer depletion, increased salinity, and potential contamination through nutrient leaching. Surface water quality can also deteriorate due to sedimentation, chemical runoff and altered hydrological cycles. The high variability in seasonal rainfall (wet and dry seasons) further complicates water quality management, particularly in tropical regions where high-flow events can transport substantial nutrient and pesticide loads into water bodies. Specific trends for northern Australia In northern Australia, the predominant irrigation method is surface irrigation, which has been shown to significantly affect water quality. Studies from regions like the Burdekin Haughton Water Supply Scheme and Ord River Irrigation Area highlight the elevated concentrations of nutrients and pesticides in irrigation runoff. While dilution effects during high-flow events (e.g. in the wet season) may help reduce contaminant concentrations, the ecological risks remain significant. Long-term monitoring A major limitation in advancing sustainable irrigation practices in northern Australia is the lack of comprehensive, long-term water quality data. This lack hinders efforts to understand baseline water quality conditions and how irrigation and agricultural expansion have affected these systems over time. Furthermore, while there have been some studies on the impacts of irrigation on cotton farming, the effects on other crops remain under-researched. Additionally, digital databases containing valuable historical water quality data are fragmented and incomplete, limiting accessibility. Contents Director’s foreword .......................................................................................................................... i The Victoria and Southern Gulf Water Resource Assessment Team ............................................. ii Shortened forms .............................................................................................................................iii Units ............................................................................................................................... iv Preface ............................................................................................................................... v Executive summary ......................................................................................................................... ix 1 Introduction ........................................................................................................................ 1 1.1 Agricultural expansion and water resource development in northern Australia . 1 1.2 Agriculture and water quality ................................................................................ 1 1.3 Natural processing of water contaminants ........................................................... 3 1.4 Concentrations versus loads ................................................................................. 4 1.5 Report aims............................................................................................................ 4 2 International irrigation development review ..................................................................... 5 2.1 Methods ................................................................................................................ 5 2.2 Results and discussion ........................................................................................... 8 2.3 Environmental factors influencing water quality ................................................ 11 2.4 Irrigation factors influencing water quality ......................................................... 17 3 Water quality data in northern Australia ......................................................................... 24 3.1 Background .......................................................................................................... 24 3.2 Baseline water quality data ................................................................................. 24 3.3 Existing water quality modelling studies in northern Australia .......................... 25 4 Knowledge gaps ................................................................................................................ 27 References ............................................................................................................................. 29 Appendices ............................................................................................................................. 39 Figures Preface Figure 1-1 Map of Australia showing Assessment areas (Victoria and Southern Gulf catchments) and other recent CSIRO Assessments ........................................................................ vi Preface Figure 1-2 Schematic of the high-level linkages between the eight activity groups and the general flow of information in the Assessments ..................................................................... vii Figure 2-1 Flow chart of the literature identification, screening, eligibility and inclusion process and outcomes.................................................................................................................................. 7 Figure 2-2 Number of studies reporting a) surface water and b) groundwater quality in northern Australia ........................................................................................................................................ 10 Figure 2-3 Inundation in northern Australia under future sea-level rise (2030 to 2040)............. 16 Tables Table 1-1 Water quality variables reviewed and their impacts on the environment, aquatic ecology and human health.............................................................................................................. 2 Table 2-1 Search terms for elements of the primary question (and secondary questions if relevant) .......................................................................................................................................... 5 Table 2-2 Search string used for electronic searches ..................................................................... 6 Table 2-3 Inclusion and exclusion criteria applied to the search returns ....................................... 6 Table 2-4 Number of studies featured within the body of evidence according to climate ........... 8 Table 2-5 Number of studies featured within the body of evidence according to location (country) .......................................................................................................................................... 8 Table 2-6 Number of studies featured within the body of evidence according to study type and location............................................................................................................................................ 9 Table 2-7 Minimum, mean and maximum observation period (in years) from the body of evidence, according to study type. ................................................................................................. 9 Table 2-8 Number of studies within the body of evidence reporting surface water and/or groundwater quality, according to climate................................................................................... 10 Table 2-9 Number of studies featured within the body of evidence according to the factors found to affect surface water and groundwater quality .............................................................. 11 Table 2-10 Mean change in groundwater and surface water quality parameters from the wet season to the dry season attributed to hydrological factors. Values are from studies conducted within the wet-dry tropics. ........................................................................................................... 13 Table 2-11 Climate change factors affecting water quality globally ............................................ 15 Table 2-12 Irrigation methods and the number of studies reporting these methods in each climate zone .................................................................................................................................. 18 Table 2-13 Reported changes in water quality parameters, grouped by climate ........................ 19 Table 2-14 Irrigation mechanisms found to affect surface water and groundwater quality and the proportion of studies within the body of evidence that documented them ......................... 21 Table 2-15 The impact of fertiliser application upon surface water and groundwater quality in the wet-dry tropics and northern Australia .................................................................................. 22 1 Introduction 1.1 Agricultural expansion and water resource development in northern Australia Globally, water resource development has a known impact on downstream surface water and groundwater resources (Ayyandurai et al., 2022; McIntyre et al., 2011). The degree of this influence depends on a range of factors, such as the extent of agricultural development and the farming practices used, the soil profile and underlying geology, the local climate and time of year, changes in flow (e.g. from dam construction or groundwater extraction) and local land management practices (e.g. to prevent runoff from entering surface water and groundwater networks). Northern Australia is a focus for agricultural expansion and water resource development (Australian Government, 2015). Therefore, to understand the environmental responses and socio- economic consequences of water resource development, and to ensure that northern Australia’s water resources are developed sustainably, investigations into water resource availability and reliability, environmental impacts, implications for local Indigenous cultures and heritage, and social and economic prospects are necessary (Morán‐Ordóñez et al., 2016). These investigations are of particular importance in northern Australia due to its rich cultural heritage and diversity of terrestrial and aquatic species (Arthington et al., 2015; Finlayson et al., 2006; Hansen et al., 2022; Williams et al., 2003), including economically important coastal fisheries (Staples and Vance, 1985) and species of international conservation significance, such as migratory birds and sharks and rays (Dawkins, 2022). Habitat and water quality protection are therefore critical to the long-term conservation of northern Australia’s unique flora and fauna, including species of recreational, commercial and Indigenous cultural value. 1.2 Agriculture and water quality Irrigated agriculture is vital to global food security: it covers only 20% of cultivated land but accounts for 40% of global food production (Food and Agriculture Organization, 2020). However, while agricultural expansion and water resource development may provide food security and other social and economic prospects, agricultural activities, such as irrigated agriculture, can have significant negative environmental impacts on both surface water and groundwater quality (see Table 1-1). This is mainly due to the inefficient application of fertilisers and pesticides, and the high volumes of irrigation water required. Crops worldwide use less than 50% of the fertiliser nitrogen applied (Bijay and Craswell, 2021), and irrigated agriculture is responsible for approximately 60% of fresh water withdrawals globally (Food and Agriculture Organization, 2021). These inefficient practices mean that excess nutrients and pesticides are available for leaching and/or transport via runoff from irrigation and rainfall events into local surface water and groundwater bodies. Nitrogen, phosphorus and potassium are the three primary nutrients used in agricultural fertilisers. In sufficient quantities, both nitrogen and phosphorus can stimulate high phytoplankton and algal growth, potentially leading to eutrophication and hypoxia or anoxia of receiving water bodies and threatening aquatic life (Carpenter et al., 1998). Pesticides, used to increase agricultural productivity, can harm downstream aquatic ecosystems, flora and fauna. In mechanisms similar to nutrient transport, pesticides can enter surface water and groundwater bodies via infiltration, leaching and runoff from irrigation and rainfall events. These chemicals can be toxic to non-target species such as aquatic life and humans, affecting the nervous and immune systems, photosynthesis and growth (Cantin et al., 2007; Kaur et al., 2019; Naccarato et al., 2023). Pesticides can be carcinogenic (Mohanty and Jena, 2019), and they can cause multiple sub-lethal effects that can disrupt the ecological balance of aquatic systems and degrade aquatic communities (Giglio and Vommaro, 2022; Miller et al., 2020; Wang et al., 2022). Other water quality variables that can have a significant effect on the health of aquatic species, communities and ecosystems include salinity, pH and suspended sediments. Increased salinity, indicated by electrical conductivity and total dissolved solids, can interfere with osmoregulatory processes, harming species not adapted to saline conditions (Hart et al., 2003). Variations in a water body’s pH can negatively affect an organism’s biochemical processes, leading to altered behaviour, function, growth and even reduced survival (United States Environmental Protection Agency, 2024). In aquatic ecosystems, elevated loads of suspended sediment can smother habitats and benthic invertebrates, affect the feeding and respiratory systems of aquatic species, and reduce light penetration, affecting photosynthetic activity (Chapman et al., 2017). Table 1-1 Water quality variables reviewed and their impacts on the environment, aquatic ecology and human health 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 1.3 Natural processing of water contaminants While elevated contaminants and water quality parameters can harm the environment and human health, there are several processes by which aquatic ecosystems can partially process contaminants and regulate water quality. Denitrification, for example, is the process of anaerobic microbial respiration which, in the presence of carbon, reduces nitrogen to nitrous oxide and dinitrogen gas (Martens, 2005). Therefore, denitrification is a naturally occurring process that can remove nitrogen from a water body, reducing nitrogen concentrations. Pesticides can also be naturally removed from water via chemical oxidation, microbial degradation or UV photolysis, although some chemically stable pesticides are highly persistent, and their microbial degradation is slow (Hassaan and El Nemr, 2020). Phosphorus, however, does not have a microbial reduction process such as denitrification. Instead, if not temporarily taken up by plants, phosphorus can be adsorbed to the surface of inorganic and organic particles and stored in the soil, or deposited in the sediments of water bodies such as wetlands (Finlayson, 2022). However, this phosphorus can be remobilised into solution and re-adsorbed, resulting in ‘legacy’ phosphorus that can affect water quality for many years (Records et al., 2016). 1.4 Concentrations versus loads When monitoring and reporting on water quality variables, it is important to understand the ecological significance of reporting concentrations versus loads. Measuring concentrations of contaminants and water quality characteristics indicates the relative health of a water body at a particular time. Therefore, it is important to monitor concentrations of water quality variables over time to understand a water body’s baseline water quality and any broader trends. Concentrations are a useful means of measurement as they have biological significance. For example, the nitrogen species ammonia becomes acutely toxic (96-hour LC50) to the freshwater amphipod Eulimnogammarus toletanus at concentrations of 0.65 mg/L but is acutely toxic to salmon fry (Oncorhynchus gorbuscha) at only 0.08 mg/L (Camargo and Alonso, 2006). Understanding contaminant loads – that is, the total amount of contaminants entering a system over time – allows natural resource managers to understand the mass and/or volume of contaminants entering a system and the cumulative effect of contaminant inputs. Therefore, monitoring both contaminant loads and concentrations is key to developing comprehensive water quality management strategies. 1.5 Report aims This report seeks to understand baseline water quality trends in northern Australian catchments and the relative risks of agricultural water resource development, specifically irrigation practices and environmental factors, on surface water and groundwater quality. The key aims of this chapter are to: • review international studies to provide a context of the environmental factors and characteristics of irrigation schemes that have been found to influence surface water and groundwater quality, particularly in wet-dry tropical catchments • collate available baseline water quality data for rivers across northern Australia • collate existing water quality data and synthesise existing studies in the vicinity of irrigation areas across northern Australia • synthesise results of existing water quality modelling studies in northern Australia and models used • synthesise findings of studies in northern Australia that have related changes in water quality to changes in environmental conditions • outline some of the knowledge gaps. 2 International irrigation development review Understanding the intricate interplay between irrigation scheme characteristics, the environment, and surface water and groundwater quality is essential to sustainable water development and resource management. This review seeks to elucidate the diverse factors and characteristics inherent to irrigation practices and their environment that significantly affect both surface water and groundwater quality. By analysing and synthesising the global literature, particularly findings from wet-dry tropical climates, this study aims to uncover patterns and insights that may inform the sustainable development and management of water resources in northern Australia. 2.1 Methods The method adopted for this review is an evidence synthesis, a form of ‘rapid review’ that identifies, compiles and combines relevant knowledge from multiple sources. This method involves constraining the search effort, while still applying methods to minimise author bias in the searches and evidence synthesis. 2.1.1 Published literature search and eligibility Searches were performed using the literature search database, Scopus, and grey literature from the Northern Australia Water Resources (NAWR) digital library (Hyperlink to: North Australia Water Resources Digital Library ). Search terms A list of the search terms used to inform the online Scopus database search is provided in Table 2-1. Table 2-1 Search terms for elements of the primary question (and secondary questions if relevant) For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Search strings The search string used to conduct the online searches is presented in Table 2-2. The search string detailed in Table 2-2 features fewer subject-specific search terms than those collated in Table 2-1. This was done to reduce the number of search outputs (maximum search outputs were 5,847,516 studies) and maintain relevance to the topic. Several subject areas were also excluded from the searches to increase the relevance of the outputs obtained. Table 2-2 Search string used for electronic searches For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Inclusion and exclusion criteria Several inclusion and exclusion criteria were established to guide the initial and secondary screening processes and to minimise author biases in study selection. Inclusion and exclusion criteria are presented in Table 2-3. Table 2-3 Inclusion and exclusion criteria applied to the search returns 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 Screening process and data extraction The initial screening involved reviewing each study’s title and abstract to determine its eligibility for inclusion in the body of evidence, according to the inclusion and exclusion criteria (Table 2-3). This process reduced the 1353 studies from the Scopus search, NAWR digital library and author libraries to 481 (Figure 2-1). Figure 2-1 Flow chart of the literature identification, screening, eligibility and inclusion process and outcomes A flow chart showing the process of literature identification and screening for the current literature review. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au The second screening reviewed each document fully to determine its eligibility according to the inclusion and exclusion criteria. Secondary screening and data extraction, in which characteristics of the study and its results were entered into a data sheet, were conducted simultaneously for efficiency. In total, 78 studies formed the body of evidence for this review (see Figure 2-1 and Appendix A). 2.2 Results and discussion 2.2.1 Characteristics of the body of evidence Study climate conditions For the global component of the literature review, the studies featured within the body of evidence covered multiple climates and latitudes but not the polar regions (Table 2-4). The body of evidence features a large proportion of tropical wet-dry climates (n = 38 studies), due to the tropical wet-dry focus of this review. Temperate (n = 12), Mediterranean (n = 11) and subtropical climates (n = 9) also feature prominently within the body of evidence. Table 2-4 Number of studies featured within the body of evidence according to climate For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Study locations Of the 78 studies, 12 studies were from India, 9 studies were from the USA and 28 studies were from Australia, of which 26 studies were from northern Australia (Table 2-5). Table 2-5 Number of studies featured within the body of evidence according to location (country) 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 Study types Most studies within the body of evidence were observational (n = 56; 72%), with water quality measurements made in the field; 12% were experimental (n = 9) and the remaining 16% were modelling studies (n = 6) and reviews (n = 3) (Table 2-6). Table 2-6 Number of studies featured within the body of evidence according to study type and location For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Study duration The mean observation period (i.e., the timeframe over which data was collected) was approximately 3 years. Studies ranged from 9 days to 65 years in length (Table 2-7). Table 2-7 Minimum, mean and maximum observation period (in years) from the body of evidence, according to study type. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Study seasonality Of the38 studiesconducted withintropical wet-dry climates, 33stipulated the seasonality of theresearch. Most(64%)reportedwater qualityresults during both thewet and dryseasons,while15%reportedwater quality resultsfrom a singledryseasonand 21% from a singlewet season(Table2-8). Water quality in surfacewaterand groundwater bodies Only11 ofthe 78studiescharacterisedwaterquality in both surfacewatersand ground waters. Most studieswithinthe body of evidencefocused oneithergroundwater(n=37studies) orsurface water quality(n=28) alone.Within the body of evidence, studiesconducted within thewet-dry tropicsfocusedmainly on either groundwater quality (n=17)or surface water quality(n=15)withfewerstudies(n=6) reporting water qualitydata in both(Table2-8). Table2-8Number of studies within the body of evidence reporting surfacewaterand/orgroundwater quality, according to climate CLIMATEBOTHGROUNDWATER ONLYSURFACE WATERONLY Desert2 3 Mediterranean1 5 4 Subtropical 53 Temperate2 4 6Tropical wet 2Tropical wet-dry6 17 15 In northern Australia,the majority of surfacewaterand groundwater studies within the body of evidence arefrom the Ord River Irrigation Area (Figure2-2). a)A map of the tropic of capricorn and capricorn Description automatically generated Figure2-2Number of studies reporting a)surface waterand b)groundwater qualityin northern Australia 10|Reviewofwaterqualitystudies Environmental and irrigation factors influencing surface and groundwater quality Of the 78 studies within the body of evidence, 67 reported either an effect of the environment, irrigation or both upon surface water and/or groundwater quality. Irrigation factors were the most reported (by 40% of studies) followed by environmental factors (36%) and a combination of the two (24%) (Table 2-9). Table 2-9 Number of studies featured within the body of evidence according to the factors found to affect surface water and groundwater quality For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au 2.3 Environmental factors influencing water quality 2.3.1 Climate, geography and water use From the body of evidence, it is apparent that, in temperate countries, the use of groundwater or surface water resources or both for agriculture is region-specific (see Appendix B). In subtropical studies the trend differed, with groundwater used for agriculture in all regions featured within the body of evidence except for agricultural areas close to river basins where surface water was used (Appendix B). Interestingly, no records within the body of evidence discussed the environmental factors affecting water quality within subtropical regions. Within the body of evidence, groundwater was the dominant source of irrigation water in the sub-tropics. The most reported water quality parameters in temperate regions were nutrients and electrical conductivity (EC), whereas in subtropical regions EC was the dominant water quality parameter reported (Appendix B). Findings and trends from the wet-dry tropics and northern Australia In northern Australia west of the Great Dividing Range, few studies have assessed the impact of environmental conditions and irrigation practices on water quality. To expand the evidence base, several studies from the Burdekin River catchment in Queensland have been incorporated into this synthesis. The three jurisdictions of Australia’s wet-dry tropics form part of this literature review: Queensland, the Northern Territory (NT) and Western Australia (WA). The major river basins featured are the Burdekin and Fitzroy in Queensland, the Daly River in the NT and the Keep and Ord rivers in WA. 2.3.2 Seasonal hydrology, rainfall and first-flush events Rainfall, hydrology and first-flush events are well documented as pivotal environmental factors shaping surface water and groundwater quality across climates. Rainfall can both dilute and concentrate pollutants depending on the volume and intensity of the events. Studies show that above-average rainfall dilutes salt concentrations, decreasing electrical conductivity in groundwater systems (Koç, 2008). In surface water bodies, similar dilution effects were observed for nutrients like total phosphorus during increased streamflow events (Skhiri and Dechmi, 2012). However, high-intensity rainfall and high-flow events can also mobilise contaminants, leading to spikes in nutrient concentrations, especially when irrigation and agricultural fertilisation interact with rainfall patterns (Albus and Knighton, 1998; Shinozuka et al., 2016). These dynamics underscore the importance of understanding local hydrological events and incorporating vegetation management to mitigate the impact of contaminant mobilisation. Findings and trends from the global literature Table 2-10 outlines the mean percentage changes in water quality parameters from the wet to dry season in two distinct climate zones (subtropical and tropical wet-dry), highlighting the influence of seasonal hydrology, rainfall and first-flush events on surface water and groundwater quality. The table also presents the number of observations for each measurement. Note that many measurements have only one corresponding observation from the body of evidence. In the subtropical climate of Bamako City, South Africa, hydrological factors predominantly led to a decrease in surface water concentrations of chloride (-12.5%), sodium (-52.3%), EC (-27.6%) and total dissolved solids (-29.8%) from the wet season to the dry season (Sangaré et al., 2023). Nitrate concentrations, on the other hand, increased by 36.1%, while phosphate exhibited a drastic decline of 99.9% (Sangaré et al., 2023). These fluctuations in nutrient concentrations reflect the complex interaction between water flow and nutrient retention in different seasonal conditions. A key management practice identified to reduce the high contaminant loads of high-flow and rainfall events is the establishment and management of vegetation cover. In California, USA, pesticide concentrations were three times lower in runoff from experimental treatment plots planted with resident vegetation (grasses) than runoff from bare soil treatments (Joyce et al., 2004). Therefore, it is essential to understand and manage the dynamics of high-flow events to safeguard water quality in both surface water bodies and groundwater systems. Minimising the impact of contaminant mobilisation during such periods requires best management practices. Findings and trends from the wet-dry tropics and northern Australia In studies from tropical wet-dry climates (Table 2-10), hydrological factors, such as monsoonal rains and wet season flushing were found to reduce the electrical conductivity of surface water and groundwater bodies. This resulted in the reported mean electrical conductivity of groundwater and surface water bodies being 2.8% and 176% higher, respectively, in the dry season relative to the wet season (Ayyandurai et al., 2022; Bennett and George, 2011, 2014; Palanisamy et al., 2023; Sangaré et al., 2023; Townsend, 2019). Despite the increase in mean EC in both surface and groundwater systems from the wet season to the dry season, hydrological factors were attributed to a 33.4% and 9.4% decrease in the mean concentration of total dissolved solids from the wet season to the dry season in groundwater and surface systems, respectively (Bennett and George, 2011, 2014; Palanisamy etal., 2023; Sangaré et al.,2023).Mean concentrations of total suspended solidsalsodeclinedin surface water systems by 91.1% from thewet season tothe dry seasondue to hydrological factors(Townsend, 2019). From the body of evidence,nutrientconcentrationshave been found todecrease during the lowflows of thedry season, withhydrological factorsattributed toreductions inthe meanconcentrations of ammonium (76.5%; Townsend and Douglas, 2017), nitrate(95.5%; Sangaré etal., 2023; Townsend andDouglas, 2017), total nitrogen (89.3%; Bennett and George, 2011) andtotal phosphorus (81.6%; Bennett and George,2011)in surface water bodies from thewet season tothe dry season.In thedry season,thisisattributed tolow flows and higher residence timesfacilitating denitrification. Inthe wet season,highconcentrationsofnitrogenand phosphorusspecies have beenattributed to nutrients beingflushed fromthesoilandtransported to nearbysurface water bodies. Table2-10Meanchange in groundwater and surface water qualityparametersfrom the wetseasontothedryseasonattributed tohydrological factors. Values are from studies conducted within the wet-dry tropics. WATERBODYWATER QUALITY PARAMETERMEAN % CHANGE FROM THE WETSEASON TO THE DRYSEASON(NUMBER OF OBSERVATIONS) GroundwaterBicarbonate (HCO3−)31.2 (1) Electrical conductivity2.8 (3) Total dissolved solids-33.4 (1) Surface waterAmmonium (NH4+)-76.5 (1) Electrical conductivity176.6 (6) Nitrate (NO3−)-95.5 (1) Total dissolved solids-9.4 (4) Totalnitrogen-89.3 (2) Totalphosphorus-81.6 (2) Total suspended solids-91.1 (1) Findings and trendsfrom the wet-dry tropicsand northern Australia The wet-dry tropics ofnorthern Australia exhibitdistinct seasonal patternsin water qualitydue toextreme shiftsbetween thewet and dry seasons.Several studies fromnorthern Australia havehighlighted the strong link between hydrology and water qualityoutcomes. In theDaly River in theNT, for example,nutrientconcentrations in surface waters show considerable seasonal variation. Nitrate and ammonium concentrations in the Daly River peakduringthewet season and graduallydecrease during thewet-dry transition and into the dry season.This variability isdriven bywet- seasonhigh flows,which transport nutrientsfrom surroundingagricultural lands intoreceivingsurfacewater bodies(Townsend andDouglas, 2017). In contrast,the dry season is characterisedby groundwater-fed discharge,which typically results in increased salinity, as indicatedby ECvalues. The lower Keep River inWAfurther illustrates these seasonal dynamics.During thewet season, baseline levels of totalnitrogen and totalphosphorus are1.3to13times higherthan thedefaulttrigger valuesfor tropical Australia(Total nitrogen:0.3 mg/L; Totalphosphorus:0.01 mg/L; Bennett and George, 2014). These trigger values,established bythe Australian and New Zealand Chapter2 International irrigation development review|13 Environment and Conservation Council and the Agriculture and Resource Management Council of Australia and New Zealand, are concentrations that, if exceeded, indicate a potential environmental problem, ‘triggering’ a management response. Similarly, turbidity and TSS levels during the wet season have been reported to exceed default trigger values (Turbidity: 2-15 NTU; TSS: 2-15 mg/L) as much as 22-fold (Bennett and George, 2014). These heightened levels of nutrients and sediments during high-discharge periods in the wet season can lead to adverse ecological impacts, such as algal blooms, which pose risks to water quality and ecosystem health. In contrast, during the dry season, reduced water volumes lead to increased EC in both the Daly and Keep rivers, with some pools in the Keep River having EC values up to 167 times higher than default trigger values (2-25 mS/m; Bennett and George, 2014). Moreover, there is an inverse relationship between discharge and conductivity during events in the Daly River, where higher flows dilute EC values (Townsend, 2019). These trends demonstrate that in the tropical wet-dry climate of northern Australia, water quality is highly responsive to seasonal hydrology: wet-season flows facilitate nutrient and sediment transport, and dry-season conditions lead to higher salinity and reduced water quality due to concentrated ions and dissolved solids. Understanding these patterns is essential for effective management of water resources, especially in the context of agricultural expansion and irrigation development in the region. 2.3.3 Climate change Findings and trends from the global literature A significant challenge to managing water quality is the impact of climate change on the global hydrological system. Increases in surface temperature are affecting rates of evaporation and transpiration and altering the frequency and intensity of rainfall events and storms (Zaitchik et al., 2023). From the body of evidence, few studies have focused on assessing the impacts of climate change on water quality (Table 2-11). In temperate regions, only two studies (one in Portugal and one in Uzbekistan) have addressed this issue, both based on modelling climate change impacts. Interestingly, few observational datasets were included from the climate change impact modelling in Portugal and none of more than 4 months. However, the modelling study from Uzbekistan included long-term datasets over the past 65 years (Table 2-11). Noting the large uncertainty associated with such modelling studies, the outcomes of the climate change modelling on water quality from Portugal suggest that local increases in temperature would be associated with increased potential evaporation evapotranspiration, resulting in increased salt concentration in the soil and water resources and directly affecting crop productivity (do Nascimento et al., 2024). Contrastingly, the climate modelling results from Uzbekistan suggest that the changing climate and associated rainfall would lead to a decrease in land-based runoff to river ecosystems, reducing the nutrient load in surface waters used for agricultural purposes (Jarsjö et al., 2017). CLIMATE Table 2-11 Climate change factors affecting water quality globally For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Findings and trends from the wet-dry tropics and northern Australia In the tropical regions, there is a single study from Vietnam that has used data from the past 30 years to model climate change impacts on water quality (Table 2-11). The outcomes suggest that, with a reduction in rainfall due to climate change, there would be a reduction in water flow, leading to low levels of nutrients in the surface waters used for agriculture in Vietnam (Whitehead et al., 2019). In northern Australia, projections suggest significant shifts in temperature, rainfall patterns, storm activity and sea level as a result of climate change. Expected temperature increases ranging from 1.3 C (under the IPCC’s intermediate climate change scenario, or Representative Concentration Pathway: RCP4.5) to 5.1°C (under the ‘worst case’ climate change scenario: RCP8.5) by 2090 will exacerbate heatwaves and affect evaporation and transpiration rates (Brown, 2018). These changes are likely to alter seasonal weather patterns and influence the local hydrological cycle, potentially altering the frequency and intensity of rainfall and storms. In terms of precipitation, projections indicate increased intensity and variability, and extreme El Niño and La Niña events are anticipated to become more frequent (Brown, 2018). Such changes in rainfall patterns can lead to variable runoff and river discharge, influencing the EC, total dissolved solids and nutrient loads in surface water and groundwater bodies. Additionally, sea levels are expected to rise between 0.27 m (under RCP4.5) and 0.87 m (under RCP8.5) above the 1986 to 2005 level by 2090 (Brown, 2018; CSIRO and Bureau of Meteorology, n.d.). Such rises in sea level are expected to increase the frequency and severity of coastal inundation, threatening low-lying areas and coastal habitats (Figure 2-3). Within the body of evidence, there were no studies investigating the effects of climate change on surface water or groundwater quality in northern Australia. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Figure 2-3 Inundation in northern Australia under future sea-level rise (2030 to 2040) Orange circles highlight potential changes to the coastline. Saltwater intrusion and erosion could see these areas extend further inland (CSIRO Oceans and Atmosphere, n.d.). 2.3.4 Landscape factors Findings and trends from the global literature The global literature reveals that environmental factors, particularly soil type and irrigation mechanisms, significantly affect water quality, irrespective of seasonal rainfall, hydrology or geology (Appendix C). In temperate regions, regardless of whether groundwater or surface water is used for agriculture, the application of fertilisers and the resulting runoff typically increase nutrient levels (e.g. total phosphorus or nitrogen) and salinity in receiving water bodies (Appendix C). However, in subtropical and tropical climates, there is a noticeable lack of detailed research on how different soil types and irrigation methods affect water quality. Limited available data suggest that the use of fertilisers and anthropogenic chemicals, coupled with runoff from seasonal rainfall, plays a more frequent role in degrading water quality than other factors. While rainfall and hydrology are major contributors, additional environmental factors – such as land use patterns and the slope of agricultural lands – also significantly influence water quality, though more research is needed to understand their global relationships. Beyond irrigation and rainfall, landscape features like soil type, hydrogeochemistry and geomorphology also exert a substantial influence on surface water and groundwater quality. Hydrogeochemical processes, such as weathering and mineral dissolution, can affect surface water pH and ion concentrations, particularly bicarbonate levels (Zikalala et al., 2021). In groundwater systems, the permeability of geological formations plays a critical role; highly permeable formations can facilitate the transport of nitrates, leading to water quality degradation in irrigation zones (Chaudhuri et al., 2012). Factors like slope steepness and altitudinal gradients also influence water quality, with steeper slopes increasing runoff volumes and sediment losses (Ashraf et al., 1999). Additionally, soil type is a key determinant of groundwater quality. Compared to finer-textured soils, coarser, sandy soils promote greater infiltration due to their high macroporosity, which in turn leads to higher nutrient concentrations in groundwater (Aziane et al., 2020). Findings and trends from the wet-dry tropics and northern Australia In tropical wet-dry climates, such as in the Rangit River basin in India, the dissolution of carbonate and silicate minerals has been found to influence surface water quality by introducing ions such as calcium, magnesium, sodium, potassium, and chloride (Gupta et al., 2016). Similarly, in the Godavari district, the weathering of silicate minerals and marine clays contributes to fluctuations in groundwater conductivity and ion concentrations (Gurunadha Rao et al., 2013). These hydrogeochemical processes, particularly the dissolution of calcite and dolomite during monsoon seasons, are critical in determining groundwater quality, influencing calcium concentrations and, by extension, the sodium adsorption ratio. High sodium adsorption ratio values, driven by an imbalance of calcium and magnesium relative to sodium, can lead to soil salinisation and sodicity, further complicating water and soil management (Palanisamy et al., 2023). In northern Australia, water quality is heavily influenced by the interaction between soil types, seasonal rainfall and associated runoff. Queensland’s sandy and clayey agricultural soils are especially vulnerable, with runoff leading to increased levels of nutrients and electrical conductivity, which subsequently raises groundwater salinity and decreases overall water quality (Appendix E). Similar effects are observed in WA, where seasonal environmental factors play a dominant role in influencing water quality (Appendix E). Despite these known influences, there is a marked lack of scientific research focusing on the specific environmental factors affecting water quality in northern Australia. This gap underscores the need for more comprehensive studies to assess how soil types and seasonal hydrological events drive nutrient and contaminant transport in the region. Overall, while global studies offer insights into the hydrogeochemical processes influencing water quality, northern Australia requires targeted research to better understand the unique environmental and soil-driven factors affecting its water resources. 2.4 Irrigation factors influencing water quality 2.4.1 Irrigation method Apart from natural rainfed irrigation, the two main irrigation techniques used globally are pressurised irrigation systems and gravity-flow distribution systems. Each method has distinct applications and trade-offs, depending upon the pedological, geomorphological and hydrological context and the crop to be cultivated. To summarise, pressurised irrigation systems can irrigate large areas with a high degree of precision, offering water-efficiency rates between 80% and 95% (Brouwer et al., 1988). Micro irrigation, or trickle irrigation delivers water directly to plant roots, minimising water losses from evaporation and runoff(Brouwer et al., 1988). Spray irrigation systems mimic natural rainfall by spraying water over crops; however, these systems are prone to water loss through wind drift and evaporation (Sarwar et al., 2021). Surface irrigation systems, including basin, border, and furrow irrigation methods use gravity to distribute water across fields and are often less efficient (Brouwer et al., 1988). Lastly, surge irrigation seeks to improve the efficiency of surface irrigation methods by intermittently supplying water to furrows, reducing runoff and the volume of water used (Kifle et al., 2008). Findings and trends from the global literature Of the 78 international and northern Australian studies reviewed, 34 specify the irrigation method used in the study location. Surface irrigation methods (n = 30), in particular furrow irrigation (n = 20), are the most commonly reported irrigation method. There are no discernible trends among climate zones and the irrigation methods employed (Table 2-12). Table 2-12 Irrigation methods and the number of studies reporting these methods in each climate zone For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au There was no distinct trend associating micro, centre-pivot and spray irrigation systems with surface or groundwater quality due to the low number of studies (n = 8) investigating surface water and groundwater quality in regions using these systems, aside from the application of fertilisers and pesticides (Albus and Knighton, 1998; Aziane et al., 2020; Grundy, 2012; Huebsch et al., 2013; Joyce et al., 2004; Skhiri and Dechmi, 2012; Van Antwerpen et al., 2012; Zikalala et al., 2021). The global literature highlights several key trends in the relationship between irrigation practices and water quality, particularly the effects of surface irrigation systems across different climate zones (Table 2-13). In Mediterranean climates, before-and-after studies showed no change in chloride levels, but nitrate levels decreased by 33.3% between low- and high-flow conditions. Electrical conductivity also decreased by 20% (Causapé et al., 2006). Table 2-13 Reported changes in water quality parameters, grouped by climate Results are separated into before-and-after studies and studies reporting water quality in low-flow versus high-flow conditions. For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au †NA = data not available. Findings and trends from the wet-dry tropics and northern Australia Very few, if any, of the reviewed studies investigated the impacts of irrigation methods on surface water and groundwater quality. This presents a significant gap in the literature and our understanding. In the lower Burdekin, the herbicides ametryn and atrazine showed decreases of 83.3% and 66.7%, respectively, between low- and high-flow events (Davis et al., 2013). Conversely, in the Ord River Irrigation Area, pesticides like fipronil and quintozene exhibited sharp increases with fipronil rising by 112.5% and quintozene by 200% (Moulden et al., 2006). This illustrates how irrigation practices, coupled with climate conditions, can either reduce or exacerbate water quality variables, posing challenges for water management in agricultural settings. The data also reveal that while some nutrients and pesticides are diluted by hydrological processes, others persist or even increase in concentration depending on specific climate and flow conditions. 2.4.2 Irrigation area The USA has the largest irrigation area within the body of evidence. USA irrigation systems range from 4 ha to 181,300 ha, indicating its heavy reliance on irrigation for agriculture. Australia and Spain also display wide ranges in irrigation area; for example, Australian studies report irrigation areas ranging from 9 to 100,000 ha, reflecting the diversity of agricultural practices in the country. Findings and trends from the wet-dry tropics and northern Australia In northern Australia, conclusions regarding water quality changes associated with different irrigation areas are limited, due to the low number of studies. Paddocks from seven farms, distributed across the Burdekin delta, were sampled during high and low flow events over the course of 5 years. Overall, the results showed substantial declines in pesticide levels from low-flow (typically during the dry season) to high-flow events (typically occurring during the wet season), with ametryn decreasing by 83%, atrazine by 67%, and diuron by 56% (Davis et al., 2013). 2.4.3 Aquifer salinisation The widespread use of irrigation practices significantly influences surface water and groundwater quality in agricultural regions worldwide. In groundwater systems in particular, irrigation activity can lead to the secondary salinisation of aquifers via seawater intrusion The over-extraction of groundwater for anthropogenic uses, such as irrigation, can lower the watertable. In coastal aquifers, this can lead to seawater encroachment and saline intrusion, increasing groundwater EC and total dissolved solids (Ayyandurai et al., 2022; Khezzani and Bouchemal, 2018; Sarkar et al., 2021; Taşan et al., 2022). A second mechanism by which irrigation activity can lead to aquifer salinisation, is through the leaching of irrigation water into groundwater systems. Leaching of irrigation water can l increase the height of the watertable, bringing salts into the plant root zone. When water from the soil is taken up by plants or evaporated, these salts can accumulate in the soil (Kulmatov et al., 2018). In wet-dry tropical climates, implementing irrigation schemes can increase evapotranspiration due to increased crop transpiration during irrigation periods (do Nascimento et al., 2024). This increase in evapotranspiration has been found to increase groundwater concentrations of total dissolved solids and EC (Ali et al., 2008). Moreover, any salts accumulated in the soil can be leached back into the groundwater during rainfall events or irrigation periods (Ortiz and Jin, 2021). 2.4.4 Irrigation water quality The quality of the irrigation water source also plays a crucial role in influencing surface water conductivity (Causapé et al., 2004). For example, in the Great Menderes Basin, Turkey, a negative feedback was established in which irrigation waters were draining into the Great Menderes River, the irrigation supply source. Due to high groundwater salinity levels and a statistically significant relationship between groundwater and drainage salinity, significant quantities of salt were subsequently transported to the Great Menderes River from irrigation schemes in the region and accumulated in the soil profile, negatively affecting riverine ecology and causing the extinction of two aquatic species (Koç, 2008). This cumulative effect underscores the complex interplay between irrigation practices, groundwater extraction, hydrology and the consequential changes in surface water and groundwater EC. 2.4.5 Fertiliser and pesticide application Findings and trends from the global literature Fertiliser applications on irrigated land can result in elevated levels of nutrients such as total phosphorus and total nitrogen in drainage waters, increasing surface water concentrations during the irrigation season (Barbieri et al., 2021; Mosley and Fleming, 2010). Different cropping systems under irrigation can also affect groundwater nutrient concentrations. For example, nitrate concentrations in mulch-till continuous sweet corn systems are 50% lower than in ridge-till cropping systems, which highlights the importance of best management agricultural practices in groundwater quality management (Albus and Knighton, 1998). Among the observations that found an impact of irrigation practices on surface and/or groundwater quality, approximately 35% documented co-occurring environmental impacts and approximately 65% documented water quality impacts of irrigation practices alone. The most commonly reported irrigation mechanism affecting water quality was fertiliser application (15 observations; Table 2-14), followed by fertiliser application and the leaching of nutrients into groundwater bodies (6 observations). Table 2-14 Irrigation mechanisms found to affect surface water and groundwater quality and the proportion of studies within the body of evidence that documented them For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Findings and trends from the wet-dry tropics and northern Australia In the wet-dry tropics, fertiliser application was reported to affect nutrient concentrations (potassium, total phosphorus, phosphate, total nitrogen, nitrate and ammonia), as well as sodium concentrations, electrical conductivity and total dissolved solids. However, while fertiliser application may have added nutrients to the system, these trends are masked by numerous additional and contextual factors influencing water quality in these areas and their water bodies. The results presented in Table 2-15 highlight significant variations in the impact of fertiliser application on water quality in the wet-dry tropics and northern Australia. Globally, changes in nitrate concentrations show substantial variation, particularly in regions like India, where nitrate levels fluctuate drastically between pre- and post-monsoon periods. For example, nitrate levels in India showed both significant increases (up to 891.7% according to Gurunadha Rao et al. (2013)) and decreases (−24.7% and −47% according to Ayyandurai et al. (2022) and Palanisamy et al. (2023), respectively) from pre- to post-monsoon. In Mexico, nitrate concentrations increased by 106.3% from pre- to post-monsoon (Sedeño-Díaz et al., 2022). These global trends suggest that seasonal hydrological patterns, land use practices and fertiliser management significantly influence nutrient leaching and water quality, particularly in regions with heavy rainfall or distinct wet and dry seasons. In contrast, northern Australia shows relatively less variability, trending towards reduced nutrient concentrations following the irrigation season or over time. Total nitrogen, for instance, saw reductions of 36.9% during the transition from irrigation to pre-wet season (Smith et al., 2007), and up to 90.9% in alluvial soils between 2009 and 2010 (Grundy, 2012). Phosphorus however, exhibited a slight increase of 8.6% during the transition from irrigation to pre-wet season (Smith et al., 2007). Table 2-15 The impact of fertiliser application upon surface water and groundwater quality in the wet-dry tropics and northern Australia Abbreviations as follows: not available (NA). 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 2.4.6 Conclusion The development of irrigation resources in northern Australia comes with significant environmental and hydrological challenges. The findings from this report highlight the impacts of environmental and irrigation factors on water quality, particularly in regions where seasonal rainfall, soil characteristics and agricultural runoff converge to influence both surface water and groundwater systems. The limited research in northern Australia underscores the need for a more comprehensive understanding of how irrigation practices affect water resources across varying soil types and under future climate change. Current data reveal that nutrient runoff, pesticide contamination and salinity are key concerns that, if not properly managed, could degrade local ecosystems and reduce water quality for agricultural and community use. Sustainable water resource development involves prioritising long-term water quality monitoring and the implementation of best management practices, such as optimising fertiliser use and increasing the efficiency of pesticide application. Moreover, improving our understanding of the potential climate change impacts on water quality will increase our ability to manage water resources sustainably in a changing climate. Such efforts will require open data sharing and access, as a collaborative, informed and adaptive approach is essential to balancing the region’s agricultural growth with the protection of its valuable and vulnerable water resources. 3 Water quality data in northern Australia 3.1 Background Water resource development in northern Australia requires careful management as it can lead to a range of environmental impacts, including changes to flow regimes (both surface water and groundwater), land use, river channel connectivity and natural waterway conditions, including water quality. The rivers, floodplains, coastal and nearshore regions of northern Australia support a diverse range of species that hold recreational, commercial and cultural values. To understand the potential risks to the natural river catchment environment that is associated with water resource development, this review examines international and national studies to summarise the environmental factors and characteristics of irrigation schemes that have been found to influence groundwater and surface water quality. 3.2 Baseline water quality data The data availability from various government databases is presented in Appendix F. In addition to a few databases, there are several technical reports that investigated water quality in northern Australia, but their data are not accessible or publicly available. Moreover, the region lacks comprehensive water quality databases, particularly for surface water bodies. Such databases are necessary for assessing long-term trends and understanding the historical influences on current water quality variables. These are particularly important because, in many river systems, antecedent hydrological conditions influence water quality variables today. Therefore, past data, access to older reports and long-term monitoring studies are vital in understanding observations made today (Zikalala et al., 2021). However, much of this older knowledge is stored in reports that have not yet been digitised and so are unavailable for inclusion in large-scale reviews. The simplest form of surface water and groundwater quality monitoring in northern Australia includes long-term datasets of water temperature and rainfall, with other physicochemical and abiotic water quality parameters recently added to the suite of variables frequently monitored (see Appendix F). The interactive map from the Bureau of Meteorology’s Australian Water Data Service (http://www.bom.gov.au/waterdata/) provides detailed information on multiple sites across Australia that have long-term data loggers and river gauges, providing data on water volumes in storage, river and groundwater levels and water quality, as well as local water uses and restrictions. This website provides an online platform for the public to analyse and visualise regional water quality. Furthermore, the Australian Water Resources Information System (AWRIS) has set priorities for the next 5 years (2023 to 2028) to build: (i) a new national water data hub, (ii) a single platform for public water information, and (iii) a hydrological model integration and enhancement. AWRIS will also provide sustainable decision support, water information and data leadership, and water and hydro-climate science leadership. The only sediment core-based long-term (100 years) monitoring of water quality available for northern Australia is in WA. Long-term monitoring data on drinking water quality has been collected in the NT but the data are not publicly available. For northern Queensland, long-term datasets (>4 million water quality records) that cover chemical, physical and biological properties of water and sediment are available for various rivers and aquifers from 1968 onwards. An interactive map on pesticide concentrations in water and sediment samples and Pesticide Risk Metric (https://prmdashboard.des.qld.gov.au/) across Queensland is also available from the Department of Environment, Science, and Innovation (https://apps.des.qld.gov.au/water-data- portal/map). Most states and territories in Australia have a water quality monitoring system in place, but there is a significant lack of up-to-date data available in a common platform for public usage. GIS files with the locations of monitoring sites are readily available for most states and territories in Australia, as are reports and peer-reviewed publications, but the associated water quality data are not. In summary, only limited surface water and groundwater quality data for northern Australia are available to the public. These datasets have significant gaps in spatial and temporal coverage and in the water quality parameters monitored, and they are typically not maintained. Looking forward, AWRIS’s priorities to centralise and improve national water data capabilities may begin to resolve the lack of collated and publicly available water quality data for northern Australia. 3.3 Existing water quality modelling studies in northern Australia Water quality modelling is a key component of water resource development and management, but few water quality modelling studies have been conducted in northern Australia. The studies include is a mix of geochemical, soil salinity, water quality and integrated parameter models (Appendix G). However, these modelling studies are based on datasets with less than 5 years of data and, except the recent water quality model of Lillicrap et al. (2015), were developed almost 2 decades ago. Lillicrap et al. (2015) is part of a report series that assesses the surface water chemistry of the 8000 ha Weaber Plain (Goomig Farmlands) in northern WA (Lillicrap et al., 2015; Lillicrap et al., 2011). The reports were commissioned in 2008 as part of the Ord River Irrigation Expansion project connecting the Weaber Plain to the Ord River Irrigation Area by constructing an irrigation supply channel. Initial groundwater-level simulations of the project revealed that introducing irrigation would lead the groundwater levels of the Weaber Plain to rise, potentially affecting soil salinity and surface water quality (Kellogg Brown and Root Pty Ltd, 2010, 2011). To eliminate this risk, groundwater management plans were developed to pump groundwater from beneath the Weaber Plain into the main irrigation supply channel. As part of the Ord River Irrigation Expansion project approvals process, several groundwater pumping scenarios were modelled to understand the impact of groundwater pumping on the water quality of the main irrigation supply (Lillicrap et al., 2015; Lillicrap et al., 2011). The AquaChem™ hydrochemical model was used to simulate groundwater and supply channel mixing and predict salinity levels resulting from the different pumping scenarios. These scenarios included ‘expected’ and ‘worst case’ conditions, varying in the volume of groundwater pumped and the flow rates within the supply channel. Results indicated that, under both scenarios, total dissolved solids in the groundwater under Weaber Plain, initially at 1162 mg/L, could be reduced to levels suitable for irrigation (178 to 192 mg/L) upon mixing with supply channel water. Despite the significance of these findings, accessing technical reports, particularly those written before 1990, remains challenging. Due to their age and never having being digitised, a substantial number of historical technical reports are either inaccessible or lost, complicating efforts to understand baseline water quality dynamics across northern Australia. Efforts to enhance the accessibility and preservation of these critical reports are crucial. Current repositories like the Northern Australia Water Resources Digital Library, CSIRO’s Research Publications Repository and others contain subsets of available reports but lack integration and comprehensive coverage. Establishing a unified, centrally managed repository is required to streamline access, optimise resource allocation, and ensure these valuable studies are readily available to researchers, policymakers and the public. Collaboration among stakeholders and organisations managing these repositories is essential to achieve this goal effectively, addressing challenges related to funding, staffing, maintenance and digital storage capacity. While existing water quality modelling studies provide valuable insights into the impacts of agricultural development on northern Australia’s water resources, the fragmented nature of available data highlights the need for systematic digitisation and consolidation efforts. Establishing a unified repository would not only enhance accessibility but also facilitate informed decision making and sustainable management of water resources in this ecologically significant region. 4 Knowledge gaps Water resource management in irrigated agriculture faces numerous challenges and knowledge gaps at both international and national levels. This report highlights several key areas where further research is needed to improve our understanding of the influences of environmental factors and agriculture on water quality, particularly in tropical wet-dry regions. The following gaps highlight the complexities in managing water resources in northern Australia effectively. Herbicide runoff and pesticide concentrations There are significant uncertainties regarding the dynamics of herbicide runoff and temporal variations in soil pesticide concentrations. Understanding how these chemicals move through the environment, particularly after irrigation or rainfall, is critical for protecting water quality. Soil amelioration and groundwater quality The long-term impacts of repeated applications of soil amelioration agents, such as gypsum, on deep drainage and groundwater quality are not well understood. Research is needed to determine how these practices may affect both surface water and groundwater systems over time. Lack of long-term monitoring data One of the most critical gaps is the absence of robust and long-term water quality monitoring data, particularly because of the importance of antecedent hydrology, agriculture and fertiliser use in influencing current and future water quality conditions. Accurate simulation of water quality across a river basin, for example, is particularly challenging due to the lack of consistent datasets. This hampers the ability to predict trends and make informed decisions about water resource management. Fertiliser efficiency There is a pressing need to enhance fertiliser use efficiency through best management practices and use of slow-release fertilisers. Current practices in some areas has resulted in nutrient runoff, which can degrade water quality. Predicting future fertiliser use trends globally adds another layer of complexity to this issue. Climate change and hydrological uncertainty Climate change compounds existing challenges in water resource management by altering global patterns of evaporation, evapotranspiration and precipitation. These changes introduce new uncertainties and reduce the predictability of the hydrological cycle, affecting groundwater recharge and complicating efforts to achieve sustainable water management. There is also a lack of understanding about how climate change will inevitably affect water quality. Knowledge gaps in northern Australia In northern Australia, water quality research is limited, particularly in regard to how irrigation practices affect water quality beyond cotton farming operations. Most studies focus on cotton, leaving gaps in the assessment of other agricultural systems. Additionally, the region lacks comprehensive water quality databases, especially for surface water bodies. 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Appendices Appendix A Author(s), publication year and title for reports and journal articles included within the body of evidence 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 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 For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Appendix B Country-specific water quality variables in various global regions derived from the body of evidence Abbreviations as follows:not available(NA), not specified (NS),electrical conductivity (EC),total dissolved solids (TDS). Nutrientsincludephosphate, total phosphorus, nitrate, ammonia and total nitrogen.Major ionsinclude bicarbonate,sulfate, chloride, sodium, calcium and magnesium, minor ionsincludefluoride,and minerals includesilica. COUNTRYLOCATIONSEASON (DRY OR WETOR BOTH) WATER RESOURCE(GROUNDWATER, SURFACE WATERBOTH) RAINFALL (MM)IRRIGATION OR ENVIRONMENTAL EFFECTS ONWATER QUALITYWATER QUALITYVARIABLESREFERENCESTemperate PortugalAlentejoBothGroundwater517 IrrigationMajor ionsdo Nascimento et al.(2024) USACaliforniaBothSurface water470 IrrigationNutrientsZikalala et al.(2021) CaliforniaBothGroundwater290 IrrigationNutrients, ECHarter et al.(2002) ColoradoNASurface water320 BothMineralsBern et al.(2017) TurkeyAlaçamNAGroundwater784 IrrigationECTaşan et al. (2022) Great Menderes BasinIrrigation seasononlyBoth656 Environment, IrrigationECKoç(2008) Soke plainNAGroundwater963 BothECSomay and Gemici(2012) SpainBardenas irrigation districtBothGroundwater419 BothNutrients, ECCausapé J et al. (2006) Petrola EndorheicNot specifiedBoth400 IrrigationNutrientsValiente et al.(2018) Basin SevilleBothGroundwater650 BothNutrients, ECDe Miguel et al.(2013) UzbekistanNavoiregionDryGroundwater90 IrrigationSalinityKulmatov et al. (2018) KziljarBothSurface water100 EnvironmentNutrientsJarsjö et al. (2017) ChinaSongnen plainNot specifiedBoth400 IrrigationNutrientsLiu et al.(2021) IrelandCorkBothGroundwater1200 IrrigationNutrientsHuebsch et al.(2013) KoreaNamwon-siBothBoth1313 BothNutrientsYoon et al.(2006) Appendices|45 COUNTRYLOCATIONSEASON (DRY OR WETOR BOTH) WATER RESOURCE(GROUNDWATER, SURFACE WATERBOTH) RAINFALL (MM)IRRIGATION OR ENVIRONMENTAL EFFECTS ONWATER QUALITYWATER QUALITYVARIABLESREFERENCESSubtropical TunisiaZaghouan regionDryGroundwater296 IrrigationNutrients, EC, TDS,major ionsFarhat et al.(2024) Kebili ProvinceBothGroundwater100 IrrigationECBouarfa et al. (2009) AlgeriaNorthern AlgeriaNSGroundwater250 IrrigationECBouarfa et al. (2009) SouthCrocodile riverBothSurface water880 IrrigationECvan der Laan et al. (2012) Africa Lomati riverBothSurface water760 IrrigationECvan der Laan et al. (2012) Pongola riverBothSurface water610 IrrigationECvan der Laan et al. (2012) Tropical IndiaNew DelhiBothGroundwater567 IrrigationECPradhan et al.(2010) AndhraPradeshBothGroundwater456 BothECGurunadhaRaoet al.(2013) SikkimWetSurface water1150 EnvironmentpH, EC, TDS, nutrients, major ionsGupta et al.(2016) Tamil NaduBothGroundwater1446 BothpH, EC, TDS,major and minorAyyandurai et al. (2022) ions, nutrientsTamil NaduBothGroundwater908 BothpH, EC,major ionsPalanisamy et al. (2023) Tamil NaduWetGroundwater1512 EnvironmentpH, EC, TDS,major ions, nutrientsBalamuruganet al.(2020) JharkhandBothGroundwater1363 BothNutrientsSingh et al.(2018) UttarPradeshWetGroundwater650 EnvironmentMineralsGautam and Rai(2023) Andhra PradeshDryGroundwater1000 EnvironmentEC,major ions, nutrientsSwarna Latha and Rao(2012) MaharashtraWetGroundwater950 EnvironmentEC, TDS,majorionsPanaskar et al. (2016) Tamil NaduWetGroundwater1210 BothNutrients,major ions,mineralsElango et al.(2003) MexicoTehuacan ValleyBothGroundwater450 EnvironmentEC,nutrients, major ionsSedeño-Díaz et al.(2022) San Luis PotosíBothSurface water444 IrrigationTDSGonzález-Acevedo et al.(2016) 46|Reviewofwaterqualitystudies Appendix C Environmental factors affecting water quality globally Abbreviations as follows: not applicable (NA), nitrogen (N), phosphorus (P), sodium (Na), magnesium (Mg) and potassium (K). For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Appendix D Water quality variables in tropical wet-dry regions of Australia derived from the body of evidence Abbreviations as follows: not applicable (NA), not specified (NS), electrical conductivity (EC), total dissolved solids (TDS), dissolved organic carbon (DOC), nitrogen (N) and phosphorus (P). 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 Appendix E Environmental factors affecting water quality in Australia Abbreviations as follows: not applicable (NA), electrical conductivity (EC) and nitrogen (N). For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au Appendix F Baseline water quality databases available for northern Australia Abbreviations are as follows: not applicable (NA), not specified (NS). 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 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 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 Appendix G Global and Australian water quality modelling studies For more information on this figure, table or equation please contact CSIRO on enquiries@csiro.au