1. Abstract No. M02 1. Introduction Many of Australia’s water management areas are over-allocated or highly developed, particularly in the Murray-Darling Basin – MDB (AWR 2005). Impacts of climate change could reduce surface water availability by 13% in the south and 9% in the north of the Basin by 2030 (CSIRO 2008). This has prompted a greater consideration of alternative water sources, such as brackish groundwater, which often require treatment prior to use (e.g. desalination). 3. What is brackish groundwater? For the purpose of this investigation, brackish groundwater is defined as having an EC of 1 500 – 7 000 μS/cm. An advantage of brackish groundwater is that it is less expensive to treat than other alternative sources of water such as seawater, grey water or effluent. 2. Purpose  To do preliminary mapping of the location and volume of brackish groundwater in the MDB using existing groundwater data;  To identify areas which are suitable for brackish water irrigation; and  To model crop yields to determine the impact of applying brackish water. Table 1. Approximate areas potentially suitable for brackish water irrigation 4. How much brackish groundwater is available? 1 Glynnis Barodien, 2 Eloise Nation and 3 Kemachandra Ranatunga 1 Bureau of Rural Sciences, Department of Agriculture, Fisheries and Forestry, GPO Box 858, Canberra, ACT 2601 2 Water Division, Bureau of Meteorology, GPO Box 1289, Melbourne VIC 3001 3 Water Division, Bureau of Meteorology, GPO Box 2334, Canberra ACT 2600 glynnis.barodien@brs.gov.au  The GIS mapping techniques used worked well in the MDB because of data availability. Investigations are needed into appropriate mapping methods in areas with insufficient data of reasonable quality.  Suitability mapping showed areas must have appropriate soil texture, depth of watertable and rainfall, as well as access to brackish water.  Preliminary investigations have shown that brackish groundwater is a promising alternative source of water for irrigation.  The economic and environmental consequences of treating brackish groundwater are important factors that need investigation. Figure 2. Potential suitable areas for brackish water irrigation in the Murray hydrogeological basin (southwestern region). EC in µS/cm Approximate area as a percentage Approximate cumulative area in ha <200011.53,584,300 <400017.55,472,750 <6000216,473,500 <8000237,083,200 Figure 3. Percent reduction in yield with different saline levels in the irrigation water; (A) for canola and (B) for field peas. Table 2. Summary of EPIC results <2000 μS/cm <4000 μS/cm <6000 μS/cm<8000 μS/cm Figure 1. Estimated volume of brackish groundwater in the MDB Brackish Water as an Alternative Water Source for Agriculture in the Murray-Darling Basin  A groundwater salinity map with EC of 1 500 – 7 000 μS/cm was used as part of a weighted multi-criteria analysis to identify areas suitable for brackish water irrigation.  The analysis was run with water EC of <2000, <4000, <6000 and < 8000 μS/cm, along with physical factors such as coarse textured soil, deeper watertables and higher annual rainfall in the Murray hydrogeological basin.  The result was a series of maps showing suitable areas for brackish water irrigation when EC is <2000, <4000, <6000 and < 8000 μS/cm, Figure 2.  Table 1 shows the approximate percentages and extent of potential suitable areas (ha).  Groundwater salinity was mapped using existing groundwater data (bores, groundwater management units (GMUs), stream connectivity and the groundwater salinity contours of the MDB.  Groundwater volume was calculated based on specific yield and storativity. Specific yield was estimated based on standard values for the dominant lithology of unconfined aquifers and storativity was estimated for confined aquifers using the storativity equation (Singh et al. 1999).  Mapping of groundwater volume indicates that there are large volumes of brackish groundwater in the southwestern MDB in the Murray hydrogeological basin, the Murrumbidgee and in the far north of the MDB, Figure 1. 6. Conclusions Ideally, irrigation water is <800μS/cm; water between 800 and 2300μS/cm is generally considered marginal for irrigation. However, tolerance to salinity varies greatly between crops and is also dependent on factors such as climate and soil characteristics. 5. Brackish water irrigation Crop Type Salinity level in irrigation water (μS/cm) Percentage reduction in yield Wheat2 500 - 4 000 10 - 40 % Barley2 5 00 - 4 000 6 - 25 % Canola2 500 - 4 00010 - 17 % Field Peas2 500 - 4 000 48 - 85 % The Environmental Policy Integrated Climate (EPIC) modelling framework was used to model the impact of using brackish (up to 5000 µS/cm) water on barley, wheat, canola and field peas in the Murray hydrogeological basin over a 20 year period. Figure 3 shows the percentage yield reduction and is further summarised in Table 2. References AWR 2005, WRON Alliance for the National Water Commission. CSIRO (2008). Water availability in the Murray-Darling Basin. A report to the Australian Government from the CSIRO Murray-Darling Basin Sustainable Yields Project. CSIRO, Australia. 67pp. Singh, VP, Won Seo, I & Sonu, JH (Eds) 1999, Hydrologic Modelling: Proceedings of the International Conference on Water, Water Resources Publications, Colorado.