Groundwater yields in south-west Western Australia

ion throughout the project area is very non-uniform because the aquifers are variable in extent, depth and salinity, and demand is uneven (Figure 7). The area near Perth has the highest use because of the needs of drinking water, peri-urban horticulture, private and public parks, and gardens and industry. This area has deep aquifers, especially north of the Swan River, which are now fully exploited. Concentrations of use also occur on the Swan Coastal Plain south of Perth associated with irrigation and industrial demands. The main land uses in the groundwater part of the project area are dryland agriculture (56 percent); native vegetation including forests and national parks (38 percent); urban (3 percent); softwood plantation forestry (1 percent); open water and estuaries (1 percent); and irrigation and other summer wet areas (1 percent). Over two-thirds of the Northern Perth Basin is under dryland agriculture but less than one-third of the Southern Perth Basin has this high recharge land use. The proportion of rainfall that becomes recharge is greatly affected by soil type, vegetation cover and depth to watertable. It is highest when soils are sandy; there is little or no perennial vegetation; and the watertable is neither very shallow (so that potential recharge is rejected and evaporation losses are high) nor very deep (so that roots can intercept it before it reaches the watertable). Much of the Swan Coastal Plain is occupied by dunal sands with clayey soils fringing the scarp and major rivers. Most of the plain has been cleared of vegetation, except on the Gnangara groundwater system north of Perth and the coastal strip between Jurien Bay and Dongara. The depth to watertable is 3 m over 22 percent of the southern half of the Perth Basin (comprised of the Central Perth Basin, Peel-Harvey Area and Southern Perth Basin), whilst 14 percent of the area is between 3 and 6 m and 10 percent is between 6 and 10 m (Figure 6). These areas support groundwater dependent ecosystems. Given these circumstances, recharge under the Swan Coastal Plain is generally high except where watertables are very high, there is perennial vegetation or the soils are clayey. Recharge under the Scott Coastal Plain is also favoured by high rainfall, sandy soils and agricultural clearing in some areas but is even more limited by high watertables, especially in the west. The Dandaragan Plateau and Arrowsmith Region are largely cleared for dryland agriculture and have moderately deep watertables which encourage recharge. However, soils of these regions are more gravelly and clayey than the coastal soils and the region’s rainfall is lower. Most of the Blackwood and Darling plateaux are covered with native vegetation or plantations. Their soils are more gravelly and clayey than in all other areas, and watertables tend to be deep. Therefore despite high rainfall, recharge is significantly inhibited. Figure 6. Depth to watertable in the southern half > of the Perth Basin Figure 7. Groundwater abstractions in the Perth Basin > 6 Groundwater y ie lds in south-west Western Austral ia December 2009 7 Change in groundwater levels between 2008 and 2030 Results from the three groundwater models (PRAMS, PHRAMS and SWAMS) which cover the 20,000 km2 southern half of the Perth Basin were combined to estimate the change in groundwater levels between 2008 and 2030 under climate and development scenarios (Figure 8). The effect of vegetation, soil type, abstraction and depth to watertable on groundwater levels is evident. Groundwater levels continue to rise under the Dandaragan Plateau despite this being the driest modelled area because the area is cleared, the watertable is relatively deep, soils are reasonably permeable and abstraction is modest. Groundwater levels under the western Swan Coastal Plain are estimated to fall by less than 3 m by 2030 in all except the Gnangara Mound (within the Gnangara groundwater system) which is vegetated and has relatively high levels of abstraction (Figure 7). This is due to sandy soils and lack of vegetation. However, in areas with high watertables, potential recharge is lost through drainage and evapotranspiration. Despite the reduction in rainfall, the shallow aquifers refill each winter under all scenarios except the dry extreme future climate and the future development