Climate change impacts on water yields and demands in south-western Australia

Abstract

Summary A climate shift in the mid 1970s reduced rainfalls in south-western Australia by 10–15% and inflows into reservoirs that supply the city of Perth (population 1.8 m) by more than half. The region has a Mediterranean climate, similar to other areas in the world experiencing reductions in rainfall and rises in temperatures. Rainfall–runoff modelling has indicated that streamflows may reduce by a further quarter by 2030 or by half if a dry future climate is experienced. Groundwater levels on the coastal plain in south-western Australia have fallen since the mid 1970s where unconfined aquifers are covered with perennial vegetation, including under the main water supply aquifer for Perth. Modelled projections are that groundwater levels in most areas will continue to fall through to 2030 under most future climate scenarios. Projected streamflows and groundwater levels indicate reduced water availability but these need to be converted to projected water yields, i.e. the amount of water that can be diverted for consumptive use. This paper reports how projections of future streamflow and groundwater levels were used to estimate 2030 divertible water yields for a 62,500 km 2 area in south-western Australia. These yields were then compared with estimates of water demands in 2030 to identify areas of water surplus and deficit under clearly defined assumptions. The methods used to define future yields are based on sets of rules that could be varied by water managers if desired. Surface water yields are estimated to decrease by about 24% (possible range of −4% to −49%) which is similar to the projected reduction in runoff (−25% with a range of −7% to −42%). Groundwater yields are projected to fall by only about 2% (range of +2% to −7%) because of reductions in evapotranspiration and drainage losses as watertables fall where groundwater levels are close to the surface. In addition, recharge remains relatively high under cleared areas used for non-irrigated agriculture. In some areas with high groundwater recharge and little groundwater use groundwater levels may continue to rise even under a hotter and drier future climate. While overall groundwater yields may be little affected there are very important groundwater resources, especially those covered by perennial vegetation, which will experience large reductions in available yield under most projected future climates. Under current per-capita water consumption levels, rapid population and economic growth, along with the reductions in water yields, are expected to result in appreciable water deficits developing near Perth and some regional cities in south-western Australia by 2030. Where both surface water and groundwater resources are available for use, groundwater is anticipated to become increasingly used in future.