Climate change and long term water availability in South-Western Australia - an experimental projection

The population of the Perth-Bunbury region in Western Australia is predicted to increase to 3.1 million by 2050. Water supply is a key issue, as below-average rainfall since the mid-1970s has led to about 40% decline in the streamflow. General Circulation Models (GCMs) project a further decrease in rainfall leading to diminished water resources in the future and posing a threat to water supply and the environment. In this experimental study we assess the impact of climate change at Serpentine Reservoir using data from eleven GCMs which contributed to the latest Intergovernmental Panel on Climate Change (IPCC) assessment report. Data from two emission scenarios (A2 and B1) were used and downscaled, using a state-of-the-art statistical downscaling model, to a 5 km resolution compatible with catchment modelling. The LUCICAT rainfall-runoff model was calibrated for the Serpentine catchment and then changes in runoff were projected using the downscaled rainfall data. Land use and potential evaporation were not changed for the future rainfall-runoff modelling. Nearly all GCMs projected reductions in rainfall by mid (2046-2065) and late (2081-2100) 21st century compared to 1981-2000 period. There was a significant variation in projected rainfall reductions between different GCMs and emission scenarios. Under the A2 climate scenario, there could be a further 14-24% reduction in rainfall, and this would result in a 49-69% reduction in reservoir inflow by the mid to end of the 21st century. Rainfall reduction under B1 scenario would be around 12% and corresponded to streamflow reduction of about 45-46%.