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 future and posing a threat to water supply and 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 contrasted 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 rainfallrunoff
modelling. Nearly all GCMs projected reductions in rainfall by mid (2046-2065) and late (2081-2100)
21st century. 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
would result in 49-69% reduction in reservoir inflow by the mid to end of 21st century. Rainfall reduction under
B1 scenario would be 11.7-12.1% respectively and corresponded to streamflow reduction of 44.5-45.8%
respectively.