Sensitivity of Urban Water Security Based on Various Global Circulation Models and Emission Scenarios

The intrinsic link between the urban water cycle and climate variables, in particular rainfall, is forcing water supply managers to revise future water plans to incorporate the likely impact of climate change. However, there is great uncertainty surrounding how climate change will affect urban water security due to difficulties in projecting rainfall and the variability in rainfall projections between the various Global Circulation Models (GCMs) and the Special Report on Emissions Scenarios (SRES) scenarios produced by the Intergovernmental Panel on Climate Change. The purpose of this study is to develop a method to incorporate rainfall projections under a range of climate change scenarios into an urban water supply model in an attempt to estimate the effects on the security of supply. We illustrate the approach for Adelaide’s Southern water supply system up to the year 2100. The future Adelaide supply system includes reservoirs fed from catchments and water pumped from the River Murray, household rainwater tanks, stormwater reuse schemes and a desalination plant. Monthly percentage change factors for Adelaide’s rainfall at five-year intervals for four GCMs and three SRES scenarios were obtained from CSIRO’s OzClim website. These were superimposed on 1000 stochastic daily rainfall time series, which were used as inputs for a planning model of Adelaide’s Southern water supply system. Water security, in terms of the average annual shortfall of supply, was then determined for each of the 12 combinations of climate change possibilities. The average annual shortfall for Adelaide’s Southern system is greatly affected by the rainfall sequence, the GCM used and, to a lesser extent, the SRES scenario. These findings indicate that urban water managers should consider planning for the likely impacts of climate change using multiple stochastic rainfall sequences and a broad range of GCM projections and SRES scenarios.