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 supply security due to the difficultly in projecting rainfall and the variability in rainfall projections between the various Global Circulation Models (GCMs) and SRES emission scenarios.

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 determine the effects on the estimated security of supply. We illustrate the approach for Adelaide’s Southern water supply system up to 2050. The Adelaide 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 5-year intervals for 23 GCMs and 6 emission scenarios were obtained from CSIRO’s OzClim website. These were superimposed on 1000 stochastic daily rainfall timeseries, which were used as inputs for a planning model of Adelaide’s Southern urban water supply system. Water security, in terms of the reliability and average vulnerability (shortfall) of supply, are then determined for each of the 138 combinations of climate change possibilities.

The probability functions for system reliability and vulnerability are greatly affected by the selection of GCM, while choice of emission scenario produces smaller variability in the results. These findings indicate that the potential influence of climate change on urban water supply security is extremely uncertain at this point in time. Therefore, until there is greater agreement between GCMs, urban water managers should consider planning for climate change using a wide range of GCM projections and emission scenarios.