The impact of climate change on water demand.

Climate change impacts weather and changes in weather is the single largest factor influencing fluctuations in water demand. Therefore, it would be natural to expect climate change to impact on demand. However, most climate models typically do not produce a single forecast, but rather produce an ensemble of equally likely scenarios. The high variation between these projections lead to different estimates of demand, leaving decision makers with the task of selecting one projection to use. On the other hand, there exists no scientific way of doing this, as they are all equally likely, meaning selecting one would be as good as the other. This paper describes the way Sydney Water approached the problem, which was to use a two-staged process. This meant developing a mathematical solution to integrate the output of global models with Sydney Water’s demand forecasting model, and developing an approach based on the risk tolerance thresholds of the decision it informs to select the most appropriate output from a climate model.

Figure 1. Annual Demand. Red CCMA3.1, Green CSIRO-MK 3.0, Blue ECHAM5, Brown-MIRCO3.2 (reproduced from Barker et al (2018b)