Urban Water Supply Drought Security: A Comparative Analysis of Complimentary Centralised and Decentralised Storage Systems

In Australian urban water supply, large reservoirs are traditionally used to ensure adequate drought security. In recent years there has been a trend towards decentralised storage in the form of rainwater tanks harvesting rainfall from small roof catchments. In the urban setting the rainwater tank is usually too small to provide high reliability of supply. Rather the tank is used to compliment mains water supply. This study asks the question, under what conditions do urban rainwater tanks make the most significant contribution to urban water supply drought security? To answer this question in a way that maximizes insight, a Monte Carlo study is conducted involving 2400 combinations of climate variability and physical configuration of large reservoirs and urban rainwater tanks. The results are summarised using non-dimensional variables. It was found that rainwater tanks make the greatest contribution to drought security when the centralised storage is stressed (i.e., it experiences a high level of regulation and/or high streamflow variability) and when per-capita domestic demand is minimised. Moreover, the contribution of rainwater tanks to system drought security is more robust in the presence of climate change with declining rainfall.