Rainwater tanks are a viable alternative water resource option used around the world. Their effectiveness is a function of local climate, uptake, roof and tank size and demand characteristics. Watercare carried out an assessment of rainwater tank potential in the context of the Auckland supply-demand balance to compare them with other water resource options.
The assessment considered their benefits at Watercare’s two relevant Levels of Service, which drive the selection of water sources in Auckland. Ambitious uptake rate scenarios for new and existing properties were developed, which modelled between 23% and 66% of Auckland households having a tank installed by 2050. A synthesized 1000 year rainfall sequence was applied to determine the conjunctive yield of the tanks and Watercare’s lake supply system for the different scenarios of rainwater tank use.
The work modelled a range of scenarios including the implementation of a programme installing large rainwater tanks on a widespread basis throughout Auckland. Outputs demonstrated that the most favourable scenario would result in tanks supplying up to 16% of the forecast demand at the drought level of service and 35% at the peak level of service. The capital cost of implementation of such a programme would be of four times that of a river source able to supply 100% of the forecast demand at both levels of service. This paper solely addresses the water resource benefits of rainwater tanks; any wider benefits are excluded.