The stormwater retention performance of rainwater tanks at the landparcel scale

The use of tanks to harvest and store rainwater has the potential to simultaneously: 1) augment potable water supplies and 2) restore some aspects of pre-development flow regimes in receiving waters. However, the use of rainwater tanks to achieve these multiple objectives has not been well quantified. Such quantification is required to assist the development of computer models of urban water systems capable of up-scaling the effects of rainwater harvesting at the land-parcel scale to simulate catchment-scale responses. In this paper, we quantify how the use of rainwater tanks in a temperate climate (740 mm average annual rainfall) can achieve these multiple objectives at the land-parcel scale, based on water use measurement from houses with a range of tank volumes and demands. We use these new empirical data to model a range of typical rainwater tank scenarios. It is shown that tank yield can be substantial and is not significantly reduced when tanks are configured for passive irrigation, even though this design modification significantly improves the capacity of the tanks to retain rainfall events. We also find that the use of tanks alone cannot completely restore the natural retention capacity of typical land-parcels. Our results suggest that typical rainwater tank scenarios can concurrently assist in restoring pre-predevelopment flow regimes and reliably augment potable supply. If retention capacity is limited by tank volume or a lack of demands, tanks could be allowed to partly drain to the garden for passive irrigation or be configured to overflow to infiltration-based retention systems.