A Prototype Analysis for Determining the Stormwater Retention and Water Supply Benefits of Cisterns

Cisterns are usually considered for their water supply benefit. This paper evaluates how effective cisterns are as flood control measures as well as water supply. It considers a problematic situation that is becoming common in the southwest – residential areas on small lots where over half the lot is impermeable. The study uses a simple mass- balance relationship with daily input of rainfall to the cistern and daily use of the water by two citrus trees. It uses the 105 years of daily rainfall measurements at the University of Arizona to calculate daily site runoff and to determine if water will be available to irrigate the trees. The cisterns were shown to be capable of reducing runoff in comparison to a site without a cistern. Likewise, the simulation showed that cisterns will provide most of the water for the trees in an average year.

One measure of smart growth from a hydrologic perspective is for post-development runoff to be maintained at pre-development levels. This simulation showed that whether runoff is maintained at pre-development levels depends on soil types and the runoff from impermeable areas not harvested by the cistern. Cisterns can reduce runoff to pre- development values for the 2 to 10 year events for soils with high runoff potential. For more permeable soils, both water harvesting in earth works and a cistern may be necessary to maintain runoff volume at pre-development levels.

In addition to assessing the benefits of water harvesting for the types of residential housing currently being built, this kind of analysis could be used to determine how to lay out developments that result in smart growth from a hydrologic perspective. Future assessments should integrate the storage capacity of earthworks into the analysis to determine the integrated benefits of cistern and earthworks water harvesting on landscape water supply and flood control.