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.