Rainwater harvesting (RWH) is a renewable water supply option for nonpotable use, most commonly used for landscaping irrigation. Water rights in Colorado prohibit all RWH except residential rain barrels and a pilot project program that allows centralized rainwater harvesting for new development. Development of a natural catchment creates impervious surfaces, thereby increasing runoff, with a subsequent decrease in infiltration and losses to evapotranspiration; pilot projects are allowed to harvest a volume equal to the predevelopment runoff losses that would have occurred on new impervious areas at the site. To support this administrative policy, a tool was developed for the efficient calculation of daily allowable harvest at nearly any project site in Colorado. A reliable and useful policy tool requires the incorporation of hydrologic science with widely applicable, user-friendly design, a challenging balance of rigor and accessibility that is welcomed by engineers and policymakers alike. The daily allowable harvest is determined for each soil group as a percentage of infiltrated rainfall less the groundwater return. Horton’s infiltration method is used to model rainfall‒runoff for a range of soil parameters (NRCS hydrologic soil groups) and precipitation events (0.25- to 25-year return periods and 15-min to 24-h durations). For most events, the percent infiltration is 90% of the precipitation depth; this ratio decreases when precipitation exceeds the infiltration rate. Results are simplified in a spreadsheet tool for policy application, with allowable harvest rules binned by event duration and frequency. Simulations using the tool for a 2010‒2017 precipitation record from Colorado’s Front Range showed that RWH can supply up to 50% of the annual demand for traditional landscaping and over 100% of the water-smart landscaping demand.
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