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Stormwater Harvesting and Reuse Model Summary Figures for a 1 acre watershed (CN 74) with either 85%, 65%, or 30% impervious, 5 months of irrigation, and a storage basin sized for a range of rainfall events (Figure 14 of the [2013 Stormwater Harvesting: Accounting of Benefits and Feasibility report). Several key design drivers were observed from these model test runs. 1) Percent annual volume reductions due to irrigation were higher for cistern storage systems than pond storage systems because there was no evaporation from the cistern surface, but overall volume reductions were slightly greater for pond storage systems. Percent annual volume and phosphorus reduction tended to increase as the percent imperviousness decreased because at high percent imperviousness there are not enough green spaces to apply water. 2) Stormwater harvesting ponds in sites with low percent imperviousness tended to have greater variability in percent annual volume and phosphorus reduction due to high evaporation rates and large differences in pond surface area as volume increased. In sites with very low percent imperviousness, stormwater harvesting and irrigation use is not always cost-effective due to low stormwater generation. 3) The percent annual volume and phosphorus reduction of stormwater reuse increased as the basin volume increased from 1” to 2” rainfall events. Basin volumes designed for larger rainfall events provided additional annual phosphorus and volume reductions but at incrementally smaller amounts.
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