Optimizing rainwater harvesting systems for the dual purposes of water supply and runoff capture

Rainwater harvesting (RWH) systems recycle and reuse runoff to meet demand. RWH may also provide an additional benefit, runoff reduction or capture. Most studies of RWH have focused upon water supply exclusively. We evaluate decentralized RWH systems across a wide range of land uses and locations in Virginia for water supply and runoff capture, using the Rainwater Analysis and Simulation Program (RASP) model. RASP simulates an RWH system using storage volume, roof area, irrigated area, and indoor nonpotable demand as inputs. A lifecycle cost-benefit model of RWH was developed. Water supply and runoff capture reliability were assessed in each simulation. Near-optimal solutions were identified for each case and location using a nonlinear metaheuristic algorithm. While positive net benefits were not achieved in any of the land-use cases or locations, distinct differences were observed. A 20% reliability target was achieved, or nearly so, for commercial, office, and high density residential cases, which also had the lowest runoff capture unit costs. In all cases, water supply reliability was binding. Statistical analysis indicated that costs variability can be characterized by two locations in Virginia; for water supply or runoff capture, three locations are necessary, and within specific locations, some land-uses could be combined. Cost effectiveness analysis of each land use provided a means of identifying least cost solutions for water supply and runoff capture. Net benefits were found to be very sensitive to water and wastewater charges.