Effects of Stormwater Capture and Use on Urban Streamflows

Cities across the globe manage stormwater to enhance water supplies. Capturing and using stormwater in urban watersheds can have benefits for groundwater recharge, reduced pollutant loading in downstream watersheds, and habitat management. In California, metropolitan areas in the southern coastal regions of the state have for decades captured an average of 493 Million Cubic Meters (400,000 acre-feet) of runoff annually to recharge groundwater. But in a state with highly managed watersheds and seasonal precipitation, capturing stormwater for water supply goals can affect urban streamflows. Using a model with simulation and optimization of regional urban water resources management in Los Angeles County (Artes), we analyze the potential effects of increasing stormwater capture and infiltration on urban streamflow volumes. Results indicate that for many watersheds in LA, further increasing stormwater capture and use would significantly reduce urban streamflow volumes, especially in downstream basins. But in some basins, streamflows are increased to preferentially direct water to existing stormwater capture basins. Results illustrate potential tradeoffs in water supply, in-stream water flows, and aquatic habitat that must be considered when looking to increase use of local water sources through more stormwater capture.