The rainwater and spillover from sprinklers that flows off roads, yards and rooftops  — much of it eventually emptying into waterways or the ocean — could help boost California’s water supply. 

The state's urban areas shed 770,000 to 3.9 million acre feet of runoff a year that could be captured, according to the Pacific Institute. That’s enough to supply between 2.7 and 13.7 million households for a year.

The potential is highest in Southern California, which has lots of pavement that sends rainwater and irrigation runoff into storm drains. Collecting this runoff and feeding it into aquifers — or eventually treating it and sending it to taps — would avoid wasting it.

Some local agencies have been corralling stormwater to replenish aquifers for years, with dozens more projects in the works. 

The Fresno Metropolitan Flood Control District, for instance, captures runoff across 400 square miles in Fresno County. The water is used to fill more than 150 ponds, where it trickles through the soil to refill groundwater stores. In bone-dry 2021, storm flows accounted for almost all of the district’s groundwater recharge.  

Santa Monica has been a leader in treating urban runoff, and plans to upgrade a recycling facility built near its famous pier more than 20 years ago. The plan is to treat the collected runoff and stormwater so it’s clean enough to be injected directly into Santa Monica’s groundwater basin. 

Strategies for using stormwater also include installing permeable pavement in yards and communities and building basins that let it drain into the soil instead of flowing into storm drains or streets.

But barriers remain to capturing more of the flows. These include high costs and a lack of funding, concerns about impacts to water quality, and lengthy planning and approval processes.