California's Stormwater Potential

A new study reveals that if California could collect and treat more stormwater in cities, it could provide enough water to supply a quarter of the state’s urban population.

2 minute read

March 4, 2024, 9:00 AM PST

By Clement Lau

View of Hollywood Reservoir with palm trees in foreground and Los Angeles neighobrhoods in background.

Lake Hollywood reservoir in Los Angeles, California. | cherylvb / Adobe Stock

Cities across the United States face a range of water challenges, including intensifying impacts of flooding and drought, water scarcity risks, constraints on traditional water supplies facing peak water limits, and water pollution. Innovative ways are needed to address these challenges and improve long-term water resilience.

Stormwater capture is a promising approach. Specifically, It can help narrow the gap between water supply and demand by augmenting and diversifying water supplies. It can also mitigate impacts on communities from intensifying flooding and drought and reduce water pollution. In addition, stormwater capture offers additional co-benefits, such as greening communities and cooling urbanized areas. 

As reported by Rachel Becker in this article, a new assessment by the Pacific Institute, in partnership with 2NDNATURE, quantifies the volumetric potential of urban stormwater runoff across the country. The study estimates urban areas have the potential to generate 59.5 million acre-feet of stormwater runoff annually on average. This is equivalent to an annual average of over 53 billion gallons per day. It equals 93 percent of municipal and industrial water withdrawals in 2015, the most recent year with available data.

With respect to California, the analysis found that the Golden State sheds nearly 2.3 million acre-feet of precipitation from pavement, roofs, sidewalks and other surfaces in cities and towns annually. If it were captured and treated, that would be sufficient to supply over a quarter of California’s urban water use or almost 7 million Southern California households each year. 

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