California fails to capture massive amounts of stormwater rushing off city streets and yards that could help supply water for millions of people.

That's according to a nationwide report by the Pacific Institute, which ranks California in 9th place among states with the most estimated urban runoff.

In 2018, LA County voters approved Measure W, a parcel tax to generate about $300 million per year for stormwater capture projects.

Almost 2.3 million acre-feet of precipitation slicks off pavements and other surfaces in California cities and towns every year.

It's enough to supply more than a quarter of the state's urban use, if all of it could be captured and treated.

Capturing runoff can be costly, requiring elaborate construction projects to trap, clean up, and hold massive volumes of water.

Cities like Los Angeles are intentionally designed to protect against flooding by funneling stormwater away from communities and into channels, and then out to sea.

In the Los Angeles, Long Beach, and Anaheim area alone, the amount of rainfall that flows off pavement is roughly equal to the total water used in Los Angeles and some surrounding areas in a year.

There are efforts to hold on to more stormwater without risking flooding.

LA County already collects a lot of runoff, including almost all of the San Gabriel River's flows.

Meanwhile, the Los Angeles Department of Water and Power is working to funnel water off streets and into planted areas, as well as into underground basins and wells.

There are plans to ramp up the effort and expand stormwater capture beneath parks in the San Fernando Valley, too.

For CalMatters, I'm Rachel Becker.