Capturing Storm Runoff Could Supply Water to Millions of Californians a Year, Study Finds

California fails to capture massive amounts of stormwater rushing off city streets and surfaces that could help supply water for millions of people a year, according to a new analysis released today.

The nationwide report, by researchers with the Pacific Institute, ranks California ninth among states with the most estimated urban runoff. Rainwater flows off streets and yards into storm drains that eventually empty into waterways and the ocean — carrying pollutants picked up along the way.

The analysis found that California sheds almost 2.3 million acre-feet of precipitation from pavements, roofs, sidewalks and other surfaces in cities and towns every year. If captured and treated, that would supply more than a quarter of California’s urban water use (PDF), or almost 7 million Southern California households each year.

Los Angeles came in first in the West and 19th nationwide among 2,645 urban areas for amounts of runoff. An average of about 490,000 acre-feet a year of rainfall flows off the pavement in the Los Angeles-Long Beach-Anaheim area — roughly the amount that the city of Los Angeles and some surrounding areas (PDF) use in a year.

“What we’ve recognized, and are recognizing, is that stormwater is a resource that can be harnessed,” said Heather Cooley, co-author of the study and director of research at the Pacific Institute.

In recent years, former President Donald Trump and other Republican politicians and lawmakers have criticized California for “wasting” water that flows out to sea. At the Conservative Political Action Conference last week, Trump said a California congressman told him, “‘No, we don’t have a drought. We have so much water you don’t know what to do.’ But they send it out to the Pacific. We’re not going to let them get away with that any longer.”

But there are many reasons why stormwater flows into the ocean: Capturing it can be costly, requiring elaborate construction projects to trap and clean up or hold massive volumes of water.

And cities like Los Angeles are intentionally designed to protect people from floods by funneling large volumes of stormwater into channels and then out to sea.

“The whole area is designed with storm drains to capture all the flows so that people don’t get flooded, people’s property don’t get flooded,” said Adam Ariki, interim deputy director at Los Angeles County Department of Public Works. “We’re really trying to capture as much of it as possible. And with time, that number is going to go higher and higher and higher.”

In many cases, water containing oil, trash and other pollutants must be treated before it can percolate into aquifers pumped for drinking water. In others, lack of open space limits where runoff could be allowed to seep naturally into the ground.

Also, in some controversial cases, particularly the Bay-Delta in Northern California, experts say stormwater must flow into rivers and the ocean to support fish (PDF) and other wildlife. Growers and others in the Central Valley criticize those flows and call for more reservoirs, saying the water is wasted.

Sometimes, there’s just too much rain at once to capture all of it. “Some flows may need to be sacrificed or allowed to go to the ocean because you can only capture so much of it, especially like last year,” Ariki said. “That’s the challenge.”

Water officials and experts agree that capturing more stormwater before it flows into drains is a top priority to help boost California’s water supply.

Los Angeles County already collects about 200,000 acre-feet of runoff a year, including about 95% of the San Gabriel River’s flows — enough to fill about 100,000 Olympic-sized swimming pools. Last year, among the wettest in California, the county captured around 630,000 acre-feet of storm flows, Ariki said.

Fourteen dams capture flows off the mountains (PDF) that are then slowly released into 27 spreading grounds, where they percolate into the ground to feed aquifers. However, capturing water from concrete rather than mountainsides can be more challenging.

In most California cities, runoff flows into storm drains, not treatment plants. San Francisco, with its combined sewer system that treats both stormwater and sewage, and Santa Monica, with its recently upgraded facilities for treating and injecting stormwater into the aquifer, are rare exceptions. Pollution of ocean waters can sicken people and disrupt ecosystems.

“It is absolutely unsafe to swim in many locations for 72 hours after a rain event because of the pollution coming from our storm drain system,” said Tracy Quinn, president and CEO of Heal the Bay, an environmental nonprofit that focuses on Santa Monica Bay.

In 2018, Los Angeles County voters approved Measure W, a property tax of 2.5 cents per square foot of impermeable surface, to generate about $300 million per year for stormwater capture projects.

Environmentalists are calling on the program to replace more hardscapes with parks and usable greenspace by 2045 — a move that also could help communities, especially in highly urbanized cities, better weather the extreme heat and floods of climate change.

Los Angeles is already working to funnel water off streets and into planted areas as well as underground infiltration chambers and wells. And it has plans to ramp up the effort and expand stormwater capture beneath parks in the San Fernando Valley, as well, said Art Castro, manager of the Los Angeles Department of Water and Power’s watershed management group.

“If you look at the density of the city, we can no longer build spreading grounds that are roughly 150 acres big,” Castro said. “Parks are going to be the next big opportunity that we have, and if you think about it, parks are in almost every community, in every watershed.”