Shea Rouda via Unsplash
Harvesting Urban Storm Runoff

Edward Ring

Director, Water and Energy Policy

Edward Ring
March 6, 2024

Harvesting Urban Storm Runoff

In a normal year, by the end of March downtown Los Angeles receives 13 inches of rain. Last year 27.8 inches fell, and through March 3 of this year, 21.3 inches has already fallen. This suggests that both this year and last year, over 1.0 million acre feet of rainfall hit the region. Even in an average year, rainfall totals about a half-million acre feet.

If Californians could somehow capture more of this runoff, it would tip the balance from scarcity to abundance in a state that has coped with chronic water shortages for several decades. A 2022 study by the Pacific Institute concluded that California’s urban “stormwater capture potential is 580,000 AFY in a dry year to as much as 3.0 million AFY in a wet year.” But can engineers design systems to capture whatever the skies deliver?

More generally, is it possible for California’s coastal megacities to become completely independent of imported water through a combination of runoff harvesting, wastewater reuse, and desalination? Orange County Water District, with a service population of 2.5 million, is the furthest down the path to water independence.

In an average year, they capture about 75,000 acre feet of baseflow from the Santa Ana River, in addition to harvesting 55,000 acre feet of storm runoff. So-called incidental percolation from rain contributes another 60,000 acre feet per year to their groundwater basins, and the agency built the biggest water recycling plant on the West Coast, allowing it to reuse 130,000 acre feet of wastewater every year. With a total demand for water at 390,000 acre feet, OCWD only has to import 70,000 acre feet per year from the State Water Project.

To capture more storm runoff, OCWD’s current approach is to create more opportunities for incidental percolation by encouraging conversion of impermeable surfaces to permeable surfaces. By doing this, the district estimates they can increase annual rainfall driven aquifer replenishment from 60,000 acre feet to 80,000 acre feet per year. This would lower their water import requirement to 50,000 acre feet per year. If the California Coastal Commission had approved the proposed Huntington Beach desalination plant, which was designed to produce 55,000 acre feet of fresh water per year, 2.5 million people living in northern Orange County would be on track to be completely independent of imported water.

While desalination and wastewater recycling ought to be part of a diverse and resilient portfolio of water supply infrastructure investments, the big numbers are still found in what falls out of the sky. Which brings us back to Los Angeles County. Can enough megatonnage of atmospheric river rainfall be harvested to slake the thirst of this megapolis?

It’s a tough problem. Not only because these intermittent deluges deliver torrents that are barely contained in a 400 foot wide and 35 foot deep concrete culvert they still call the Los Angeles River. Also, because it’s not just how much water has to be processed, it’s what’s in the water. Consider this excerpt from Los Angeles Waterkeeper, “LA’s water watchdog,” describing what happens during a major storm:

“In Los Angeles, our concretized LA River and all its tributaries turn into the city’s largest sewer, carrying pesticides and herbicides from our homes, oils, and grease from our roads, heavy metals and other toxins from Los Angeles’ businesses, and trash, bacteria, and other contaminants from local communities straight into our waterways.”

That’s quite a spew. In Orange County, runoff travels over less mileage of contaminated surfaces on its way to aquifer storage, and those contaminants are filtered as they percolate, diluted within the aquifer, then treated again when pumped up for use. Many of the aquifers in the Los Angeles Basin, on the other hand, are contaminated.

Despite the additional challenges, Los Angeles County is pursuing many of the same strategies as Orange County. On average the county successfully harvests 200,000 acre feet per year of stormwater, about 15 percent of the total demand. In the very wet 2022-23 rainy season, LA County Public Works estimated that stormwater capture at groundwater recharge facilities totaled over 500,000 acre feet.

At the same time, the Los Angeles Dept. of Water and Power has begun groundwater remediation with the ultimate goal of relying on these massive aquifers to store millions of acre feet of imported water, recycled wastewater and storm runoff. In the meantime, long-standing efforts are now accelerating to “unpave” the city, especially upstream where the runoff doesn’t hit as many surface contaminants.

It isn’t clear, even with all of this, whether or not Los Angeles can ever become water independent without also relying on a combination of imported water and desalination. But independence of imported water is exactly what they’re planning. Through aggressive conservation programs, total water demand in Los Angeles County has dropped from nearly 2 million acre feet per year at its peak around 20 years ago to an estimated 1.35 million acre feet. Today, Los Angeles County recycles 134,000 acre feet of wastewater per year, with plans to increase processing capacity to just over 500,000 acre feet. The county intends to double its stormwater harvesting capacity, and currently withdraws an additional 270,000 acre feet per year from its aquifers through natural recharge.

The difference between total demand and the contributions from these various sources is made up for by imports from the State Water Project and the Colorado Aqueduct. Water imports into Los Angeles County have averaged around 800,000 acre feet per year in recent years, but with completion of planned projects and additional conservation those imports are projected to drop to under 100,000 acre feet per year.

The investments required for California’s cities to achieve near self-sufficiency in water supply are daunting. But these investments often bring the additional benefit of making the city a healthier and more alluring place for people and wildlife. These positive externalities generate intriguing cost-benefit questions. How much is it worth to plant new green roofs on top of old concrete buildings? What about buildings designed strong enough to actually grow trees on their roofs? Rewilding a city increases storm and drought resiliency, but also provides a deeper human benefit. Just like replacing concrete with mass timber, astroturf with grass, and underground culverts with daylighted streams, more nature, along with more nurturing architecture, are how cities become welcoming habitats instead of human warehouses.

The people designing the water future for California’s South Coast cities are doing an impressive job, but they might wish to consider the positive externalities of surplus water. What if Los Angeles County planned to increase their planned water supply capacity by 17 percent, instead of planning to reduce it by 17 percent? Both outcomes are well within the scope of feasibility, even if the more generous choice might cost more. But what is it worth, for example, to continue to use recycled wastewater to guarantee perennial flow in the Glendale Narrows?

It’s not just the kayakers who benefit. It’s the sightseeing public, the diners on the overlooking terraces, the grateful residents, the lucrative revitalized local culture. What is the economic and human benefit of creating a plethora of urban/wildland interfaces deep inside a city, using entirely artificial sources of water? How much water is actually lost, if after traversing the Narrows, this water finds its way back into spreading basins that double as birdwatching habitat? What if, in a prodigiously extravagant gesture, the entire Los Angeles River were to achieve perennial flow, all the way to the ocean, all year around?

Many of California’s most cherished natural assets are artifacts of human intervention. These managed gems can be enhanced, and new ones can be created, without attenuating the amenities that make life pleasant. It is not a zero sum game. It just depends on how you invest the money, how you design your monumental plumbing, how you harvest the pouring rain.

Want more? Get stories like this delivered straight to your inbox.

Thank you, we'll keep you informed!