Harvesting Southern California rain
can reinvigorate its cities
By Edward Ring | July 27, 2023
Anyone who has experienced traffic on the Santa Monica Freeway coming to a halt as torrential rain floods the lanes, pelting the windshield with drops so big and so plentiful that visibility is reduced to a few feet, knows the truth of Albert Hammond’s iconic hit. It doesn’t rain very often along the Southern California coast, but when those storms do come in, some of them are whoppers.
Nowadays, of course, we call them atmospheric rivers, an apt description. They originate in the tropical Pacific, where the warm ocean evaporates stupefying amounts of moisture. When a strong fast wind hits that airborne moisture just right, a few days later in faraway Los Angeles it will pour. And man, it pours. According to National Geographic, the average atmospheric river is about 500 miles wide and 1,200 miles long.
Some are much bigger, such as in the very wet winter of 2017, when an atmospheric river over 5,000 miles long dumped enough water to fill Lake Oroville in northern California to overflowing. The neglected spillway couldn’t handle the torrent that year and began to come apart, triggering the evacuation of 180,000 people.
In Los Angeles, where, in case you’ve been living on Mars, it poured like crazy this past winter, the phenomenon of atmospheric rivers is compounded by the tight geography of the watershed. From the 9,000 foot crest of the San Gabriel Mountains, an immovable object where rivers of rain make landfall, it is only 30 miles to the Long Beach Harbor, where the Los Angeles River meets the sea. There’s a reason the Los Angeles River was turned into a gigantic concrete culvert in a series of projects started in the 1930s and completed by 1960. Without that unobstructed channel, atmospheric rivers would routinely turn the City of Angels into a swamp.
Intermittent but ridiculously intense episodes of rain are California’s opportunity and curse. The curse is obvious. Even in California’s vast interior, where incoming storms hit the Sierra Nevada Mountains and are dispersed over a crest 430 miles long and descend through thousands of square miles of watershed, there are still disasters like the catastrophic levee break at Jones Island in 2004, or the near disaster at Lake Oroville in 2017. But in the packed coastal cities, there’s no margin for error.
The opportunity, however, is tantalizing. If Californians could somehow capture all this runoff, there would be abundant water in a state that has coped with chronic water shortages for several decades. A 2022 study by the Pacific Institute evaluated the opportunity to harvest storm runoff in California’s coastal cities. The authors 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.” The Pacific Institute based their estimates on the average amount of rainfall hitting California’s urban areas. But can engineers design systems to capture whatever the skies deliver?
Steve Sheldon, a director and former president of the Orange County Water District, used a metaphor to describe the challenge. “You can’t build a freeway with so many lanes that it is smooth flowing at 5 p.m. every day during rush hour,” he said, “it would be too big the rest of the time.” John Kennedy, OCWD’s executive director of engineering and water resources, was more explicit, saying “we would have to build billions of dollars of facilities that would only be used in very wet years.”
Kennedy emphasized how stormwater capture is very specific to each region, and in that regard, OCWD’s service area in the northern half of Orange County is fortunate. In an average year, they capture about 75,000 acre feet of baseflow from the Santa Ana River, in addition to harvesting another 55,000 acre feet of storm runoff.
So-called incidental percolation from rain contributes 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, less than 20 percent.
Orange County also has the benefit of uncontaminated aquifers with an estimated storage capacity in excess of 60 million acre feet. The county is able to divert storm water into 1,500 acres of OCWD owned percolation ponds, where up to 2,500 acre feet per day will settle into underground aquifers. During heavy storm runoff this is only about 25 percent of what’s coming down the river, but it’s unlikely OCWD will be investing in more land for percolation ponds, considering they operate in some of the most densely populated, expensive real estate on Earth.
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 importation 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, northern Orange County would be completely independent of imported water.
For an urban area on the relatively arid Southern California coast to achieve water independence is an extraordinary feat. 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 systematically pursuing many of the same strategies as Orange County, but on a much larger scale. On average the county has successfully harvested 200,000 acre feet per year of stormwater, about 15 percent of the total demand. In this most recent and rather extraordinary 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.
The number of ways to increase the percentage of permeable surfaces in a city as big as Los Angeles is only limited by one’s imagination. Solid concrete driveways can be replaced with a durable combination of gravel and pavers. Underground culverts, surreptitious tributaries feeding the LA River can be “daylighted” and lined with plants that filter contaminants at the same time as the original pipe or concrete is replaced with gravel, allowing for percolation as well as runoff along the entire length.
Cisterns with permeable bottoms can be buried underground in parks. Fed by storm drains, they can fill up with storm runoff, then slowly empty as the water percolates. Along major watercourses, treatment wetlands and recharge basins can be integrated into neighborhood and regional parks.
It isn’t clear, even with all of this, whether or not Los Angeles can ever harvest all of its storm runoff, or, more to the point, can ever become water independent without relying on a combination of imported water and desalination. But over the next few decades, 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. Bruce Reznik, executive director of the influential advocacy group LA Waterkeeper, claims additional conservation measures could bring total demand down to 1.12 million acre feet per year, a drop of another 17 percent. But where will the water come from?
Today, Los Angeles County already 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 to eventually average 270,000 acre feet per year. While most all of this stormwater is stored in aquifers, the county currently withdraws an additional 270,000 acre feet per year from aquifers through existing 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 well under 100,000 acre feet per year.
What California’s south coast cities are doing to achieve water independence is impressive, and stormwater capture is on track to become the primary source. In Orange County, between base-flow capture, storm-water capture and capture through natural percolation, 49 percent of their water comes from local rain. That total is estimated to increase to 54 percent. In Los Angeles County, harvesting local rain currently supplies 30 percent of their total demand, with that increasing to 48 percent when planned projects are completed.
There are obvious, life affirming synergies that come with many stormwater-harvesting projects. In South Los Angeles, the Franklin D. Roosevelt Park has a new turf soccer field that covers a filtration system that is fed by storm drains and directs water into an underground aquifer instead of straight to the river. Bringing back actual living grass playing fields – which percolate – instead of yet another toxic, fuming, volatilizing, plasticine heat-island generating outdoor rug, is a tremendous example of how retaining water and rewilding urban spaces are mutually reinforcing benefits.
These externalities point to a deeper question. When measured purely according to the value of the additional water captured, investments in stormwater harvesting quickly reach a point where, as Kennedy at OCWD put it, “the juice isn’t worth the squeeze.” But what if those investments are bringing the additional benefits of living systems that make a city a healthier and more alluring place for people and wildlife?
Positive externalities generate intriguing cost-benefit equations. 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? Notwithstanding how in Los Angeles those roofs would compete with helipads, swimming pools and the now ubiquitous solar panels, is the juice worth the squeeze?
In a provocative interview published in 2022 by the Yale School of the Environment, urban ecologist Eric Sanderson describes ways to “weave nature back into the urban fabric.” Sanderson’s ideas may be on the fringe of what is practical, but his concept is sound. Rewilding a city can introduce resiliency from storms and droughts, 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 water supply 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.