Ringside: Harvesting Urban Storm Runoff

via californiaglobe.com

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.