Can California Better Use Winter Storms to Refill its Aquifers?

With new rules coming into effect, farmers and municipalities using groundwater must either find more water to support the aquifers or take cropland out of use. To ease the pain, engineers are looking to harness an unconventional and unwieldy source of water: the torrential storms that sometimes blast across the Pacific Ocean and soak California.

The general long-term forecast for California as climate change intensifies: more frequent droughts, intermittently interrupted by years when big storms bring rain more quickly than the water infrastructure can handle.

This bipolar weather will have profound implications for the state’s $50 billion agriculture industry and the elaborate network of reservoirs, canals, and aqueducts that store and distribute water. A system built for irrigation and flood protection must adapt to accommodate more conservation. “The effects of climate change are necessitating wholesale changes in how water is managed in California,” according to the state Department of Water Resources.

During droughts, farmers and municipalities pumped groundwater to augment sparse surface supplies. After nearly a century of heavy use, many aquifers are badly depleted. And California’s five-year-old law regulating groundwater basins is nearing a moment of truth: in 2020, sustainability plans are due from agencies managing critically depleted aquifers. They must either find more water to support the aquifers or take cropland out of use. The law – the Sustainable Groundwater Management Act, or SGMA – is beginning to bite. A 2019 study from the Public Policy Institute of California predicted that at least 500,000 acres of farmland will eventually be idled.

To ease the pain, engineers are looking to harness an unconventional and unwieldy source of water: the torrential storms that sometimes blast across the Pacific Ocean and soak California. “We need to capture the atmospheric rivers,” said Tim Quinn, the former director of the Association of California Water Agencies. But, he added, “We don’t have the right conveyance facilities. There’s not enough storage to capture the atmospheric rivers.” Quinn wants to capture stormwater and divert it to aquatic “parking lots,” as he calls them, where water can trickle back into depleted aquifers. This serves several ends: restoring a depleted natural resource, reducing land subsidence and providing for future water supply.

For agencies or individuals seeking to create new recharge basins, a new Stanford study offers a tool. The study, done by Rosemary Knight, an Earth Sciences professor, and a former Ph.D. student Ryan Smith — now an assistant professor at the Missouri University of Science and Technology — can help those building recharge ponds avoid the problems that plague the delivery system of the Friant-Kern Canal. Over-pumping made the land below it subside, torquing the structure and reducing its capacity. The study shows how a combination of satellite and aerial remote sensing, which map the underground sand and clay layers, can be used to predict how recharge can reduce subsidence. Combining the data sources allows creation of maps of the link between recharge — or continued groundwater pumping — and subsidence.

Recharge operations are nothing new; dedicated basins have dotted the San Joaquin Valley and the Sacramento Valley for years. Kern County’s Rosedale-Rio Bravo Water Storage District dates to 1959. Such operations helped raise the total of water recharged for the San Joaquin Valley to 6.5 million acre-feet in 2017 — a year, like the current one, soaked by atmospheric rivers. But the total amount of inflow to surface waters was closer to 30 million acre-feet, PPIC reported. While the volume of rain and the volume of recharge were exceptional, researchers say more recharge was possible. An annual average of about 500,000 acre-feet of recharge potential is available – about a quarter of the average annual groundwater deficit from over-pumping.

One way to capture more of the atmospheric rivers is allowing dams to release reservoir water in anticipation of a storm. The Army Corps of Engineers, which operates the reservoirs, has strict rules on how much water must be held and when it can be released. Now the Corps is working with tools developed by the Scripps Institute of Oceanography at the University of California at San Diego that facilitate what are called Forecast-Informed Reservoir Operations.

Across California’s farm country, the hard reality of a future without guaranteed access to groundwater is sinking in. Jerry Gragnani, a neighbor of Don Cameron’s who is 70 years old, recently sold 8,000 acres of his farm, keeping 3,000 acres for permanent crops like nut trees. “There’s no future without recharge,” he said. “I don’t think there’s a farmer around here that doesn’t realize that. We know if you don’t recharge the water, it’s going to run out. Once the aquifer is closed, it’s closed forever.” He added, “It’s a good time to get out.”

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