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Can California’s floods help recharge depleted groundwater supplies?

The drenching storms that hit California in recent weeks represented a long-sought opportunity for Helen Dahlke, a groundwater hydrologist at the University of California, Davis. Dahlke has been studying ways to recharge the state’s severely depleted groundwater by diverting swollen rivers into orchards and fields and letting the water seep deep into aquifers. But carrying out such plans requires heavy precipitation—which had been scarce.

This week, however, water managers began to turn theory into practice. In the Tulare Irrigation District, which supplies water to more than 200 farms south of Fresno, officials started diverting water from the San Joaquin River into 70 fields as well as specially constructed ponds. Each day, some 1.5 million cubic meters of water—roughly equivalent to 600 Olympic-size swimming pools—has been pouring onto the landscape. “We are in full [groundwater] recharge mode,” Aaron Fukuda, the district’s general manager, wrote in an email. Similar flooding is underway in the Madera Irrigation District north of Fresno.

Over the past decade, Dahlke’s experiments with submerging small plots have suggested intentional flooding can replenish aquifers without damaging either groundwater quality or crops. But she says bureaucratic hurdles and organizational inertia have blocked widespread use of the practice—despite state laws and policies designed to encourage it.

“My frustration is growing!” Dahlke says. “This always looks so easy when you write these scientific papers, and give presentations, but to really implement [flooding] on a widespread scale is very hard.” She and others hope this winter’s floods will encourage more of the state’s water managers to embrace the practice.

California’s farmers and others often extract far more water from aquifers than normally seeps in from the surface. The idea of using working farms to slow or reverse the trend was born in 2010, when independent hydrologist Philip Bachand and farmer Don Cameron flooded some of Cameron’s vineyards. The vines thrived, and the water replenished the aquifers beneath Cameron’s land.

Four years later, California adopted a landmark law, the Sustainable Groundwater Management Act (SGMA), that promotes the practice. It requires farmers to treat aquifers like bank accounts, clamping down on overdrafts but also allowing those who deposit water into them to make bigger withdrawals later.

The most catastrophically depleted aquifers lie in the San Joaquin Valley, the nation’s largest single source of tree nuts, fruit, and vegetables. In places, groundwater extraction has caused the land to sink by several meters, and declining runoff from the Sierra Nevada means growers can no longer depend on a steady supply of river water. In this region, Dahlke says, capturing water during wet years and storing it underground for later use will be a matter of survival. The looming shortage “is just getting really scary,” she says.

But several obstacles have stood in the way of recharge projects, experts say. Some districts need state permits and getting them is time-consuming. The SGMA’s limits on extraction are only kicking in now, so farmers haven’t had much incentive to spend the money required to flood their fields. “If you’ve chosen to somewhat ignore this law, you’ve been able to,” says Sarah Woolf, a water consultant and farmer.

Still, the recent floods are prompting new interest. In the Madera Irrigation District, General Manager Thomas Greci says farmers seem increasingly open to drenching their fields. “I have been shocked to see the number of growers coming in to sign up to take this water,” he says. And other irrigation districts have been calling, asking how it’s done, says Dina Nolan, the district’s assistant general manager. “It was, frankly, quite shocking to me,” she says. “I was like, ‘You’ve never promoted this?’”

California’s high waters are now receding, but the opportunity to capture runoff will likely continue through the spring as a hefty mountain snowpack melts. Many farmers, however, won’t be inclined to drown their fields when it’s time to plant or pollinate their crops. “Only certain crops are compatible [with flooding] at that time of year,” says Daniel Mountjoy, director of resource stewardship with the nonprofit Sustainable Conservation.

When all is said and done, Dahlke estimates this year’s intentional floods will counterbalance less than 10% of the San Joaquin Valley’s typical annual groundwater deficit. But she hopes the experience will prepare the state to do better when the next deluge arrives. With that in mind, she’s hoping to soon launch a study aimed at identifying easier ways of using California’s extensive irrigation infrastructure to steer 1 trillion liters of floodwater into the state’s aquifers. The goal, she says, is “to go more large-scale.”


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