Can California floods help recharge depleted groundwater supplies?  |  Science

Can California floods help recharge depleted groundwater supplies? | Science

The torrential storms that have hit California in recent weeks presented 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 rainfall, which had been scant.

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 began diverting water from the San Joaquin River to 70 fields as well as specially constructed ponds. Every day, around 1.5 million cubic meters of water is dumped onto the landscape, roughly the equivalent of 600 Olympic-sized swimming pools. “We are in full [groundwater] recharge mode,” Aaron Fukuda, the district general manager, wrote in an email. Similar flooding is occurring in the Madera Irrigation District north of Fresno.

Over the past decade, Dahlke’s experiments submerging small plots have suggested that intentional flooding can replenish aquifers without harming groundwater quality or crops. But she says bureaucratic roadblocks 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 seems so easy when you are writing scientific papers and giving presentations, but to actually implement [flooding] on a large scale it is very difficult.” She and others hope this winter’s flooding will encourage more of the state’s water managers to adopt the practice.

California farmers and others often draw much more water from aquifers than normally percolates 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 below the Cameron land.

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

The most catastrophically depleted aquifers are in the San Joaquin Valley, the nation’s largest single source of nuts, fruits, and vegetables. In some places, groundwater extraction has caused the land to sink several meters, and diminishing runoff from the Sierra Nevada means producers can no longer rely on a constant 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 getting really scary,” she says.

But several obstacles have stood in the way of recharging projects, experts say. Some districts need state permits and obtaining them takes a long time. SGMA extraction limits are barely being enforced now, so farmers haven’t had much incentive to spend the money needed to flood their fields. “If you’ve chosen to ignore this law for a bit, you’ve been able to,” says Sarah Woolf, a water consultant and farmer.

Still, the recent floods are generating new interest. In the Madera Irrigation District, General Manager Thomas Greci says farmers seem increasingly open to wetting their fields. “I was surprised to see how many growers signed up to drink this water,” he says. And other irrigation districts have been calling to ask how it’s done, says Dina Nolan, the district’s assistant general manager. “Frankly, it was quite a shock to me,” she says. “I was like, ‘Have you never promoted this?'”

California high waters are now receding, but the opportunity to capture runoff is likely to continue through the spring as heavy snowpack melts on the mountains. Many farmers, however, will not be inclined to drown their fields when it comes 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 management for the nonprofit organization Sustainable Conservation.

When all is said and done, Dahlke estimates that intentional flooding this year will offset 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 comes. With that in mind, she hopes to soon launch a study aimed at identifying easier ways to use California’s extensive irrigation infrastructure to direct trillions of gallons of floodwater into the state’s aquifers. The goal, she says, is to “go bigger.”

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