Droughts exposed California’s thirst for groundwater. Now, the state hopes to refill its aquifers

California’s Central Valley—one of the richest agricultural regions in the world—is sinking. During a recent intense drought, from 2012 to 2016, parts of the valley sank as much as 60 centimeters per year. “It isn’t like an earthquake; it doesn’t happen, boom,” says Claudia Faunt, a hydrologist with the U.S. Geological Survey. But it is evidence of a slow-motion disaster, the result of the region’s insatiable thirst for groundwater.

For decades, farmers have relentlessly pumped groundwater to irrigate their crops, draining thick, water-bearing clay layers deep underground. As the clays compress, roads, bridges, and irrigation canals have cracked, causing extensive and expensive damage. In 2014, when NASA scientists flew radar equipment over the California Aqueduct, a critical piece of water infrastructure, they found that one section had dipped 20 centimeters over 4 months. Such sagging can leave canals carrying less water—an “ultimate irony,” says Graham Fogg, a hydrogeologist at the University of California (UC), Davis, because they were built in part to slacken demand for groundwater. Excessive pumping also jeopardizes water quality, as pollutants accumulate within groundwater and the clays release arsenic. Worst of all, the persistent pumping means that, one day, aquifers might run out of usable water. “If you pump too hard,” Fogg says, “you’re playing with fire.”

Now, California has launched a landmark effort to save its groundwater. In 2014, deep in drought, the state passed a law to protect its aquifers; since then, local water managers have developed sustainability plans for those deemed the most imperiled. The plans for some particularly hard hit regions, just released for public comment, call for ending the groundwater deficit mainly by allowing precipitation to refill aquifers, but also by curtailing demand. The state is funding scientists to gather better data on the crisis; researchers estimate that in the Central Valley, half of the aquifers are dangerously depleted, but they don’t know the extent of the damage. Meanwhile, geologists are working to identify the best places to replenish aquifers by flooding farm fields, including some with especially permeable geology.

Groundwater science is taking on a new urgency as California and other regions around the world face growing threats from drought—and are increasingly drilling wells to make up for missing rain and snow. Globally, aquifers are “highly stressed” in 17 countries that hold one-quarter of the world’s population, according to the World Resources Institute. Water and food supplies for billions of people are under threat.

California is a case study in the challenges of protecting those resources. Farm interests, which use the most groundwater, often resist limiting withdrawals, whereas environmentalists demand more water be returned to rivers and the Sacramento-San Joaquin delta; the first lawsuit challenging California’s sustainability plans was filed last month. Demand for groundwater is growing where farms have expanded into areas with little surface water. Across the state, climate change is making precipitation less reliable. “A lot of people are looking to California to see how the law plays out,” says Ellen Hanak of the Public Policy Institute of California (PPIC). The hope, she adds, is “there’s just so much local innovation in California that it can be a model for folks elsewhere.”

California once served as a global model for another type of innovation: massive water projects. Los Angeles and other cities clamored for more water than they could get locally. The San Joaquin Valley in the southern Central Valley, the state’s largest and most lucrative agricultural zone, had fertile soil and plenty of sunshine, but never enough water. Farmers had to make do with what nature provided—and what they could pump from the ground.

In the 1930s, the federal government began to build a network of dams, pipelines, and canals that moved water from the state’s wetter north to farms in its semiarid south. Local projects sent water to urban centers. With the taps turned on, California’s farms and cities flourished.

But the imported water didn’t relieve the pressure on groundwater for long. Thanks to rural electrification, more farmers could pump as much as they wanted. There were no regulations, no limits. And pumps have become ever more powerful, with the best able to guzzle up to 5000 liters per minute from aquifers. Now, in a wet year, about 40% of the water used in the state comes out of the ground; during a drought, the proportion swells to 60%. In some farming areas, the dependence is even greater during dry years (see map).

California drying NASA’s GRACE satellites detect the gravitational pull of water masses in aquifers, reservoirs, and snowpack. In 2014, GRACE data showing water loss (below, red indicates loss) helped dramatize the draining of aquifers and galvanize state lawmakers to protect groundwater. version="1.0" encoding="utf-8"? 2002 2005 2008 2011 2014 2017 Change in groundwater storage (km³) 8 6 4 2 0 0 –2 –4 –6 –8 –10 –12 0 13 Wetter conditions Groundwater used in 2014 (km³) Groundwater as % of total water used Wet year 2011 Drought 2014 Drier conditions 29% 38% 18% 74% 30% 11% 12% 65% 8% 20% 90% 39% 19% 21% 75% 5% 24% 25% 54% 81% San Joaquin Valley region San Joaquin Valley region Tallying groundwater losses A thirsty valley Subtraction outraces addition California’s north receives abundant precipitation, so it relies less on groundwater during droughts than the drier farmland of the San Joaquin Valley to the south (left). In that valley, pumping has taken increasing amounts of groundwater (right), with withdrawals during dry years exceeding replenishment during wet years. June 2002 Sierra Nevada San Francisco Los Angeles June 2008 June 2014

Rates of groundwater extraction are unsustainable in many parts of the state, says Jay Famiglietti, a hydrologist at the University of Saskatchewan. During wet years, enough water from rain and gushing streams sinks into the ground to partially refill aquifers, he says, but levels can fall even lower during the next drought. “It’s like a tennis ball bouncing down the stairs, it’s just going in one direction,” Famiglietti says.

The trend became especially worrisome during the 2012–16 drought. In the San Joaquin Valley, deep irrigation wells lowered groundwater levels—already 250 meters below the surface in places—putting it out of reach of shallower wells that provided thousands of people with drinking water. Elsewhere, environmental groups feared that springs, streams, and rivers would run dry as groundwater levels fell.

In response, state legislators introduced proposals to regulate groundwater withdrawals. The bills were fiercely opposed by farm groups, which worried about declining land values. But the push gained momentum from new satellite radar images that dramatically depicted the state’s subsidence problems. “The images really drew attention to a system that’s out of balance,” says Rosemary Knight, a geophysicist at Stanford University.

Lawmakers were also alarmed by images of water loss (above) from NASA’s Gravity Recovery and Climate Experiment (GRACE), which surveys surface and groundwater by measuring how its mass tugs on a pair of satellites. GRACE measurements, combined with other data, indicated that in 2010 Central Valley aquifers held 20 cubic kilometers less water than they had in 2003.

The Sustainable Groundwater Management Act, which became law in September 2014, was “an incredible step” for a state that had long resisted groundwater regulation, Famiglietti says. But it only requires California’s some 260 groundwater sustainability agencies (new organizations set up under the law, often made up of local water districts) to stabilize, not to increase, groundwater levels. And it allows increased pumping if needed during drought, as long as no major problems result. Still, the law has forced a statewide rethink of groundwater policies. In January, the new agencies in 21 basins deemed critically overdrawn had to submit plans for achieving groundwater “sustainability” within 20 years. (Other agencies must submit their plans by 2022.)

The push to develop the plans has, in places, revealed an astounding lack of data. Many districts, for instance, aren’t sure how much water is being removed from the ground because California doesn’t require all pumps to have meters. (Local rules or court orders require metering in some basins to help resolve disputes.) In the absence of hard data, researchers have for years estimated flows by examining electricity records—groundwater pumps are energy hogs—and by mapping the extent and types of irrigated crops. Information on subsidence is also helpful. “It’s pretty amazing,” says hydrogeologist Andrew Fisher of UC Santa Cruz. “We’re in a position now of not knowing what a lot of the big groundwater flows are or how they vary.”

Reducing pressure on groundwater isn’t easy or quick. One obvious tactic is to reduce demand. Some parts of California have lessened their reliance on groundwater by incentivizing efficiency and imposing requirements such as water-saving showerheads and toilets. Planting water-efficient crops helps—grapes and young almond trees use much less water than alfalfa, for example. So does leaving fields fallow, a strategy farmers have used to cope with past droughts.

But for some parts of California, such measures aren’t practical, in part because of a massive expansion of profitable vineyards and orchards—tree nut acreage alone increased 85% between 2008 and 2018. The groves and vineyards cannot be fallowed like other fields, although they can survive with less water than normal. And farmers are reluctant to rip them out, because they are expensive to plant, can take years to mature, and have relatively long life spans.

Still, researchers say truly protecting groundwater in California will require cutbacks in agriculture, which on average makes up about 80% of commercial and residential consumption. To stabilize groundwater in the San Joaquin Valley, farmers will likely have to reduce irrigated cropland by more than 200,000 hectares, or 10%, according to a 2019 report from PPIC. Not surprisingly, such prospects worry farmers across the state, says Chris Scheuring, a water lawyer with the California Farm Bureau Federation. “We are absolutely hoping for mitigated outcomes that get us to sustainable management without causing a lot of pain.”

To slow the rate of depletion with less pain, a few districts are counting on proven methods for recharging aquifers. For decades, some water districts have filled dedicated ponds in wet years so that the water percolates into the ground. Others flood farm fields when water is plentiful. Vineyards can tolerate spring flooding, and some crops, like alfalfa, do well with flood irrigation. Building culverts and berms to move and hold the water can be expensive, however. In urban areas, where land is scarce or the upper layers of sediment or rock aren’t very permeable, officials pump water into the ground instead of removing it.

To expand such practices, researchers have been searching for areas ripe for recharge, based on factors such as soil type, land use, and aquifer geology. A UC Davis team identified 1.5 million promising hectares by reviewing existing data, they reported in California Agriculture in 2015. Some of the best places are valleys, now buried, that once existed in the Central Valley and were filled with coarse sediments during the last ice age. These sweet spots may be able to drain 60 times as much water as average sites, Fogg says. Researchers have discovered just three of these buried valleys, but Fogg says many others must exist given the region’s geological history.

Knight is using geophysical techniques to find such promising recharge areas. A helicopter-mounted instrument sends electromagnetic signals into the ground, measuring the electrical properties of buried sediment to create 3D maps of geologic formations that are as much as 300 meters deep. After that, smaller devices can be towed through fields or orchards for higher resolution images. The maps can help managers identify areas where water will quickly soak in—avoiding ponding that can lead to crop diseases or undermine trees.

The maps also show where water is most likely to reach deep layers where pumping is causing subsidence. “The level of complexity that we’re capturing is amazing,” Knight says. California’s water resources agency recently committed $12 million to using the helicopter-mounted system in groundwater basins throughout the state.

Restoring groundwater could become even more important because of climate change. The state has long relied on abundant mountain snow to provide a reliable, year-round source of surface water. Its many reservoirs were designed to fill with snowmelt by July, and then release the water to satisfy the peak demand during the hot summer. But because of a warming trend, the annual snowpack is already becoming thinner and melting sooner. And climate scientists predict more and more precipitation in California will fall as rain, rather than as snow in the mountains. All that means reservoirs will fill sooner and water will have to be released earlier in the spring, before it’s needed. In the summer, farmers would likely have to rely even more heavily on groundwater.

To adapt to that future, officials are pondering a new arrangement in which dam operators would release water ahead of rainstorms. That would make room for the storm water, and the discharge would allow downstream sites to put more into the ground. The idea sounds simple, but involves significant changes in regulations, operations, and in some cases infrastructure, Fogg says. Still, several pilot projects are underway. In the American River watershed, a flood control agency wants to retrofit some upper reservoirs. If the strategy is implemented there and in an adjacent Sierra Nevada river basin, Fogg and colleagues estimate about one-third of a cubic kilometer of water could be stored underground each year. That’s 10% to 25% of the annual statewide deficit, Fogg says.

Recharge water that comes from mountain reservoirs often has high quality. But a different source, stormwater runoff from urban or managed landscapes, could pose a problem: preventing contaminants—including farm fertilizers—from seeping into groundwater. Fisher has been studying ways to remove certain contaminants by adding biomatter such as wood mulch and almond shells to the soil at recharge sites. His team has found that the materials can promote the growth of microbes that remove nitrate, a common pollutant. “If we’re going to be putting hundreds of thousands or millions of acre feet of water in the ground every year, we should be taking every opportunity to make that water cleaner on the way in,” Fisher says.

Recharge isn’t the whole solution. In the San Joaquin Valley, scientists estimate recharge alone can eliminate, at best, just 25% of the groundwater deficit—in part because there is so little surface water to begin with in the region. So, any additional savings will likely have to come from reduced pumping, with its political challenges, as well as shifting water to the most productive croplands while leaving others uncultivated. That will require new canals and other infrastructure, and a new level of coordination. Across the state, multiple government and private entities will need to work together on managing supply and demand at the scale of entire basins, in order to minimize the economic cost of using less water. “There has to be policy innovation or financial innovation to get people to move away from this myth that we still have an unlimited groundwater supply and that we’re just never going to hit bottom,” Famiglietti says.

Bridget Scanlon, a hydrologist at the University of Texas, Austin, is optimistic that innovation will occur. “California has opportunities to move towards more sustainable management, and I think they are,” she says. Fogg is hopeful, too, but adds a cautious note. “Civilization has never been very successful at controlling water demand,” he notes.

Luckily, California’s recent winters have provided enough precipitation to allow aquifers to recover a bit. The state may not find out whether it learned the lessons of the last drought until the next one.