LOS ANGELES — For the first time, scientists have found a direct link between the snow that falls on California’s mountains and the groundwater that lies beneath Central Valley farmlands.

Analyzing two decades of measurements from NASA satellites, the researchers discovered that after a snowy winter in the Sierra Nevada, the melting snow that runs off the mountains substantially raises water levels underground.

The study documents just how important years of heavy snow are for recharging California’s groundwater aquifers. It also reveals that a year with rain but little snow doesn’t benefit the state’s underground water nearly as much.

“It really does seem that the snow, rather than the rain, is what drives the groundwater recharge,” said J.T. Reager, the study’s lead author. “If it rains, a lot of that rain is going to run right off, whereas if it snows, that snow sits there and slowly melts over the entire spring and summer, and provides this nice steady, infiltrating trickle that provides water to farmers, puts more water in reservoirs and also recharges the ground.”

In the Central Valley, groundwater has been dropping for decades as wells and pumps pull up water to irrigate crops. Scientists estimate that since the early 20th century, the Central Valley has lost more groundwater than the total volume of Lake Tahoe.

The study backs up something farmers have noticed for years: that the water in their wells sometimes rises in the months after heavy snows in the mountains.

Reager, a water scientist at NASA Jet Propulsion Laboratory, worked with colleagues to analyze satellite data and estimate how much recharge has occurred annually in California since 2004. They found that the state’s aquifers have had a net gain in only six of 18 years, and years with lots of snow led to the biggest rises in underground water.

A prime example was 2023, when a series of atmospheric rivers brought California one of the largest accumulations of snow ever recorded.

As the deep snow melted, runoff flowed into reservoirs and canals, even inundating roads and fields. It enabled farmers to turn off their wells, and as snowmelt spread across the land, it soaked underground.

According to the study, 2023 brought by far the biggest rise in the state’s aquifers of the last two decades — enough to recoup the groundwater lost to overpumping during the severe drought of 2020-22, but still only a fraction of the water depleted over decades.

The researchers found that much less water made its way underground in years when the precipitation fell as rain rather than snow, such as 2004 and 2010. In most years, heavy pumping led to declines in groundwater.

“It’s still coming down,” Reager said. “It just doesn’t naturally recharge the amount that we take out.”

The study, published Thursday in the journal Geophysical Research Letters, says the only other years of groundwater gain in California were 2005, 2006, 2011, 2016 and 2019.

The researchers examined measurements from two U.S.-German satellite missions, called Gravity Recovery and Climate Experiment (GRACE) and GRACE-Follow On.

Other research based on the satellite data has shown that large areas of the world are getting drier and losing fresh water, and the Central Valley is one of the regions where depletion is happening the fastest.

In an effort to curb the worsening decline and prevent more wells from running dry, California legislators in 2014 passed the Sustainable Groundwater Management Act.

New local agencies became responsible for halting chronic groundwater declines. Many of them are planning to build recharge sites where they can bring in water by canal and let it sink underground.

Other research has shown, however, that these efforts probably can offset only a small fraction of the unsustainable extraction in the valley.

Researchers at the Public Policy Institute of California have estimated that as much as 900,000 acres of farmland in the San Joaquin Valley may need to be left dry by 2040.

“Measuring groundwater recharge over large areas is extremely difficult,” said Jay Famiglietti, a hydrologist and professor at Arizona State University who co-authored the study. The satellite data enabled a month-by-month analysis, he said, that can be important for tracking progress under the state’s groundwater law.

The researchers said the data show that an average snow year isn’t enough to raise aquifer levels. They calculated that a net gain in underground water occurred only in years when the peak snowpack reached 120% of average.

With climate change, droughts are getting more severe, winters are getting warmer, and average snow lines have moved higher in the mountains.

The researchers said as warming winters bring more precipitation as rain and less as snow, that could hinder efforts to replenish groundwater.

“We’re really going to lose these recharge-capable years,” Reager said. “That’s kind of a red flag for the future.”

Christine Gemperle, an almond farmer near Modesto, said she’s hoping the strong El Niño might bring enough runoff to help replenish the groundwater in her area. She has seen water levels come up in wells after snowy winters. She has set aside some of her land where she uprooted an old almond orchard to take in any excess water and help recharge the aquifer.

“I want to be able to put back in more, or at least the same amount that I’ve taken out,” she said.

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