The ground is sinking in parts of California as the continued drought strains reservoirs, increasing reliance on the state’s already precarious groundwater reserves depleted by years of well-pumping.

In just one year, from October 2020 to September 2021, satellite-based estimates showed entire towns in the Central Valley, including in Kings and Tulare counties, sinking by nearly a foot. The maximum loss recorded during that time was 1.1 feet on the northwestern edge of Tulare County.

The sinking, known as land subsidence, happens when excessive pumping dries out the water reserves underground and collapses the space where water used to be. Experts say it’s a century-old problem in California that regulators have tried to slow with sustainability measures. But with the changing climate, they face an uphill battle.

“It’s a latent issue that’s been building over a long time, and we’re kind of seeing a lot of fallout from that,” said Andrew Ayres, an environmental resources and economics researcher at the Public Policy Institute of California, a nonpartisan think tank.

Top of story: The San Joaquin Valley has experienced some of the worst land subsidence in the state. Above: The Delta Mendota canal has been awarded more than $3 million for repairs.

Land subsidence threatens infrastructure, including roads and canals, he said. Damage from it led to a $3.3 million repair project at the Delta Mendota canal in San Joaquin Valley, which delivers water to 1.2 million acres of farmland and 2 million people in the region. The repair project, the funding for which was announced in April, is part of a larger effort by the state water department to address deficiencies in California’s water conveyance systems.

“As we pump groundwater out of the aquifer, the water exists in these spaces between various layers and pieces of rock,” Ayres said. “If you pump out enough water, those places will get compressed and this leads to a loss in long-term storage.”

Even if an aquifer is recharged with rain or by other means, it won’t be able to hold as much water as it used to, Ayres added. As aquifer levels drop, the process of pulling water from them becomes more difficult and costly.

“It might be impossible to access any remaining groundwater supplies,” Ayres added.

The problem existed long before major infrastructure and sustainability requirements were put in place. The U.S. Geological Survey says that between the 1920s and 1970s, significant land subsidence occurred in about half of the San Joaquin Valley, or about 5,200 square miles, with some areas subsiding by as much as 28 feet.

The continued depletion of groundwater reserves, especially in drought years, is worrisome because of groundwater’s critical role as a buffer when there’s little rain or snowpack to replenish the state’s many surface water resources, like reservoirs.

“In drought years, (groundwater) can make up to 60% of the state’s water supply,” said Steven Springhorn, a supervising engineering geologist for the water department. That compares with about 40% in non-drought years.

Ross Franson at a flooding area near his groundwater recharge basin. Excessive pumping is drying out reserves underground.

When major surface water sources, such as the State Water Project, can’t deliver enough water during drought, local agencies must find alternative sources, like pumping it from the ground or purchasing it. The State Water Project is a massive system of dams and canals (similar to the Central Valley Project). It delivers water to about 27 million people, including farmers and city dwellers.

The State Water Project announced it expects to provide just 5% of the water requested by contractors in the coming year.

“This year, we don’t have any surface water to provide to our growers,” said Kristin Sicke, general manager of the Yolo County Flood Control and Water Conservation District, which provides irrigation water to farmers in western Yolo County, as well as delivering water to dozens of smaller municipal and industrial customers.

But the groundwater situation is also dire. Sicke said the district is anticipating record-low groundwater levels this year — beyond the historical low point set during the 1976-77 drought, she said.

Many wells, not just in western Yolo County but across the state, are reporting similarly grim groundwater levels. As of early May, more than 60% of wells in California that reported data within the past year indicated below- normal levels of water, data shows.

“This is a problem, especially for rural communities that tend not to have very deep wells,” said Ayres, of the policy research group. “It’s also a problem for ag users who, you know, maybe drilled a well 15 years ago when groundwater tables were a lot higher than they are today.”

There was heavier reliance on pumping in the past before key sustainability measures, such as the state’s Sustainable Groundwater Management Act of 2015, created tighter regulations around groundwater, he said. Much of the most persistent overdraft happened then, and the long-term consequences will continue to play out.

In recent years, many farmers have been choosing to fallow ground without planting instead of resorting to pumping from wells, he said. “In part, that’s because they kind of see the writing on the wall and are acting to control the negative impacts on the groundwater aquifer. In other cases, it’s because groundwater sustainability agencies have already adopted a constraint on how much groundwater people can pump, so they don’t have an option.”

Without water, crops can’t grow, said Ryan Jacobsen, CEO of the Fresno County Farm Bureau. The uncertainty of future conditions will make securing water supply more challenging. “Not knowing if this is year three of three or year three of 10 makes it more difficult for preparation purposes.”

Farmers, too, are seeking sustainable solutions, including investing in sophisticated technology for efficiency. “There’s definitely been a very large push to utilize every drop possible.”

The scarcity of water underscores the importance of monitoring groundwater, said Springhorn of the water department. Rigorous monitoring helps inform drought response and advanced planning as water agencies and agricultural communities navigate drought. Among the efforts to protect groundwater reserves include improved monitoring of groundwater systems, Springhorn said.

Sicke, the water manager in Yolo County, said her agency may need to reconsider its revenue structure, which currently relies more on surface water availability, should these prolonged drought conditions turn out to be the “new normal” for the state.

But for now, Sicke said she remains hopeful for a natural recovery. “We have seen the recovery historically when rain has come. We’re trying not to forget. Right now, it’s hard.”

Yoohyun Jung is a San Francisco Chronicle staff writer. Email: yoohyun.jung@sfchronicle.com Twitter: @yoohyun_jung

Yoohyun Jung is a data reporter for The San Francisco Chronicle. Most recently, she worked as a data journalist for Honolulu Civil Beat, a watchdog news organization covering the Hawaiian islands. Born in Seoul, Yoohyun began her career in Arizona, where she worked for two of the state's largest newspapers covering various beats, including criminal justice and education. She is also an alumna of Reveal from the Center for Investigative Reporting Investigative Fellowship and The New York Times Student Journalism Institute. 

Read more about the data team and their work.