A California aquifer may have crossed the point of no return
Satellite data reveal in unprecedented detail how groundwater reservoirs permanently lose their ability to store water
- 27 Jul 2026
- 3:00 PM ET
- By Christian Elliott
The Lake Oroville Reservoir in California's Sacramento Valley was just 28% full in October 2021 amid a record drought. Groundwater pumping around the same time caused land across the valley to subside by more than half a meter in 2 years. California Department of Water
In 2020, a historic drought struck California’s Central Valley, where one-quarter of the food produced in the United States is grown, causing farmers to pump vast amounts of water from wells to keep their crops alive. All that pumping reduced the water pressure within the porous sediments storing that water deep underground. As a result, those sediments compacted over the next 2 years—causing land surfaces in the northern part of the Central Valley, known as the Sacramento Valley, to sink by more than half a meter.
This kind of subsidence is typically reversible, or “elastic.” Rainfall seeps into the ground and refills the empty sediment pores, causing the surface to rebound. But in 2021, the aquifer compacted so quickly and extensively that it likely permanently lost its capacity to store water, researchers report today in the Proceedings of the National Academy of Sciences.
The degree of change demonstrated in the study “seems really unbelievably evident,” says Pietro Teatini, a hydraulic engineer at the University of Padua who was not involved in the work.
This isn’t the first time researchers have identified an aquifer that has become inelastic, or lost its ability to rebound. The Central Valley’s San Joaquin region to the south subsided by as much as 9 meters in the 20th century, some of which was permanent. And satellites have detected inelastic aquifers elsewhere, including in China, Mexico, and Iran. But this study marks the first time that transition has been documented at the regional scale in such high resolution, in a place with groundwater records that can ground-truth satellite measurements.
Stacy Larochelle, a geophysicist at the University of California, Los Angeles, tracked how the Sacramento Valley changed from 2016 to 2022 from two points of view: beneath the surface and from hundreds of kilometers overhead. First, she and her colleagues compiled centimeter-scale radar measurements of ground deformation across the region, collected by two European Space Agency satellites. The researchers also obtained groundwater measurements from more than 2500 wells across the valley going as far back as the 1940s.
Combined, the two data sources painted a grim picture. From 2016 to 2020, land height fluctuated seasonally in step with the aquifer’s water level. Then in 2021, subsidence accelerated dramatically: Some areas fell by up to half a meter per year, well out of the historic range. “One of the surprising things is how abruptly this happens … all of a sudden it just collapses,” Larochelle says. “I, for one, wasn’t expecting that.”
Some researchers are skeptical the Sacramento Valley’s subsidence is permanent. One challenge is that it is hard to prove an aquifer has compacted irreversibly, notes Manoochehr Shirzaei, a geophysicist at Virginia Polytechnic Institute and State University who has not involved in the study. Inelastic subsidence has an exacting technical definition: It occurs when an aquifer compacts to an unprecedented degree, requiring groundwater pressure levels to reach historic lows. “To be able to say with confidence that the observed deformation is inelastic, we have to put it in the context of water table,” Shirzaei says.
During the 2020 drought, however, groundwater pressures did not fall to historic lows in all the parts of the Sacramento Valley’s aquifer where Larochelle’s team infers inelastic deformation. Meanwhile, other parts that did hit record lows didn’t deform significantly. Shirzaei thinks that variation means the aquifer potentially could recover, just on a longer, multiseason timeframe.
Larochelle is less hopeful. From 2022 to 2024, she says, the Sacramento Valley received heavy rains, but preliminary observations suggest the land didn’t rebound, indicating the aquifer wasn’t able to recharge. She also notes that groundwater pressures are typically measured in an aquifer’s most permeable parts—not at depth, where permanent compaction within the Sacramento Valley aquifer is most likely to have occurred. “We don't think that data accurately represents … the parts of the aquifers that are actually deforming,” Larochelle says.
The only way to know for sure whether permanent compaction has occurred is through expensive core samples or other measurements at depth. But analyses based on satellite data are getting better all the time: The new NISAR satellite mission, for example, uses radar to track land deformation at a global scale. Such data could be especially useful for monitoring coastal aquifers, where some 60% of the world’s population live, Teatini says—especially in Indonesia, Vietnam, and Malaysia, where aquifers are known to be subsiding rapidly. “The dream is to be able to monitor groundwater entirely from space using [advanced radar] and other satellite technologies,” Larochelle says.
In the Sacramento Valley, she adds, real-time satellite monitoring could provide an early warning of subsidence—potentially enabling land managers to prevent damage by reducing pumping.
Compaction within the Sacramento Valley will likely continue for years to come. Larochelle says state officials should consider declaring the aquifer as “critically overdrafted,” enabling them to impose new limits on groundwater use as they have in the San Joaquin Valley. In an aquifer that’s permanently collapsed, artificial recharge—pumping water forcibly back underground—is the only option, according to Shirzaei. That’s expensive and carries the risk of triggering earthquakes. But if land managers continue to push aquifers past the point of no return, the stress of future climate change may leave few alternatives.
“Water resources are going to become more and more scarce, and so being able to have access to this water resource in a sustainable manner for the century to come is important,” Larochelle says. “Otherwise, we won't have it for very long.”
Clarification, 28 July, 4:40 p.m.: This story was updated to clarify that observations of the Sacramento Valley from 2022 onward are preliminary and that groundwater measurements are often made within an aquifer's most permeable layers, not necessarily its shallowest. An attribution to Manoochehr Shirzaei was also added.
doi: 10.1126/science.z1ss4u9
