Decades of overpumping groundwater have irreversibly altered layers of clay beneath California's Central Valley, permanently reducing the aquifer's ability to store water, finds a new satellite remote sensing study by scientists at Stanford University, Stanford, California; and NASA's Jet Propulsion Laboratory in Pasadena, California.

The study, published online in the journal Water Resources Research, reveals that overpumping caused land in the state's San Joaquin Valley to sink almost 3 feet (85 centimeters) during a recent drought from 2007 to 2010. As a result, the aquifer permanently lost between 336,000 and 606,000 acre-feet of natural water storage capacity. An acre-foot is equal to 326,000 gallons. In comparison, the Hetch Hetchy Reservoir that stores the primary water supply for the San Francisco Bay area has a capacity of about 360,000 acre-feet.

The San Joaquin Valley is one of the largest U.S. agricultural hubs, producing an estimated $17 billion of crops a year. The new findings come just as the state is experiencing its wettest season in years following an extended, record-setting drought.

"California is getting all of this rain, but in the Central Valley, there has been a loss of space to store it," said study coauthor Rosemary Knight, George L. Harrington professor at Stanford's School of Earth, Energy and Environmental Sciences.

Knight and her colleagues used data acquired with a satellite technology called Interferometric Synthetic Aperture Radar (InSAR) collected by the Phased-Array L-band Synthetic Aperture Radar (PALSAR) instrument on the Japan Aerospace Exploration Agency's Advanced Land Observing Satellite to measure centimeter-scale changes in elevation in the San Joaquin Valley between 2007 and 2010. The scientists compared multiple satellite InSAR images of Earth's surface to calculate how much the land subsided (sank).

"Our work is a good example of the use of Earth-observing satellites to answer down-to-Earth questions about the sustainability of water resources," said JPL research scientist and study coauthor Tom Farr.

Subsidence happens when the water pressure in the subsurface dips below a critical level when too much groundwater is removed, causing the sediments to compact. "As you pump groundwater out of an aquifer, the water pressure in the tiny pores of the sediment drops," said study first author Ryan Smith, a doctoral candidate in Knight's lab. "That reduces the ability of the aquifer to hold up the ground above it and causes it to collapse. That collapse is manifested at the surface as subsidence."

If too much water is extracted, particularly from clay layers, the compaction becomes irreversible, and the soil's ability to retain water is permanently diminished. "When too much water is taken out of clay, its structure is rearranged at the microscopic level and it settles into a new configuration that has less storage space," said Knight, who is also affiliated with the Stanford Woods Institute for the Environment.

This not only makes it more difficult to store water in the future, but also makes it harder to draw any existing water out of the ground today. "It's like trying to suck water from a really thin straw," Knight said. "The pressure that needs to be exerted to pull the water out gets greater and greater as the clay structure collapses."

The scientists only examined InSAR data collected during the drought period between 2007 and 2010. Since then, California has experienced a more severe drought, from 2012 to 2016. "Although our paper didn't deal with the most recent drought, I think it's safe to say that the latest drought may have caused at least as much, or even more, subsidence and permanent compaction in the region than the last one," Smith said.

"This is because the rate of water decline increased during that period, causing the groundwater to drop to historically low levels. Recent InSAR studies by JPL, not included in this study, also demonstrate that subsidence continued at a similar, and in some cases even greater, rate compared with what we saw from 2007 to 2010."

One way farmers in the region could alleviate the problem, Knight said, is to avoid drawing water from clay layers and instead pump groundwater from more shallow sand and gravel layers, which are more easily recharged and are less susceptible to permanent compaction.

Until recently, however, distinguishing clay layers from sand and gravel from the surface required drilling expensive wells. But Knight's group is testing a novel geophysical electromagnetic method that involves flying a helicopter equipped with instruments capable of imaging the subsurface from the air to create a three-dimensional map of clay, sand and gravel deposits.

"With the right geophysical tool," Knight said, "we can not only better understand the composition of the subsurface, but also help guide pumping and groundwater recharge efforts."

Other study coauthors include Howard Zebker, Jessica Reeves and Jingyi Chen from Stanford University and Zhen Liu at JPL. Funding for the study was provided by the S.D. Bechtel Jr. Foundation, NASA's Terrestrial Hydrology Program and the National Science Foundation.

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