Groundwater pumping has been driving record-breaking sinking of land in California’s San Joaquin Valley.
For nearly twenty years, groundwater extraction has outpaced natural recharge, that is, water is being pumped out faster than it is naturally replenished.
According to two researchers, between 2006 and 2022, the entire valley sank nearly an inch each year.
Rosemary Knight, a professor of geophysics in the Stanford Doerr School of Sustainability, and Matthew Lees (PhD ’23), a research associate with the University of Manchester, UK, wrote about their study and findings in the prestigious, peer-reviewed academic journal Nature Communications Earth and Environment (citation below). Lees was a PhD student in geophysics at Stanford when he worked on the study.
Sinking of land (subsidence) was common knowledge
Geophysicists, hydrogeologists, and water managers have known that the land has been *subsiding over the past two decades.
* “Subsiding” (verb ‘subside,’ noun ‘subsidence’) refers to the gradual sinking or settling of the Earth’s surface, often caused by groundwater extraction, oil and gas extraction, fracking, mining activities, natural gas storage, and heavy building or infrastructure loads.
However, as nobody had quantified the total subsidence before this latest study, they did not fully appreciate the true extent of its impact.
This was partly because of a lack of consistent data. Satellite radar systems, which offer the most accurate measurements of elevation changes, did not continuously monitor the San Joaquin Valley from 2011 to 2015.
The Stanford researchers have now calculated the extent of land subsidence during that period.
Prof. Knight said:
“Our study is the first attempt to really quantify the full Valley-scale extent of subsidence over the last two decades. With these findings, we can look at the big picture of mitigating this record-breaking subsidence.”
Costly Repairs and Threatened Water Supply
Millions of dollars in repairs to aqueducts and canals have been necessary due to rapid and uneven subsidence. These aqueducts and canals transport vital water supplies through the San Joaquin Valley to major cities in southern California.
The sinking of the land has also undermined the water supply to one of the world’s most agriculturally productive regions.
Dr. Lees said:
“The bill for repairing major aqueducts like the Friant-Kern Canal and the California Aqueduct is exceptionally high. But the subsidence is having other effects, too.”
“How much was last year’s flooding worsened by subsidence? How much are farmers spending to re-level their land? A lot of the costs of subsidence aren’t well known.”
Happening All Over Again
Subsidence happens when water is extracted from natural underground reservoirs known as aquifers, which consist of sediments such as sand, gravel, and clay.
These sediments, similar to a sponge or Swiss cheese (with holes), contain numerous pores filled with water.
As the water is withdrawn and these spaces empty, the sediments compress—sometimes irreversibly—reducing the aquifer’s capacity to store water in the future and causing the ground to sink.
Booming agriculture in the San Joaquin Valley, along with population growth, led to aggressive groundwater pumping from 1925 to 1970. Eventually, over 4,000 square miles of land sank more than one foot (12 inches, 30.48 cm)—an area comparable in size to New Jersey. In some parts of the valley, the land subsided dramatically, sinking as much as 30 feet.
A 1999 report titled ‘Land Subsidence in the United States,’ published by the US Geological Survey, described this phenomenon as “one of the single largest alterations of the land surface attributed to humankind.”
The problem receded after the new aqueducts were installed in the 1970s. However, it roared back with a vengeance in the early 2000s during a series of droughts, which intensified groundwater pumping and changes in land use. The droughts also meant that there was considerably less water coming in from Northern California rivers.
Prof. Knight said:
“There are two astonishing things about the subsidence in the valley. First is the magnitude of what occurred prior to 1970, and second is that it is happening again today.”
Measuring the Impact Using InSAR
To measure how quickly the land had been sinking, Lees and Knight used a method called Interferometric Synthetic Aperture Radar, or InSAR.
With this technique, we can track changes in ground elevation by sending radar signals from satellites in space. The signals bounce off the ground and return to the satellites, and scientists then analyze the reflected signals to see how the ground level has shifted.
InSAR can measure changes over areas the size of a football field, sometimes capturing updates several times a month.
For the period 2011 to 2015, there was limited satellite coverage, so the InSAR data record for the San Joaquin Valley is patchy. The researchers used elevation data from GPS stations scattered across the region.
They analyzed the spatial patterns captured in the InSAR data and used these patterns to estimate elevation changes in the large gaps between the GPS stations.
San Joaquin Valley aquifers need about 220 billion gallons of water coming in annually, either naturally or artificially, to prevent the land from sinking in the future, additional analysis suggested.
This amount is roughly 7 billion gallons less than the average annual surplus of surface water in the San Joaquin Valley, after meeting all environmental requirements.
Prof. Knight said:
“I am optimistic that we can do something about subsidence. My group and others have been studying this problem for some time, and this study is a key piece in figuring out how to sustainably address it.”
Refilling Aquifers to Prevent Ground Subsidence
Flood-managed aquifer recharge (flood-MAR), a type of water management approach, could help. Flood-MAR, which is commonly used in California, involves diverting excess water from rainfall and melting snow to locations where the water can seep into the ground and replenish aquifers.
It is not feasible to drench the entire Valley with flood-MAR water.
Prof. Knight said:
“We should be targeting the places where subsidence will cause the greatest social and economic costs. So, we look at places where subsidence is going to damage an aqueduct or domestic wells in small communities, for instance.”
“By taking this Valley-scale perspective, we can start to get our head around viable solutions.”
Citation
Lees, M., Knight, R. Quantification of record-breaking subsidence in California’s San Joaquin Valley. Commun Earth Environ 5, 677 (2024). https://doi.org/10.1038/s43247-024-01778-w