A new study shows land in California's San Joaquin Valley has been sinking at record-breaking rates over the last two decades as groundwater extraction has outpaced natural recharge.
The researchers found that the average rate of sinking for the entire valley reached nearly an inch per year between 2006 and 2022.
Researchers and water managers have known that sinking, technically termed "subsidence," was occurring over the past 20 years. But the true impact was not fully appreciated because the total subsidence had not been quantified. This was in part due to a gap in data. Satellite radar systems, which provide the most precise measure of elevation changes, did not consistently monitor the San Joaquin Valley between 2011 and 2015. The Stanford researchers have now estimated how much the land sank during these four years.
"Our study is the first attempt to really quantify the full Valley-scale extent of subsidence over the last two decades," said senior study author Rosemary Knight, a professor of geophysics in the Stanford Doerr School of Sustainability. "With these findings, we can look at the big picture of mitigating this record-breaking subsidence."
The new study, published Nov. 19 in Nature Communication Earth and Environment, offers ideas on how to stop the sinking through strategic regional water recharge and other management approaches.
Rapid and uneven declines in land elevation have forced multimillion-dollar repairs to canals and aqueducts that ferry critical water through the San Joaquin Valley to southern California's major cities. By damaging local wells and irrigation ditches, this subsidence is also exacerbating water supply issues for one of the most agriculturally productive regions in the world.
"The bill for repairing major aqueducts like the Friant-Kern Canal and the California Aqueduct is exceptionally high," said lead author Matthew Lees, PhD '23, a research associate with the University of Manchester who worked on the study as a PhD student in geophysics at Stanford. "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."
History repeating itself
Subsidence occurs as water is removed from natural reservoirs called aquifers, where water is stored in underground sediments including sand, gravel, and clay. Like a sponge, the sediments are full of pores. As those spaces are emptied, the sediments compact - in some cases permanently, altering future water-carrying capacity - and cause the ground level to fall.
In the San Joaquin Valley, which runs from east of the San Francisco Bay Area down to the mountains north of Los Angeles, booming agriculture and population growth prompted aggressive pumping of groundwater between 1925 and 1970. The result: More than 4,000 square miles - an area half the size of New Jersey - sank by over 12 inches, reaching about 30 feet in some locations, a profound landscape change that a 1999 governmental report described as "one of the single largest alterations of the land surface attributed to humankind."
The problem ebbed during the 1970s following the installation of new aqueducts. But it roared back in the early 2000s amid a series of droughts, intensified groundwater pumping, land-use changes, and reduced deliveries from Northern California rivers. "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," said Knight.
Understanding the problem
To gauge the recent subsidence rate, Lees and Knight turned to a technique known as interferometric synthetic aperture radar, or InSAR. The technique captures elevation changes across roughly football field-size chunks of land as frequently now as a few times per month by beaming radar signals from orbit. The signals reflect off the ground back to the satellites, and analysis of the received signal reveals changes in ground elevation.
The InSAR data record for the San Joaquin Valley is patchy between 2011 and 2015, due to limited satellite coverage. To fill this gap, Lees and Knight used elevation data from Global Positioning System (GPS) stations scattered throughout the region. They identified spatial patterns in the InSAR record and used these to interpolate elevation in the vast areas between GPS stations.
Additional analysis by the researchers suggests that San Joaquin Valley aquifers require approximately 220 billion gallons of water coming in each year - through natural or engineered processes - to prevent future subsidence.
This is about 7 billion gallons less than the amount of surface water left over in the San Joaquin Valley in an average year after all environmental needs are covered. "I am optimistic that we can do something about subsidence," said Knight. "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."
Replenishing aquifers to prevent sinking
A water management approach called flood-managed aquifer recharge (flood-MAR), which is being widely adopted in California, could help. It involves diverting excess surface water from precipitation and snowmelt to locations where the water can percolate down and recharge aquifers.
Drenching the whole of the Valley via flood-MAR water is not feasible. "We should be targeting the places where subsidence will cause the greatest social and economic costs," said Knight. "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," added Knight, "we can start to get our head around viable solutions."
