Groundwater pumping is making California’s San Joaquin Valley sink about an inch per year: Study

California’s San Joaquin Valley may be sinking nearly an inch per year due to the over-pumping of groundwater supplies, with resource extraction outpacing natural recharge, a new study has found.

This agriculture-rich region, located within the state’s Central Valley, has been sinking at record-breaking rates over the past two decades, according to the study, published on Tuesday in Nature’s Communications Earth & Environment.

While researchers have known that subsidence — the technical term for sinking — has been affecting the region in recent years, the total amount of collapse had not been quantified.

“Our study is the first attempt to really quantify the full Valley-scale extent of subsidence over the last two decades,” senior study author Rosemary Knight, a professor of geophysics at Stanford University’s Doerr School of Sustainability, said in a statement.

“With these findings, we can look at the big picture of mitigating this record-breaking subsidence,” Knight added.

The San Joaquin Valley, which extends from east of the San Francisco Bay Area to the mountains north of Los Angeles, became host to a ballooning farming sector and groundwater pumping between 1925 and 1970, per the study.

Over time, these activities resulted in the sinkage of more than 4,000 square miles — about half the area of New Jersey — by more than 12 inches, the authors explained.

Although subsidence slowed down following the construction of aqueducts in the 1970s, drought-induced groundwater pumping led to a resurgence during the early 2000s, they noted.

To gain a clearer picture of the recent rate of sinkage, the researchers used a tool called interferometric synthetic aperture radar, which beams signals from orbit to capture land elevation changes as frequently as a few times per month.

Combining this information with elevation data from GPS stations dispersed across the region, they identified spatial patterns for years that exhibited spottier satellite coverage.

In addition to divulging the average subsidence rate of nearly an inch per year, the scientists determined that San Joaquin Valley aquifers need about 220 billion gallons of water coming in annually to prevent future such sinkage.

That amount constitutes about 7 billion gallons less than the total surface water that typically remains in the valley after all environmental needs are covered in an average year, according to the study.

Knight therefore expressed optimism that something can be done about the subsidence issue, which could be solved through certain engineered processes.

One such approach, called flood-managed aquifer recharge, involves diverting excess surface water from snowmelt and precipitation to locations where the resource can drip down and recharge aquifers, the authors explained.

They noted, however, that saturating the entire San Joaquin Valley with such water would not be feasible.

“We should be targeting the places where subsidence will cause the greatest social and economic costs,” Knight said.

Some focal areas could include spots where subsidence could damage an aqueduct or domestic wells that hydrate small communities, according to Knight.

“By taking this Valley-scale perspective,” she added, “we can start to get our head around viable solutions.”

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