Researchers find permanent damage to Sacramento Valley groundwater storage

The Abridged version:

• Researchers from UCLA and Caltech found evidence of permanent damage to the Sacramento Valley’s groundwater aquifer, as detailed in a report released Monday.
• The damage occurred during the last major drought from 2020-22, when landowners in the region had to rely on pumping groundwater to meet their needs.
• In some parts of the Sacramento Valley, subsidence, or land sinking, reached levels similar to parts of the heavily sunken San Joaquin Valley.

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For decades, people in the Sacramento Valley have been able to rely on the water beneath their feet.

In years of drought, when limited rain and snow meant that access to surface water was restricted, locals with the means could pump water up from the earth to keep their crops and families alive.

Even as reliance on that practice began to wreak havoc farther south in the San Joaquin Valley, where overpumping has caused the ground to sink by several feet in some places, the Sacramento Valley largely avoided similar consequences.

But that could be changing.

In a report released Monday, researchers from UCLA and Caltech detailed permanent damage that occurred to the aquifer beneath the Sacramento Valley during the most recent prolonged drought.

From 2020-22, years when statewide surface water allocations decreased substantially due to drought, landowners leaned on groundwater as they had before.

This time, though, researchers found that there were consequences.

“Parts of the Sacramento Valley were subsiding as fast as what we observed in the San Joaquin Valley,” said Stacy Larochelle, one of the authors of the paper published in Proceedings of the National Academy of Sciences.

Groundwater storage

Groundwater is stored in pockets and pores of space between gravel, dirt and rocks in the ground. To get to it, people dig wells and use pumping systems that bring the water up dozens of feet to the surface.

Usually, that extraction creates a cyclical pattern, depleting and replenishing the aquifer. Water is taken out of the ground during dry summer months, and the pores and pockets are refilled by winter storms.

“If you don’t pump too much groundwater, then you’ll remain in this elastic, reversible regime, so you won’t permanently damage this pore space,” Larochelle said.

But overpumping without allowing the aquifer to recharge can compact the ground and permanently reduce the amount of space available to store water.

“At the land surface, that’s reflected as land subsidence,” Larochelle said.

That’s what happened in some parts of the Sacramento Valley during the last drought at a rate of more than a foot per year.

The consequences can be far-reaching, including diminishing water storage capacity and destroying critical agricultural infrastructure as the ground shifts.

Why now?

There have been droughts in the past, but the one that occurred during 2020-22 was the first time researchers observed this level of subsidence in parts of the Sacramento Valley.

Larochelle said that could be for a couple of reasons.

One is that satellite, radar and data tracking technology has progressed.

“If a transition like this happened in 1950, there’s no way we would have been able to image it in the way that we did in this study,” she said.

Researchers were able to pair satellite ground-mapping technology with groundwater readings from local monitoring wells to put together their findings. The method showed that well levels alone are not enough to determine whether there has been damage to the aquifer.

The other reason could be related to increased groundwater pumping, though the researchers were unable to quantify that, Larochelle said.

Erik Cadaret, assistant general manager for the Yolo County Flood Control and Water Conservation District, said that understanding groundwater pumping in the Sacramento Valley during the 2020-22 drought requires context.

“Surface water availability plays a significant factor in how much groundwater is extracted,” he said in an email.

During the 2020 and 2021 water years, contractors received a 75% allocation of Sacramento River water from the U.S. Bureau of Reclamation. Then in 2022, historic low storage at Shasta Dam dropped diversions to unprecedented lows between 0% and 18%.

“Landowners had to make a choice on what would be irrigated with groundwater versus not irrigated at all,” Cadaret said. “Therefore, a key factor in managing subsidence is how we use and manage surface water during the irrigation season and, importantly, during the winter.”