Groundwater Depletion Causes California Farmland to Sink
“This seemingly unmovable, solid surface that we live on is actually a dynamic, moving object that’s constantly changing. Those changes interact with our lives much more than most of us realize,” Lees said. “This subsidence problem is an incredible example of that.”
The first round of sustainable management plans, submitted over the past year, have in several cases come up short in dealing with subsidence. California’s Department of Water Resources has given agencies until July 2022 to address deficiencies such as a lack of information about infrastructure potentially at risk due to land subsidence, inadequate evidence to demonstrate correlation between groundwater levels and land subsidence, and insufficient historical information about land subsidence.
“The modeling carried out in this study, if done in areas throughout the state, would provide the scientific basis needed to inform sustainable management. A range of possible actions to mitigate subsidence could be rigorously assessed,” said Knight, who is the George L. Harrington Professor at Stanford Earth.
Preventing Future Sinking
The new research simulates 65 years of subsidence near the town of Hanford in the San Joaquin Valley, which makes up roughly the southern half of the vast Central Valley. Building on research published in 2019 by Knight and Ryan Smith, PhD ’18, the simulation combines satellite-based subsidence data, private well logs, and water level measurements dating back to the 1950s to make a physical model of the area’s subsurface, including layers of coarse-grained sediments and clays that compact like a squeezed-out sponge as water levels drop.
Local water agencies and conservation districts provided well logs and water level measurements and helped identify research questions for future sustainable management. Local managers, with years of experience observing how water levels in the study area rise and fall during wet and dry periods, also helped the scientists spot data errors in early iterations of the model. “Partnerships with people in the local agencies made this study possible,” Knight said. “The data they contributed and their knowledge of the system were essential.”
Consistent with modeling and observations made throughout the more closely monitored western half of the San Joaquin Valley in the 1970s, the results suggest thick clays throughout the valley will continue to compact for decades to centuries after water levels in the region’s aquifers stabilize. “This is an important observation,” the authors write, because sustainable management plans submitted for the area studied assume water levels stabilized at recent lows will be enough to prevent future sinking. “This assumption is wrong,” Knight said. The land’s subsidence will slow only if deposits to groundwater basins – from rainfall and runoff or managed aquifer recharge projects – begin to outpace withdrawals.
The new modeling also shows that compaction in the deepest layer of the groundwater system has accounted for more than 90 percent of subsidence in the study area over the past 20 years. “If your only concern is the land sinking, then you are far better off pumping your water from the shallow zone than from the deep zone,” Lees said.
While the study focuses on a relatively small area, the authors say the results are likely representative of subsiding regions throughout the San Joaquin Valley, and the methodology can be applied to understand subsidence in similar aquifer systems worldwide.
