A recent study has revealed that land in California's San Joaquin Valley has been sinking at unprecedented rates over the past two decades due to excessive groundwater extraction. The research, conducted by Stanford University and published in Nature Communication Earth and Environment, indicates that the valley has experienced an average sinking rate of nearly an inch per year from 2006 to 2022.
Rosemary Knight, a professor of geophysics at the Stanford Doerr School of Sustainability and senior author of the study, stated, "Our study is the first attempt to really quantify the full Valley-scale extent of subsidence over the last two decades." She emphasized that understanding this phenomenon is crucial for developing strategies to mitigate its impact.
The consequences of this subsidence are significant. It has led to costly repairs for canals and aqueducts vital for transporting water across California. Matthew Lees, a research associate with the University of Manchester and lead author of the study, highlighted these challenges: "The bill for repairing major aqueducts like the Friant-Kern Canal and the California Aqueduct is exceptionally high."
Subsidence occurs when water is extracted from underground reservoirs known as aquifers. This process compacts sediments like sand and gravel, causing permanent changes in ground level. In regions such as San Joaquin Valley, aggressive groundwater pumping between 1925 and 1970 resulted in significant land alterations. Although new aqueducts temporarily alleviated this issue during the 1970s, it resurfaced in recent years due to droughts and intensified pumping.
To measure recent subsidence rates accurately, researchers used interferometric synthetic aperture radar (InSAR) technology combined with Global Positioning System (GPS) data. Their analysis suggests that approximately 220 billion gallons of water need to be reintroduced into San Joaquin Valley aquifers annually to prevent further subsidence.
Knight expressed optimism about addressing this challenge through methods like flood-managed aquifer recharge (flood-MAR). This approach involves redirecting excess surface water from precipitation and snowmelt to replenish aquifers strategically. However, she noted that efforts should focus on areas where subsidence poses significant social or economic risks.
"By taking this Valley-scale perspective," Knight added, "we can start to get our head around viable solutions."
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