When drought reduced streamflows and irrigation water allotments, growers in California’s Central Valley pumped more water from their wells. Now a study by NASA and Stanford scientists says decades of overpumping permanently reduced the storage capacity of the aquifer beneath the valley by 336,000 to 606,000 acre-feet, which could exceed the capacity of the Hetch Hetchy Reservoir that is the primary water supply for San Francisco.
Overpumping during the recent drought caused land in the San Joaquin Valley to sink by up to three feet, says a NASA release. As the land sank, the subsurface particles of soil were squeezed together, leaving less space for water to fill when the aquifer is re-charged. The paper written by the team estimates a permanent loss of storage space equal to 9 percent of the water that was pumped from 2007-10 in the San Joaquin. “While this is just a small fraction of the total water stored underground in this area, this ‘water of compaction’ is an important safeguard for times of drought that, once removed, cannot be replenished,” the scientists say in a summary of their work.
“California is getting all of this rain, but in the Central Valley, there has been a loss of space to store it,” said Stanford professor Rosemary Knight, co-author of the study. One way to mitigate overpumping would be for farmers to avoid water in clay layers and to draw from more shallow sand and gravel layers, which are less susceptible to compaction, she said.