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The new study shows groundwater storage recovery has been dismal after the state's last two droughts, with less than a third of groundwater recovered from the drought that spanned 2012 to 2016.
Under a best-case scenario where drought years are followed by consecutive wet years with above-average precipitation, the researchers found there is a high probability it would take six to eight years to fully recover overdrafted water, which occurs when more groundwater is pumped out than is supplied through all sources like precipitation, irrigation and runoff.
However, this best-case scenario where California has six to eight consecutive wet years is not likely because of the state's increasingly hot and dry climate. Under a more likely, drier climate, there is less than a 20% chance of full overdraft recovery over a 20-year period following a drought.
The Central Valley produces about a quarter of the nation's food and is home to around 6.5 million people. Using too much groundwater during and after droughts could soon push this natural resource beyond the point of recovery unless pumping restrictions are implemented. The study finds recovery times can be halved with modest caps on groundwater pumping in drought and post-drought years.
"This is really threatening," said Sarfaraz Alam, a hydrologist at Stanford and lead study author. "There are many wells that people draw water from for drinking water. Since [groundwater is] always going down, at some point these wells will go dry and the people won't have water."
Measuring depletion
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