For many California farmers groundwater is an essential account: a strategic reserve for drought years, when shares of surface water shrink. Aquifers provide 30 to 40 percent of the state's water supply in normal years but close to 60 percent in drought years.
That reserve allows California's Central Valley to consistently produce a hefty share of the nation's fruits and vegetables—more than 300 different crops all told, ranging from pomegranates to almonds to asparagus to tomatoes.
But since the 1920s, when improvements in electric motors first allowed widespread groundwater pumping, the state's landowners have been making large withdrawals from their underground accounts. Recent deep and extended droughts, including the current one, have worsened the situation.