It may be a land of milk and honey, but California's Central valley - the most productive farmland in the US - is being sucked dry. The culprits? Lettuce and other green vegetables.
James Famiglietti at the University of California, Irvine, used the twin GRACE satellites to find that 20 cubic kilometres of groundwater had disappeared from beneath the valley between October 2003 and March 2010. Between 1998 and 2003, 28.5 km3 were lost, according to the US Geological Survey, meaning that about 50 km3 of groundwater had disappeared in 12 years (Geophysical Research Letters, DOI: 10.1029/2010GL046442).
That's unsustainable, says Famiglietti, and bad news for local farmers. "There is a foreseeable end to groundwater availability in California," he says. Estimates of the total reserves are rough, so the end is difficult to predict, but Famiglietti says the valley could run dry by 2100.
Growing green vegetables is profitable, but they need copious water. The problem is that water use is not regulated. "Anyone who wants to can drill a well and pump it up," says Peter Gleick of the Pacific Institute, a think tank based in Oakland, California.