California farmers in the Central Valley pumped enough water out of the ground to fill Lake Mead, which can store the entire average flow of the Colorado River for two years. It’s enough to drown the state of Pennsylvania in a foot of water.
A new study from UCLA and the University of Houston shows that’s how much groundwater was used during the most recent drought in one of the nation’s largest agricultural hubs.
The study also found the rate of groundwater withdrawal per year in the most recent drought was double that of the drought of 2007 to 2009, even though there was less land to irrigate.
“That sort of doubling of the extraction rate was attributable more or less in equal amounts to the fact that the recent drought was warmer and the evaporative amount was higher and the shift to row crops to tree crops,” says Dennis Lettenmaier with UCLA, the study’s lead author.
Over the years, farmers have shifted from row crops that can be left fallow during dry years to planting higher-value tree crops like almonds that must be watered year-round.
“Pumping groundwater during a drought is not an unreasonable strategy when there’s not enough surface water," says Lettenmaier.
But he says the strategy is unsustainable.
"This is the big issue and there’s no real plan for putting that back in.”
Laws to manage groundwater are being implemented in California. Counties, irrigation districts, farmers and other entities must form agencies by the end of June that will be tasked with managing groundwater sustainably. But limits on groundwater pumping won't likely happen until after 2020.
To track groundwater levels for the study, researchers used NASA satellite data and water balance estimates, which take into account surface water inflow from rain and snow, soil moisture, and evapotranspiration.
Lettenmaier says the study shows that between droughts there was some groundwater replenishment.
The study was published in the journal Geophysical Research Letters.