LOS ANGELES – A recent study says California’s Central Valley – one of the most productive agricultural regions in the world – lost 25 cubic miles of groundwater during the drought that ran from 2012 through last year.
Led by a group of UCLA professors, geologists studied the loss of groundwater in the Central Valley, where there are an estimated 10,000 private wells siphoning water from aquifers below the arid but soil-rich valley.
“For perspective, the amount of material associated with the 1980 eruption of Mount St. Helens was about one cubic kilometer,” said Dennis Lettenmaier, UCLA professor of geography who led the study. “So, we’re talking about 40 times that amount in the recent drought.”
Another perspective: The groundwater lost in the most recent drought would fill a tank 25 miles long, 25 miles wide and 25 miles high – or more than 44 Lake Tahoes.
Part of what accounted for the water loss was not simply a lack of rain, but the fact that the weather pattern coincided with a switch from traditional row crops to high-value but water-dependent crops like almonds, pistachios and walnuts.
Nut trees require more in-season watering than row crops, and the fields they’re planted in can’t be fallowed periodically as farmers would with other crops.
The geologists also studied the drought of 2007-2009, when the Central Valley lost over 10 cubic miles of groundwater. The switch away from traditional row crops and the increased temperatures accounted for why the rate of loss was so much higher in the last drought, despite a 7 percent reduction in irrigated land, said Lettenmaier.
Groundwater pumping remains a contentious issue in the Central Valley, where the arid climate means many farmers are dependent on water allocations from the Sierra Nevada runoff and extra water from the Colorado River Basin.
With a water scarcity in both watersheds, farmers resorted to pumping enormous volumes of water from aquifers directly beneath their farms.
The massive groundwater extraction that occurred in Central Valley began to worry environmentalists, scientists and public officials, as overpumping can result in compaction, water-storage loss and dry wells in agricultural and residential areas near the valley.
“Pumping groundwater during a drought is not an unreasonable strategy,” Lettenmaier said. “But the problem is that to have a sustainable system you have got to replenish it at some point, and there essentially is no plan to do that.”
California has taken legislative steps to address its groundwater storage problem, passing a law in 2014 that requires local water agencies in water-depleted areas to devise sustainability plans and install water meters for enforcement.
Water users who exceed a given amount during a certain period are subject to fines according to the law, but many are worried the law does not have the teeth necessary to deter farmers from getting the water they need to survive by any means necessary.
Those agencies have until 2022 to finalize those plans.
“It’s fair to assume that there’s going to be another drought, and fair to assume that there will be usage of groundwater in that drought too; the wells are already there,” Lettenmaier said. “Now that this most recent drought is in the rearview mirror, there are still questions about how much natural recovery we can expect in groundwater and how water will be managed in the Central Valley.”
California’s Central Valley comprises about 18,000 square miles, spanning the center of the state from the coastal mountains to the Sierra Nevada Mountains. Growing a cornucopia of crops including fruits, vegetables and nuts, the valley provides more than half of the produce consumed in the United States.
The researches adopted two methods to track groundwater levels, using traditional water-balance estimates that take into account rain flow, snow melt, storage capacity and evaporation, and using more technologically advanced methods.
Specifically, the team used readouts from a NASA-owned twin satellite system called GRACE (Gravity Recovery and Climate Experiment).
The two methodologies demonstrated fairly consistent readings with some variance. Researchers said they hope future studies will focus on the recovery of groundwater in the Central Valley and how much replenishment actually occurs during years when rainfall and snowmelt are robust, as they were this past winter.