UCLA-led researchers track groundwater loss during drought in California’s Central Valley.
Crops like almonds cannot be left fallow during dry years without jeopardizing the trees, which during droughts require extensive irrigation in the California Central Valley.
“So, we’re talking about 40 times that amount in the recent drought.” During droughts Central Valley farmers are forced to use wells to replace water that would typically come from the Colorado River basin and the Sierra Nevada mountains.
Higher temperatures during the more recent drought period and the transition from row to tree crops, accounted for most of the increase in groundwater loss between the two droughts, and more than offset the effects of a reduction in irrigated land, Lettenmaier said.
Groundwater usage for crop irrigation in the Central Valley is a well-documented and hot-button issue in California.
Researchers used two methods to track groundwater levels, traditional water balance estimates —which take into account surface water inflow like rainfall and snow melt, soil moisture capacity and evapotranspiration — and data from NASA’s twin satellite system called GRACE (Gravity Recovery and Climate Experiment).
GRACE estimates align with the water balance estimates, with some variance.
GRACE data estimates that groundwater loss from 2012 to 2016 was 11.2 cubic kilometers per year, compared to water balance estimates of 10 cubic kilometers per year.
“Although both water balance-based and GRACE-based groundwater volume estimates are subject to errors, the relatively small area of the Central Valley in the eyes of GRACE might also be responsible for that recovery discrepancy,” Lee said.
Researchers hope future studies will address how much actual recovery happened between droughts and whether recovery from the most recent drought is on track to replenish the system.