Monday, June 26, 2017

Significant groundwater loss in California’s Central Valley during recent droughts

A dry and empty field in Coalinga, California in the Central Valley on July 29, 2015. (Credit: California Department of Water Resources)

A new study from researchers at UCLA and the University of Houston reveals estimates of significant groundwater loss in California’s Central Valley during the recent drought and sparks questions of sustainability for the important agricultural area.

Researchers tracked net groundwater consumption in the Central Valley from 2002 to 2016, when a drought from 2007 to 2009 and a more severe drought from 2012 to 2016 hit the area. Published in Geophysical Research Letters, their findings estimate that a total of 16.5 cubic kilometers (4 cubic miles) and 40 cubic kilometers (9.6 cubic miles) of water were lost during between the 2007-2009 and 2012-2016 droughts, respectively. 

The more recent drought accounted for more than 10 cubic kilometers (2.4 cubic miles) of water lost per year. Researchers attributed this to reduced precipitation and snow melt, a change in the type of crops being cultivated, and hotter temperatures.

“For perspective, the amount of material associated with the 1980 eruption of Mount St. Helens was about one cubic kilometer,” said Dennis Lettenmaier, the UCLA professor of geography who led the study. “So, we’re talking about 40 times that amount in the recent drought.”

Decreases in groundwater worsened even with a reduction in the amount of irrigated land, which decreased 7 percent from 2007–2009 compared to the 2012–2016 drought. Higher temperatures during the more recent drought period and a transition into high-value and thirsty tree crops accounted for most of groundwater loss between the two droughts, and more than offset the effects of a reduction in irrigated land, Lettenmaier said.

Groundwater storage change over time in the Central Valley (CV) using data from NASA's twin satellite system, the Gravity Recovery and Climate Experiment (GRACE). The blue dashed lines show the rate of groundwater loss during the two drought periods. Courtesy of Xiao et al.

The study accounted for evapotranspiration, which is water released into the air through plant transpiration and soil evaporation, as well as precipitation, and surface water flowing into and out of the Central Valley.

"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 rear-view 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 is more than 18,000 square miles from the coast to the Sierra Nevada Mountains and is one of the largest agricultural hubs in the United States, providing over half of America’s fruit, vegetable, and nut crops. Researchers said they 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.

This study was supported in part by the NOAA Climate Program Office’s Modeling, Analysis, Predictions, and Projections program.


This announcement was co-developed by CPO and UCLA. Read the UCLA press release on this study

Read the paper in Geophysical Research Letters: 

Xiao, M., A. Koppa, Z. Mekonnen, B. R. Pagán, S. Zhan, Q. Cao, A. Aierken, H. Lee, and D. P. Lettenmaier (2017), How much groundwater did California's Central Valley lose during the 2012–2016 drought?, Geophys. Res. Lett., 44, doi:10.1002/2017GL073333.


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