How much groundwater did California's Central Valley lose during the 2012–2016 drought?

Abstract We estimate net groundwater storage change in the Central Valley from April 2002 to September 2016 as the difference between inflows and outflows, precipitation, evapotranspiration, and changes in soil moisture and surface water storage. We also estimate total water storage change attributable to groundwater change using Gravity Recovery and Climate Experiment (GRACE) satellite data, which should be equivalent to our water balance estimates. Over two drought periods within our 14‐1/2 years study period (January 2007 to December 2009 and October 2012 to September 2016), we estimate from our water balance that a total of 16.5 km 3 and 40.0 km 3 of groundwater was lost, respectively. Our water balance‐based estimate of the overall groundwater loss over the 14‐1/2 years is −20.7 km 3 , which includes substantial recovery during nondrought periods The estimated rate of groundwater loss is greater during the recent drought (10.0 ± 0.2 versus 5.5 ± 0.3 km 3 /yr) than in the 2007–2009 drought, due to lower net inflows, a transition from row crops to trees, and higher crop water use, notwithstanding a reduction in irrigated area. The GRACE estimates of groundwater loss (−5.0 km 3 /yr for both water balance and GRACE during 2007–2009, and −11.2 km 3 /yr for GRACE versus −10 km 3 /yr for water balance during 2012–2016) are quite consistent for the two methods. However, over the entire study period, the GRACE‐based groundwater loss estimate is almost triple that from the water balance, mostly because GRACE does not indicate the between‐drought groundwater recovery that is inferred from our water balance.