Groundwater overdraft recovery during post-drought years in California’s Central Valley

Abstract:

California has encountered a series of drought events in recent decades, which, along with changing cropping and water use patterns, have caused major groundwater depletion in the Central Valley. Especially in the recent 2007-2009 and 2013-2016 droughts, surface water shortages lead to substantial groundwater overdraft, which recovered in post-drought years at rates that varied between years and regions. However, estimates of overdraft recovery vary substantially depending on the methods used to estimate groundwater storage. Furthermore, post-drought winter precipitation is one of the principal drivers of overdraft recovery, which is strongly dependent on ENSO in California. The role of ENSO on groundwater overdraft recovery is largely unknown. Hence, we address two research questions- (1) how much groundwater overdraft has been recovered during the post-drought years? and (2) what is the role of ENSO events in the groundwater overdraft recovery? To shed more light on the nature of post-drought groundwater recovery, we examine groundwater storage changes for the periods following the droughts of 1987-1992, 2007-2009 and 2013-2016 as estimated from observation wells, a surface-groundwater simulation model (C2VSIM), GRACE observations (following the 2007-2009 and 2013-2016 droughts) as well as a water balance method. Our results show that there is general agreement as to overdraft recovery estimated from well observations and C2VSIM, however, the variability in space is higher for well measurements. Moreover, the temporal variability in overdraft recovery estimated by GRACE is higher than the water balance estimate. Analysis of groundwater recovery and precipitation pattern reveals that high precipitation during ENSO does have an important implication on post-drought groundwater recovery, however, the current agricultural and water management practices will most likely be insufficient to recover groundwater overdraft caused by future droughts.