An interdisciplinary investigation of the Central Valley's aquifer response during recent droughts.

Semi-arid, drought-prone and fast-growing regions like California are experiencing strongpressure on water resources, causing groundwater overdraft and putting at risk future watersecurity. For sustainable management of groundwater, data with sufficient accuracy andresolution are needed to evaluate the impact of human activities and climate extremeson groundwater resources. Recent developments in geodetic remote sensing andmodeling have significantly broadened our insights into groundwater resources.Gravity field observations from the GRACE mission in conjunction with hydrologicaldata allows quantifying the overall loss of groundwater in a large aquifer system,and thus providing insight into the severity of groundwater depletion. Moreover,changes in groundwater stocks cause surface deformation associated with regionalelastic loading of the Earth’s crust and localized poroelastic compaction of theaquifer skeleton, which are detectable by GPS and InSAR. The loading signal istypically much smaller than the land subsidence due to poroelastic compaction andthus masks out the loading signal adjacent to the aquifer system. However, theporoelastic signal can be used to estimate groundwater volume change in confinedaquifer units and provides insight into the mechanical properties of the aquifersystem. In this presentation, we perform an integrated multiscale analysis of various data sets toretrieve detailed information on the responses of the Central Valley aquifer system to thedrought periods of 2007-2010 (entire Central Valley) and 2012-2015 (focused on the SanJoaquin Valley). We use ∼300 continuous GPS stations, 620 SAR images acquired byALOS L-band and Sentinel1-A/B C-band sensors, and ∼1600 groundwater levelobservational wells. GRACE-based estimates of total water storage change areobtained from JPL and converted into groundwater volume loss using hydrologicaldatasets. We estimate maximum subsidence rates in the southern San Joaquin Valley ofup to ∼25 cm/yr and ∼35 cm/yr for the droughts starting in 2007 and in 2012,respectively. Using a 1-D poroelastic calculation based on deformation data, we find agroundwater loss of 21.3±7.2 km3 for the entire Central Valley during 2007-2010 and of29.3±8.7 km3 for the San Joaquin Valley during 2012-2015. The loss estimates areconsistent with that of GRACE-based estimates considering uncertainty ranges. Wefurther infer that due to overdraft during both droughts the aquifer system storagecapacity permanently reduced by up to 5%. This integrated analysis, allows usto address the question of how well the different geodetic signals agree with oneanother and what are the possible causes of disagreements. We highlight the need forinterdisciplinary efforts to integrate available geodetic and hydrological datasets to improveour understanding of aquifer systems response to drought and human activities. [ABSTRACT FROM AUTHOR]