Satellite-based monitoring of groundwater depletion in California's Central Valley

In an effort to link aquifer volume change, and related groundwater depletion, to the distribution of documented wells within the Tulare basin, we describe a constrained inversion of Interferometric Synthetic Aperture Radar (InSAR) range change observations for reservoir volume change. The constraints are in the form of a function that penalizes volume changes far from known well locations. Using this approach we are able to produce a spatial distribution of volume changes that are co-located with existing wells. The resulting distribution of reservoir volume change varies significantly from the drought year extending from September 2015 to September 2016, to the subsequent time intervals for 2017, a wet year, and 2018, a year with rainfall that was closer to historical averages. The 2.3 million acre-feet of volume reduction estimated from the InSAR data, though it is at best a lower bound on the volume of water extracted from the basin between September 2015 and 2016, does generally agree with yearly losses of 1.8 and 2.3 million acre-feet determined in previous studies. Futhermore, because groundwater losses in California's Central Valley are dominated by those in the southern San Jaoquin Valley, the estimate of 2.9 cubic kilometers in lost aquifer volume is compatible with an estimated loss of 3.1 cubic kilometers in groundwater volume reduction derived from the Gravity Recovery and Climate Experiment (GRACE) satellite mission.