Estimating the permanent loss of groundwater storage in the southern San Joaquin Valley, California

In the San Joaquin Valley, California, recent droughts starting in 2007 have increased the pumping of groundwater, leading to widespread subsidence. In the southern portion of the San Joaquin Valley, vertical subsidence as high as 85 cm has been observed between June 2007 and December 2010 using Interferometric Synthetic Aperture Radar (InSAR). This study seeks to map regions where inelastic (not recoverable) deformation occurred during the study period, resulting in permanent compaction and loss of groundwater storage. We estimated the amount of permanent compaction by incorporating multiple data sets: the total deformation derived from InSAR, estimated skeletal-specific storage and hydraulic parameters, geologic information, and measured water levels during our study period. We used two approaches, one that we consider to provide an estimate of the lowest possible amount of inelastic deformation, and one that provides a more reasonable estimate. These two approaches resulted in a spatial distribution of values for the percentage of the total deformation that was inelastic, with the former estimating a spatially averaged value of 54%, and the latter a spatially averaged value of 98%. The former corresponds to the permanent loss of 4.14x108 m(3) of groundwater storage, or roughly 5% of the volume of groundwater used over the study time period; the latter corresponds to the loss of 7.48x108 m(3) of groundwater storage, or roughly 9% of the volume of groundwater used. This study demonstrates that a data-driven approach can be used effectively to estimate the permanent loss of groundwater storage. Plain Language Summary Subsidence due to groundwater pumping from 2007 to 2010 in the San Joaquin Valley, California was mapped using satellite data known as InSAR. These data were incorporated with additional datasets, including geological information, to estimate how much subsidence was permanent. This subsidence represents a permanent loss in groundwater storage. Using these methods, we estimated that a permanent loss of 7.48 x 10(8) m(3) of groundwater storage occurred during our study period. This accounted for roughly 9% of groundwater pumping in our study area. While this is just a small fraction of the total water stored underground in this area, this 'water of compaction' is an important safeguard for times of drought that, once removed, cannot be replenished.