From Satellites to Storativity: Using Interferometric Synthetic Aperture Radar (InSAR) for Monitoring Groundwater in Central Valley California

California's Central Valley depends on groundwater to sustain a growing population and intensive agricultural practices. Multi-decadal groundwater pumping activities, recurring droughts, and revised government legislation demonstrate the need for an improved understanding and management of this resource. However, the valley's large geographic area and long history of pumping makes it difficult to monitor aquifer deformation using traditional methods. Remote sensing techniques offer a unique opportunity to track spatial and temporal changes of aquifer conditions over the entire valley and over multiple decades.

Here, we use Interferometric Synthetic Aperture Radar (InSAR) to measure the rate and magnitude of vertical deformation across the entirety of the 47,000 km² valley between 1992-2020. This deformation measured at the surface is then used to infer subsurface aquifer-system compaction. We rely on ERS-1/2, ENVISAT, ALOS-1, and Sentinel-1 satellite data, and compute correlation-based timeseries, which enable maintaining coherence even in agricultural areas. We use GNSS/GPS data for both ground truthing and to further evaluate the potential occurrence of seasonal deformation. We confirm that the aquifer's response to groundwater extraction and recharge is highly spatially and temporally variable, due the presence of loosely consolidated sediments interbedded with clay layers. We observe the maximum deformation to occur in the Tulare basin between Hanford and Corcoran, reaching 26.4 cm/yr. Finally, deformation timeseries are used as an input into the existing USGS hydrologic model (MODFLOW-OWHM and CVHM) to constrain the impact of the observed compaction on aquifer storativity.