Measuring drought impacts using a hybrid GNSS, InSAR, and GRACE joint inversion approach over California’s Central Valley

California commonly experiences multi-year droughts, which are intensified due to groundwater pumping in the Central Valley, a region of extensive farmland relying heavily on irrigation to grow crops. These large water loss signals cause surface deformation and gravity variations measurable from space that have been the focus of numerous hydro-geodetic studies. Studies of surface deformation in the region dominantly either focus on the elastic loading and unloading response of the land surface to fluctuations in water mass, or alternatively, on aquifer system deformation driven by groundwater pumping. Because these two deformation signals are opposite in sign, there is an outstanding challenge to cohesively combine these processes in order to accurately assess changes in water storage at resolutions and uncertainties sufficient for water management applications.

Here, we present a unique joint inversion approach integrating observations of surface deformation from GNSS and InSAR that does not require the separation of elastic loading and poromechanical aquifer deformation. Instead, our approach aims to identify a best-fitting solution consistent with both overlapping processes to simultaneously solve for the groundwater storage and total terrestrial water storage (TWS) loss during the drought years of 2020 and 2021 in California. Our inversion approach is further constrained with large-scale terrestrial water storage anomalies observed by the satellite gravimetry mission GRACE- follow on (GRACE-FO). Results from our inversion show that we can achieve a high-resolution and more realistic estimate of TWS loss within the Central Valley than an inversion of GRACE-FO and GNSS elastic loading displacements provide, alone. Results also reveal a groundwater volume loss of 20.4 ± 2.6 km³ in the semi-confined to confined portion of the Central Valley aquifer-system, which agrees well with a conventional GRACE-FO-derived groundwater loss (27.7 ± 5.3 km³) when considering underlying processes and uncertainties. This work reveals the potential of geodetic observations in hydro-hazards research and shows that by integrating multiple measurement systems, we can isolate storage components, like groundwater, that are notoriously challenging to separate from other dynamics, providing  insights into hydrologic processes and anthropogenic impacts at a regional scale.