Improving groundwater loss estimates using a combination of GNSS, GRACE-FO, and InSAR: Case study of California’s recent 2020-2021 drought

Over the last two decades, California, USA has undergone three multi-year intense periods of drought, with the most recent still ongoing as of August 2022. Low precipitation and unusually warm temperatures, yielding high evapotranspiration rates, low snowpack, and early snowmelt, have led to record low surface water levels in reservoirs across the state and an increased reliance on groundwater resources, resulting in widespread groundwater overdraft. Extreme states in the water cycle such as drought can be measured directly and indirectly using several geodetic remote sensing tools, including observations from the GRACE and GRACE-FO satellites and deformation recorded by Global Navigation Satellite System (GNSS) stations measuring the elastic response of Earth’s crust to changes in mass loading. In California’s Central Valley, a large, elongate sedimentary basin between the Coastal Ranges and Sierra Nevada Mountains, groundwater overdraft during drought has caused widespread subsidence. This subsidence is a poroelastic response to aquifer overdraft and has been observed using interferometric synthetic aperture radar (InSAR) and GNSS station displacements. This presentation proposes a unified geophysical model incorporating observations from GRACE, which provides regional closure of the water budget at impressive accuracy but a low spatial resolution, and indirect measurements of poroelastic and elastic deformation from GNSS and InSAR, to derive improved estimates of terrestrial water storage change (∆TWS) at a higher spatial resolution. Using a joint inversion framework that combines GRACE-FO ∆TWS with elastic deformation from GNSS station displacements and InSAR-derived vertical land motion showing poroelastic aquifer deformation over the Central Valley aquifer, we produce high-resolution maps of ∆TWS over California and Nevada and groundwater loss over the Central Valley during the 2020-2021 drought period. We find that the largest water loss occurs over the southern Central Valley, with groundwater loss of more than two meters equivalent water height. Outside of the southern Central Valley, we find that the largest TWS declines occur in the northern Sierra Nevada Mountains and northern Central Valley, with ∆TWS of ~0.5-1 meter equivalent water height. We also show that groundwater loss estimates determined using our joint inversion framework are larger, but mainly in agreement with GRACE-derived groundwater loss estimates when considering underlying processes and uncertainties.