Joint Inversion of GNSS and GRACE for Terrestrial Water Storage Change in...

Carlson, G., S. Werth, and M. Shirzaei (2022), Joint Inversion of GNSS and GRACE for Terrestrial Water Storage Change in California, J. Geophys. Res., 127.
Abstract: 

Global Navigation Satellite System (GNSS) vertical displacements measuring the elastic response of Earth's crust to changes in hydrologic mass have been used to produce terrestrial water storage change (∆TWS) estimates for studying both annual ∆TWS as well as multi-year trends. However, these estimates require a high observation station density and minimal contamination by nonhydrologic deformation sources. The Gravity Recovery and Climate Experiment (GRACE) is another satellite-based measurement system that can be used to measure regional TWS fluctuations. The satellites provide highly accurate ∆TWS estimates with global coverage but have a low spatial resolution of ∼400 km. Here, we put forward the mathematical framework for a joint inversion of GNSS vertical displacement time series with GRACE ∆TWS to produce more accurate spatiotemporal maps of ∆TWS, accounting for the observation errors, data gaps, and nonhydrologic signals. We aim to utilize the regional sensitivity to ∆TWS provided by GRACE mascon solutions with higher spatial resolution provided by GNSS observations. Our approach utilizes a continuous wavelet transform to decompose signals into their building blocks and separately invert for long-term and short-term mass variations. This allows us to preserve trends, annual, interannual, and multi-year changes in TWS that were previously challenging to capture by satellite-based measurement systems or hydrological models, alone. We focus our study in California, USA, which has a dense GNSS network and where recurrent, intense droughts put pressure on freshwater supplies. We highlight the advantages of our joint inversion results for a tectonically active study region by comparing them against inversion results that use only GNSS vertical deformation as well as with maps of ∆TWS from hydrological models and other GRACE solutions. We find that our joint inversion framework results in a solution that is regionally consistent with the GRACE ∆TWS solutions at different temporal scales but has an increased spatial resolution that allows us to differentiate between regions of high and low mass change better than using GRACE alone. Plain Language Summary Fluctuations in surface water mass cause deformation of the Earth's surface and small changes to the Earth's gravity field. Both of these effects can be observed by satellite-based observation systems and used to solve for terrestrial water storage change (∆TWS), which is the sum of the changes in groundwater, surface water, soil moisture, ice, and snow over the land surface. These observations are essential for drought monitoring and can be used to inform water management decisions. In this study, we combine deformation data with gravity-derived ∆TWS to solve for monthly ∆TWS over California and western Nevada, USA. Here, multiyear droughts cause sharp declines in TWS and risk the future productivity of the water-intensive agricultural sector.

Research Program: 
Earth Surface & Interior Program (ESI)
Mission: 
GRACE
Funding Sources: 
This study was funded by the National Aeronautics and Space Administration grants NNX17AD98G, 80NSSC20K0735, 80NSSC21K0061, 80NSSC21K0419, (Susanna Werth and Grace Carlson), and NNH19ZDA001N-FINESST (Grace Carlson). The National Science Foundation grant EAR-1735630 supported Manoochehr Shirzaei.