Retrieving global surface soil moisture from GRACE satellite gravity data

Passive microwave radiometry from space through missions such as the Soil Moisture and Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) satellites is the most reliable means for mapping global surface soil moisture (SSM). Nonetheless, microwave SSM retrievals are uncertain over densely vegetated surfaces or areas with high radio frequency interference. This paper presents a new observationally driven approach to remote sensing of global SSM based on the terrestrial water storage anomaly (TWSA) data acquired from the Gravity Recovery and Climate Experiment (GRACE) satellite. This approach rests on a physically based, yet parsimonious, model based on the Richards’ equation and the assumption that the TWSA temporal rate of change (dS/dt) approximates the land surface net water flux (NWF) as the surface boundary condition. The GRACE-based SSM is found to be in a reasonable agreement with in-situ data and highly correlated with the SMAP and SMOS retrievals, especially over wet regions where the assumption of NWF ≈ dS/dt holds valid. The GRACE retrievals contain new SSM information relative to the microwave satellite data and provide a potential solution to improve the microwave data over densely vegetated surfaces or areas with high radio frequency interference.