The Gravity Recovery and Climate Experiment satellite mission that measures changes in Earth's gravity field to detect large-scale variations in water storage. GRACE data reveal groundwater depletion, ice sheet mass loss, and continental water balance changes.
GRACE (Gravity Recovery and Climate Experiment) was a joint NASA/DLR satellite mission launched in 2002, succeeded by GRACE-FO (Follow-On) in 2018. The mission consists of twin satellites flying approximately 220 km apart in the same orbit, continuously measuring the distance between them using microwave ranging. Variations in Earth's gravity field cause minute changes in this inter-satellite distance, which can be inverted to map monthly changes in total water storage (TWS) including groundwater, soil moisture, surface water, snow, and ice. GRACE has provided unprecedented evidence of groundwater depletion in major aquifers worldwide, including the Ogallala, Central Valley (California), North India, and Middle East aquifer systems. The spatial resolution is coarse (approximately 300-400 km), limiting applications to regional and continental scales. Despite this limitation, GRACE is the only observational tool capable of measuring changes in total terrestrial water storage from space, making it invaluable for closing the water balance equation and validating hydrological models. GRACE data are distributed as spherical harmonic coefficients or gridded mass anomaly products (mascons) by NASA JPL, CSR, and GFZ.