GRACE / GRACE-FO Mascons

GRACE (2002-2017) and its follow-on GRACE-FO (2018-present) measure minute changes in Earth's gravity field to derive monthly terrestrial water storage anomalies — groundwater, soil moisture, snow and surface water combined — at basin scale (typically a few hundred km), distributed by NASA JPL as mascon solutions through PO.DAAC.

Because gravity responds to total mass regardless of depth, GRACE is the only satellite technique that "sees" deep groundwater directly — it is how the depletion of aquifers like California's Central Valley, India's Indus basin and the Middle East's aquifers was first quantified at scale. Note the 2017-2018 gap between missions and the coarse spatial resolution: this is a regional water-budget tool, not a well-scale one.