Aquifers are underground water-bearing rock layers that store the fresh water humans access through wells and springs. We have long depended on aquifers as sources of fresh water for drinking, irrigation, and sanitation. As global populations grow and societies strive to improve infrastructure and food security, our need for clean groundwater continues to increase. 

The Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-on (GRACE-FO) missions measure changes in groundwater due to consumption and recharging. These data help researchers and resource managers track the water levels and internal dynamics of aquifers. 

Each of the GRACE missions consist of a pair of satellites that orbit in tandem and track each other with microwave signals. By tracking minuscule differences in the speed and distance between the two satellites, scientists can detect tiny fluctuations in Earth's gravitational field. These minor changes in gravity reflect changes in the masses of water beneath the planet's surface, so researchers use GRACE and GRACE-FO data to derive maps of groundwater stored in aquifers. NASA shares these findings as monthly mass grids and land water content maps.