NASA’s GRACE satellites measure changes in Earth’s gravity field to track groundwater depletion, aquifer dynamics, and river discharge. By monitoring large-scale water storage shifts, GRACE provides crucial data for water management and predicting climate-related impacts like floods and droughts.
The health of our planet’s water systems is critical for sustaining life, managing agriculture, and ensuring access to clean water. Monitoring changes in water storage, whether in aquifers deep underground or river systems that provide fresh water, is challenging—especially at large scales. However, NASA’s GRACE (Gravity Recovery and Climate Experiment) satellites have revolutionized how scientists measure and monitor these water resources from space. Using advanced gravity measurements, GRACE provides unprecedented insights into the dynamics of groundwater, river discharge, and other components of the global water cycle.
Aquifers—vast underground reservoirs of freshwater—are a lifeline for many regions around the world. Yet, direct monitoring of groundwater levels is difficult and costly, particularly in developing regions or arid areas where water scarcity is a concern. This is where NASA’s GRACE mission, launched in 2002, plays a pivotal role.
GRACE detects changes in Earth’s gravity field, and since water has mass, shifts in groundwater levels alter the gravitational pull over specific regions. GRACE satellites, which fly about 220 kilometers apart, detect subtle changes in their distance from each other as they pass over areas where gravity has shifted due to groundwater depletion or recharge. By tracking these gravity anomalies over time, scientists can estimate the amount of water being lost or gained in aquifers, even at regional or continental scales.
For example, GRACE data was used to document significant groundwater depletion in northern India, where agricultural irrigation has heavily drawn down aquifer levels. According to a 2015 study published in Nature, India is losing groundwater at an alarming rate of 17.7 cubic kilometers annually due to over-extraction for farming . Similarly, in the western United States, particularly in California’s Central Valley, GRACE has revealed critical groundwater depletion due to prolonged drought and agricultural demands .
The follow-on mission, GRACE-FO (Gravity Recovery and Climate Experiment Follow-On), launched in 2018, has further enhanced this capability. GRACE-FO includes more advanced instruments that improve the accuracy of water storage measurements and enable continued monitoring of critical aquifers worldwide.
In addition to measuring aquifers, GRACE has proven useful in estimating river discharge. River discharge is the volume of water flowing out of a river system over time and is crucial for understanding freshwater availability, flood risks, and managing ecosystems. While GRACE cannot directly measure the flow of water in rivers, it can track changes in terrestrial water storage (TWS), which includes the water stored in rivers, lakes, snow, soil moisture, and groundwater. By measuring TWS, scientists can infer the water balance in a region, which is key to estimating how much water is flowing out of rivers.
For instance, researchers have successfully combined GRACE data with precipitation measurements and hydrological models to estimate discharge levels in large river systems such as the Amazon and the Ganges. A study published in the Journal of Hydrology in 2013 demonstrated how GRACE-derived water storage data was used to estimate discharge for the Amazon River Basin, where direct measurements are sparse due to its remote location . The findings showed that GRACE could help fill gaps in discharge data, allowing for more accurate predictions of water flow and potential flood events.
Furthermore, GRACE’s ability to track long-term changes in water storage allows researchers to observe patterns that contribute to flooding or droughts. For example, in 2011, GRACE data helped scientists predict the historic floods in the Mississippi River basin by revealing an unusually high amount of water storage in the surrounding regions.
Despite its successes, GRACE has some limitations. Its spatial resolution is relatively coarse, meaning it’s better suited for monitoring large regions, such as entire river basins or aquifer systems, rather than small-scale changes. To overcome these limitations, researchers often integrate GRACE data with other sources, such as ground-based river gauges, precipitation data, and other satellite missions like the Surface Water and Ocean Topography (SWOT) mission, which is a joint effort between NASA and CNES (the French Space Agency). SWOT directly measures river height and surface water levels, complementing GRACE’s ability to track water storage.
For example, in regions where river discharge data is lacking, combining GRACE’s water storage data with SWOT’s precise measurements of river surface height can provide a clearer picture of how much water is moving through a system. Such combined approaches are essential for managing water resources, predicting floods, and responding to climate change impacts on hydrological cycles.
The insights provided by GRACE have had significant real-world impacts. In addition to helping scientists track aquifer depletion in regions like India and California, GRACE data has informed water management policies. For example, following revelations of severe groundwater depletion in California, the state passed the Sustainable Groundwater Management Act in 2014, which requires local agencies to develop plans to manage groundwater sustainably . Similarly, in areas like the Middle East, where water scarcity is a geopolitical concern, GRACE has highlighted the urgent need for water conservation efforts.
GRACE data has also been widely covered in the media. In 2018, The New York Times reported on how GRACE measurements revealed that Cape Town, South Africa, was nearing “Day Zero”—when its water reservoirs would run dry . By providing a satellite-eye view of the city’s dwindling water resources, GRACE underscored the critical need for immediate action to conserve water.
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NASA’s GRACE satellites have transformed our ability to monitor and understand water dynamics on Earth. From tracking groundwater depletion in critical aquifers to estimating river discharge in remote regions, GRACE offers a unique and invaluable perspective on the global water cycle. As climate change intensifies droughts, floods, and water scarcity, the data provided by GRACE will continue to be essential for managing water resources and preparing for future challenges.
