The Gravity Recovery and Climate Experiment (GRACE) and its successor, GRACE-Follow On (GRACE-FO), monitor the Earth’s global water cycle from space. These missions use a highly sensitive technique to detect minute changes in mass distribution across the planet, rather than relying on traditional imaging. By tracking these subtle mass shifts, scientists gain insight into the movement and storage of water, including underground aquifers. The data provides a continuous, long-term record of how water is being redistributed globally.
The GRACE Mission and Measurement Principle
The GRACE and GRACE-FO missions operate using a pair of identical satellites orbiting the Earth approximately 137 miles (220 km) apart in the same orbital plane. The core measurement technique involves precisely tracking the distance between these two satellites (leading and trailing). They communicate using a microwave ranging system (K-band) sensitive enough to measure changes in separation distance down to about 10 micrometers.
When the leading satellite flies over a region with greater mass concentration, such as an aquifer, the local gravitational pull slightly increases its speed. This acceleration causes the distance between the two satellites to momentarily increase. As the trailing satellite passes over the same mass anomaly, it also accelerates, shrinking the gap between them. This precise measurement of the change in inter-satellite distance maps changes in the Earth’s gravitational field.
Connecting Mass Change to Gravity
The variations in the distance between the satellites are a direct consequence of minute changes in the Earth’s gravitational pull, determined by the distribution of mass beneath the spacecraft. Scientists translate the measured distance changes into maps showing monthly variations in gravity across the globe. Since water has mass, the movement of large volumes of water results in a measurable shift in the local gravity field.
This gravity data is expressed as a measure of Terrestrial Water Storage (TWS) change, representing the total variable mass of water on and beneath the land surface. TWS is a composite measurement that includes water stored in snow and ice, surface water bodies, soil moisture, and groundwater stored in deep aquifers. The GRACE missions measure the total change in this storage and cannot inherently distinguish between these different water components.
Isolating Groundwater from Total Water Storage
Finding groundwater aquifers with GRACE data is not a direct measurement but a complex calculation using the total TWS change. Since the satellite measures the sum of all water mass changes, scientists must isolate the groundwater component by mathematically subtracting the mass changes of the other known water reservoirs. This is achieved using the hydrologic budget equation: Groundwater Change = TWS Change – (Snow/Ice Change + Surface Water Change + Soil Moisture Change).
The subtraction relies heavily on independent hydrological models and auxiliary data sets, such as those provided by the Global Land Data Assimilation System (GLDAS). These models estimate the changes in non-groundwater components, allowing their mass effect to be removed from the total GRACE signal. Inaccuracies in these independent models directly affect the final estimate of groundwater change, making this modeling and subtraction step the primary source of uncertainty. By performing this subtraction, scientists identify long-term trends of aquifer depletion or recharge, even though the satellite’s native resolution covers areas roughly 300 to 400 kilometers wide.
Why Satellite Gravity is Essential for Aquifer Monitoring
Satellite-based gravity measurements are an indispensable tool for global water resource management because they offer the only reliable way to monitor large-scale aquifer changes globally. Ground-based monitoring, while accurate for a single point, is often impractical or impossible in remote regions or across international boundaries. GRACE data fills this gap by providing a consistent, continuous, and integrated view of water storage trends over vast areas.
This data is used for identifying regions under severe water stress, such as major aquifers in northern India or California’s Central Valley. Water resource managers and policy makers use these long-term trends to track unsustainable withdrawal rates and inform decisions about water allocation and conservation. Monitoring these resources from space allows for a better understanding of how climate change and human activities are affecting the planet’s underground water supply.