GRACE Follow-On: weigh changes in the water cycle

GRACE Follow-On, launched in May 2018, continues a method developed by the original GRACE mission. NASA and Germany’s GFZ partnered on two spacecraft that follow one another around Earth. Changing distributions of mass slightly alter their motion. Precise measurements of their separation help scientists reconstruct changes in Earth’s gravity field and investigate where water is being stored or lost.

This is a different view from a camera photograph of a reservoir or a radar measurement of its surface height. Gravity responds to mass, including water beneath the surface. The mission therefore provides information that is difficult to obtain from visible imagery, while its regional measurements cannot identify the water in one household well.

Microwave ranging measures changes in the distance between the spacecraft; GRACE-FO also carried a laser ranging interferometer to demonstrate a more precise approach. Orbit information and measurements of nongravitational forces help the processing distinguish gravitational effects from other influences on the spacecraft. The result requires a coordinated observing and analysis system, not simply two satellites flying close together.

Over land, the water-storage signal combines contributions from groundwater, soil moisture, snow and surface water. To estimate groundwater change, researchers must account for the other components using additional observations or models. A displayed water-storage map should retain its reference period, units and uncertainty. A negative anomaly means less water than that reference, not that a region contains negative water.

Consider a fictional basin where total water storage decreases by 12 units over a month. Other evidence estimates that snow and soil moisture together decrease by 5 units, with surface-water change negligible. The remaining 7-unit decrease could be attributed to groundwater under those assumptions. It is an estimate whose quality depends on both the gravity result and the component corrections.

Now suppose snow information is unreliable. The total-storage decline can remain supported while the groundwater partition becomes uncertain. A responsible interpretation reports those different confidence levels. GRACE-FO is especially useful when combined with precipitation, river, reservoir and well measurements; agreement between independent methods can strengthen the explanation of a regional change.