Satellite gravity maps pinpoint where humans shift Earth's water

Two satellites fly in tandem over the planet. When the lead one passes over ground heavier with water, its pull shifts the distance between the pair by a sliver, and instruments measure it. For more than two decades, NASA has recorded those slivers. A new study turns them into a map of where human activity is reshaping how water moves through lakes, rivers, soil and the subsurface.

The data come from the GRACE missions, short for Gravity Recovery and Climate Experiment, and their follow-on, GRACE-FO. Their routine monthly maps show only broad patterns. Because the satellites passed over most of the planet many times, the team combined all those passes and mapped long-term trends with five to 10 times more spatial detail than previous analyses. The price was less detail in time. Statistical methods then weighed human factors such as irrigation, reservoir filling and land-use change against climate.

The result identifies 40 regions where changes in water storage line up with human activity, on every nonpolar continent except Australia. In four continental regions, a human-driven factor tracked the storage trend more closely than any climate factor did. California's Central Valley, the southern High Plains of the United States and Chihuahua in Mexico show groundwater-irrigated farmland linked to declining storage. In northern India, groundwater pumping drives a steeper decline than earlier estimates suggested, and retreating Himalayan glaciers account for only 15 percent of it.

Not every human footprint is a loss. In parts of sub-Saharan Africa, water tables rose despite below-average precipitation where native savanna was replaced by millet fields. A similar pattern appeared in northern Brazil's Maranhão state, where soybean fields have replaced forests and shrubland. The researchers said this might be explained in part by the efficient water use of cereal and legume plants, and by Indigenous practices that direct water into shallow aquifers. Large reservoirs also appear on the map: the Boguchany Reservoir on Russia's Angara River, the second-largest reservoir filled in the 21st century, produced the strongest storage trend in nonpolar northern Asia.

What changes for those who manage water is the question of where to look. A map at this resolution shows which aquifers are being drawn down, where, and which practices seem to refill them. It does not prescribe a policy, and it cannot say week by week what is happening, because it gave up time detail to gain space detail. For a farmer in Chihuahua or a planner in northern India, the question moves from whether there is a problem to where exactly it sits, and how far it has gone.