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The Human Fingerprint on Earth’s Water Cycle
Humans are exerting increasing influence over how water moves through lakes, rivers, soil, and the subsurface, and a new NASA study shows where those effects are greatest. Using more than two decades of satellite measurements of subtle changes in Earth’s gravity, the researchers sharpened the picture of how—and why—terrestrial water storage has changed.
This map encapsulates their findings, depicting changes in terrestrial water storage from April 2002 to November 2025. Beyond the influence of climate variability, the scientists identified 40 regions where the changes also aligned with human activities such as irrigation, reservoir impoundment, and land-use change. These activities were significantly related to terrestrial water storage trends on every nonpolar continent except Australia.
In some areas, the effects of human activities were especially strong. “In four continental regions, an anthropogenic variable showed a stronger relationship with the storage trends than any climate variable did,” said Mary Michael O’Neill, a hydrologist at NASA’s Goddard Space Flight Center and lead author of the study.
They include well-known spots such as California’s Central Valley and northern India, where farmers’ heavy reliance on groundwater has drawn down storage—in India’s case, faster than earlier estimates suggested. Elsewhere, some types of farming and water management appear to have done the opposite.
Gravity as a Water Gauge
The data behind the map come from the GRACE (Gravity Recovery and Climate Experiment) and GRACE-FO (GRACE Follow-On) missions, each of which consists of two satellites that fly in tandem. As the lead satellite passes over an area with greater mass, the slight change in gravity tugs at it, and instruments aboard the satellites measure the change in distance between them. Since water and ice are heavy and shift around regularly, scientists can use GRACE data to infer gains and losses.
The satellites’ data are typically processed into monthly maps that show broad patterns of water storage. But over more than 20 years, the satellites have passed over most of the planet many times. By combining all of those passes, the researchers could map long-term water trends with five to 10 times more spatial detail than previous analyses. The tradeoff was a loss of temporal detail. They then applied statistical techniques to weigh the importance of various anthropogenic and climatic factors in the storage trends and to pinpoint where human activities may have contributed.
The improved spatial resolution uncovered some human impacts that had not been previously detected, said Matthew Rodell, deputy director of Earth Sciences for Hydrosphere, Biosphere, and Geophysics at Goddard and one of the study’s authors. The analysis also showed that some known trends were stronger than previously realized.
Where the Water Went
Across several continents, groundwater-irrigated farmland was strongly associated with declines in terrestrial water storage. In North America, these declines appeared in California’s Central Valley, the southern High Plains of the U.S., and Chihuahua, Mexico, where long-term droughts have increased farmers’ reliance on groundwater. Groundwater pumping also drove declines in northern India, where the refined analysis showed a steeper downward trend than previously realized. Consistent with previous findings, retreating Himalayan glaciers accounted for only 15 percent of the decline there.
Other types of agriculture had a different relationship with water storage trends. In parts of sub-Saharan Africa, for example, some increases occurred where rainfed cropland has expanded. There, swaths of native savanna were replaced by millet cultivation throughout the study period, and water tables rose despite below-average precipitation. A similar pattern emerged in northern Brazil’s Maranhão state, where soybean fields have replaced forests and shrublands. The resulting water storage trend might be explained in part by the efficient water use of cereal and legume plants and by Indigenous practices that help direct water into shallow aquifers, the researchers said.
Water storage and diversion projects also came into focus on the new map. Hundreds of large reservoirs were filled during the study period, the authors note, but the coarse spatial resolution of gravity measurements previously made them difficult to track. In the new analysis, the Boguchany Reservoir on Russia’s Angara River, the second-largest reservoir filled in the 21st century, produced the strongest water storage trend in nonpolar northern Asia.
The map also resolves smaller reservoirs and other hydrologic engineering projects. The Karakum Desert in Turkmenistan registered water storage gains stemming from the long, largely unlined Karakum Canal running through it. The canal diverts river water to support desert agriculture, but enough seeps into the ground during its journey to alter the water picture in the desert. The map’s finer resolution allows for better monitoring of such hydrologic changes in remote areas where other measurement options may be limited.
Moving forward, O’Neill wants to capture how dams, diversions, irrigation, and other controls on Earth’s water affect storage downstream, and to disentangle the influence of local and non-local activities. “I’d also like to recover more of the temporal picture,” she said. One way to do that is by combining the higher-resolution data with standard monthly products or by applying the recent technique over shorter timeframes. “This could help track how localized trends evolve in response to specific events like droughts or policy changes.”
NASA Earth Observatory map by Michala Garrison, using data from O’Neill, M.M., et al. (2026). Astronaut photograph ISS065-E-111045 was acquired on June 13, 2021, with a Nikon D5 digital camera using a focal length of 1,150 millimeters. It is provided by the ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit at NASA Johnson Space Center. The image was taken by a member of the Expedition 65 crew. The image has been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Story by Lindsey Doermann.
References & Resources
- NASA Earth Observatory (2018, May 23) Twin Satellites Map 14 Years of Freshwater Changes. Accessed October 19, 2026.
- O’Neill, M.M., et al. (2026) Spatially refined satellite gravimetry captures human signatures in global terrestrial water storage trends. Proceedings of the National Academy of Sciences, 123 (38) e2600775123.
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