Terrestrial water storage (TWS) is water held on land, both underground and at the surface. In a paper published online September 14, 2026, researchers using NASA’s GRACE and GRACE-FO satellite observations identified 40 regions where TWS trends were significantly associated with human land and water use, out of 94 regions with significant trends. The analysis found cumulative losses as well as gains across those 40 regions; it tracked more than groundwater alone. The study, by M. M. O’Neill, M. Rodell and B. D. Loomis, covers global, nonpolar observations from April 2002 through November 2025.
What GRACE measures
GRACE and its successor, GRACE-FO, use satellite-to-satellite ranging to track changes in Earth’s gravity field. Because changes in the distribution of water and other mass affect gravity, researchers can use those observations to infer shifts in terrestrial water storage.
The study analyzed Level-1B intersatellite range-rate observations with a stacked regression method. Its refined trend map has a spatial resolution of about 12,500 km²—five to ten times finer than traditional monthly-mascon maps. That gives researchers a more detailed view of regional patterns; it does not turn the result into a reading of groundwater levels at individual wells.
The 40 regions include both losses and gains
The researchers selected the 40 regions for significant spatial associations between TWS trends and human activity, including groundwater and surface-water irrigation, rainfed agriculture, deforestation and reservoir impoundment. The analysis also considered climate factors such as precipitation, vapor-pressure-deficit trends and anomalies, and glacial mass loss.
Across the 40 regions, cumulative TWS losses were 3,122 gigatons and cumulative gains were 2,432 gigatons over the study period. Among the regional estimates highlighted in the paper, TWS declined by 6.8 ± 1.3 gigatons per year in the southwestern United States, centered on California’s Central Valley. The paper also reports estimated depletion of 42.0 ± 7.7 gigatons per year in the Middle East and North Africa, and a decrease of 24.3 ± 2.5 gigatons per year in the Brazilian Highlands.
The study found significant human influence on the spatial variation in TWS trends on every nonpolar continent except Australia. That finding describes regional statistical associations, not a single cause for every trend. The authors also note that local associations may miss effects such as upstream dams, river diversions, connected aquifers and land–atmosphere feedbacks.
What the 33%, 52% and 45% figures compare
The percentages refer to different comparisons. The first two compare trend magnitudes from the refined approach with traditional monthly-mascon regression; the third compares the study’s estimates with previously published estimates of TWS declines associated with groundwater pumping.
| Comparison | Reported result | Baseline |
| Average regional trend magnitude | 33% larger | Traditional monthly-mascon regression |
| Trend magnitude in seven regions smaller than 100,000 km² | 52% larger | Traditional monthly-mascon regression |
| Previously published TWS declines associated with groundwater pumping | 45% lower on average | The study’s estimates |
The regional estimates highlighted in the paper had reported uncertainty averaging ±28% of their trend magnitudes. That uncertainty accompanies the satellite-derived regional estimates, which describe water stored across land and underground rather than groundwater levels at specific wells.
