The Colorado Basin's Hidden Water Is Draining, and the Upper Half Lost the Most

Satellite measurements of Earth's gravity show the Upper Colorado River Basin lost more groundwater than the Lower basin during the drought of 2020 to 2022. E. O. Oyedele of Virginia Tech's Department of Geosciences and colleagues put the Upper basin's loss at 17.5 ± 3.5 cubic kilometers (km³), roughly 4.6 trillion gallons, against 9.4 ± 6.5 km³ in the Lower basin. The authors describe the Upper basin as normally the more resilient of the two.

The study, published Oct. 13, 2026, in Water Resources Research, covers water years 2004 to 2023 and finds groundwater draining across the whole basin over those two decades, with losses speeding up after 2019. The authors say the results argue for water management that treats groundwater and surface water as one system, under both climate pressure and steady human demand.

None of this is visible in the readings the basin's water supply is usually judged by. River flow and reservoir levels are gauged directly; groundwater is not. The team worked instead from the GRACE and GRACE-FO satellites, which track small month-to-month changes in Earth's gravity field as water enters and leaves the ground, together with hydrological data sets.

The uncertainty on the Lower basin's figure is large next to the number itself, so the gap between the two basins is not a fixed ratio.

One part of the analysis compared how surface-water shortages and groundwater drought move together. In the Upper basin the two are closely linked. In parts of the Lower basin the link is weaker, which the authors say is consistent with groundwater pumping that operates partly independently of how much surface water is available, under a chronically limited groundwater supply.

The team is based at Virginia Tech, Indiana University Bloomington and the United Nations University Institute for Water, Environment and Health. NASA funded the work, and the paper is open access.