- The Upper Colorado River Basin lost an estimated 4.6 trillion gallons of groundwater during the 2020–2022 drought.
- Groundwater losses in the Upper Basin were nearly twice those in the Lower Basin.
- Researchers found that groundwater depletion accelerated after 2019.
- Surface water shortages and groundwater losses are closely connected in the Upper Basin.
- Scientists warn that declining groundwater reserves could further reduce water flowing into rivers.
Wednesday, October 14, 2026 — Water shortages in the Colorado River Basin are usually measured by what can be seen. Lake Mead and Lake Powell have fallen dramatically during the long-running drought, and reduced river flows have forced difficult decisions about water supplies.
But another water shortage is taking place beneath the ground.
A new study from Virginia Tech
finds that groundwater reserves across the Colorado River Basin have been shrinking for two decades. More surprisingly, some of the fastest losses are now occurring in the Upper Basin, an area traditionally considered better protected against drought.
The research, published in the scientific journal Water Resources Research and announced October 13, 2026, examined changes in groundwater storage from 2004 through 2023.
The findings reveal a shift in how drought affects underground water supplies in the American West.
Upper Basin Groundwater Losses Nearly Double.
The Colorado River Basin is divided into two major regions.
The Upper Basin includes portions of Colorado, Wyoming, Utah, and New Mexico. Mountain snow and rainfall help replenish its rivers and underground water supplies.
The Lower Basin includes portions of Arizona, Nevada, and California. Its hotter, drier climate makes replenishing groundwater more difficult.
Historically, the Lower Basin has experienced greater groundwater depletion. Researchers expected the Upper Basin to be more resistant to prolonged drought because melting mountain snow helps replace some of the water stored underground.
The latest findings challenge that expectation.
During the severe drought of 2020 through 2022, researchers estimated that:
- The Upper Basin lost approximately 4.6 trillion gallons of groundwater.
- The Lower Basin lost approximately 2.5 trillion gallons.
- The combined loss across the Colorado River Basin was approximately 7.2 trillion gallons.
These figures are estimates based on satellite observations and water-storage data, rather than direct measurements of every underground water source.
The Upper Basin’s losses were particularly striking because its groundwater reserves have historically benefited from greater snowfall and opportunities for natural replenishment.
“We generally think of the Upper Basin as more resilient because of its snowmelt-driven recharge,” said lead author Esther Oyedele, a geosciences graduate student at Virginia Tech.
“I did not expect to see so much groundwater loss there during the recent drought. What surprised me even more was that groundwater did not track surface-water shortages in the same way in the Upper and Lower Basins.”
How Satellites Measure Water Underground.
Groundwater is stored beneath the Earth’s surface in layers of rock, sand, and sediment known as aquifers.
Unlike lakes and reservoirs, these underground supplies cannot be easily measured from the surface. Monitoring wells provide valuable information, but they are scattered across a vast region.
The Virginia Tech researchers used an unusual tool to study the problem: satellites that measure tiny changes in Earth’s gravity.
NASA’s Gravity Recovery and Climate Experiment missions detect changes in the amount of water stored on and beneath the land.
Water has weight. When a region gains or loses large amounts of water, its gravitational pull changes slightly. Satellites can detect those differences.
However, the satellites do not measure groundwater alone.
To estimate underground water storage, researchers combined the satellite observations with information about water held in reservoirs, snow, and soil.
By accounting for those other sources, the team estimated how much groundwater remained and how those supplies changed over time.
The study also examined groundwater monitoring wells, although the researchers found that monitoring sites were especially limited in the Upper Basin.
This combination of satellite observations, ground measurements, and computer models allowed scientists to examine water losses across the entire Colorado River Basin.
Two Regions, Two Different Patterns.
One of the study’s most important findings involves the relationship between groundwater and surface water.
In the Upper Basin, groundwater losses closely followed shortages in rivers, lakes, and reservoirs.
When surface water supplies declined, underground water reserves also tended to shrink.
The researchers used statistical models to estimate the likelihood of groundwater drought under different conditions. In the Upper Basin, they found that the probability of groundwater drought exceeded 60% when surface water shortages reached moderate levels.
The Lower Basin showed a different pattern.
There, groundwater supplies sometimes declined before surface water shortages became apparent.
Researchers found that groundwater losses in parts of the Lower Basin were less closely connected to changes in rivers and reservoirs.
They suggested that continued groundwater pumping for farms, cities and other uses may help explain the difference.
In other words, groundwater depletion in the Lower Basin does not necessarily require a new surface-water shortage. Underground reserves may continue declining because of existing water demands.
The researchers cautioned that their statistical analysis identified relationships between water shortages, but did not establish that one condition directly caused another.
Arizona Groundwater Losses Vary by Region.
The study also examined groundwater conditions in several of Arizona’s Active Management Areas, where the state regulates groundwater use.
Researchers compared changes in surface water supplies with groundwater measurements in Phoenix, Pinal, Prescott, Tucson and Santa Cruz.
The results showed considerable differences.
Phoenix and Pinal experienced some of the largest groundwater losses among the regulated areas examined.
During the 2020–2022 drought period, researchers estimated groundwater losses of approximately:
- 211 billion gallons in the Phoenix Active Management Area.
- 238 billion gallons in the Pinal Active Management Area.
- 53 billion gallons in the Prescott Active Management Area.
Phoenix and Pinal also experienced substantial reductions in surface water supplies.
However, not every area lost groundwater.
The Tucson Active Management Area gained an estimated 53 billion gallons of groundwater, while the Santa Cruz area recorded a smaller increase of approximately 8 billion gallons.
Researchers noted that Tucson’s groundwater gains may reflect several factors, including the use of imported Colorado River water, groundwater replenishment programs, conservation measures, and reduced pumping.
The study also examined portions of Arizona outside designated groundwater management areas.
The southern region outside those regulated areas experienced an estimated groundwater loss of 1.3 trillion gallons. The northern region showed a smaller groundwater gain.
These findings illustrate the different groundwater conditions observed across Arizona during the same drought period.
A Wet Year Does Not Erase Years of Losses.
The researchers found that groundwater supplies across the Colorado River Basin have declined repeatedly since 2004.
The losses became especially pronounced after 2012 and accelerated during the drought that began in 2020.
There were periods of improvement.
The study identified wetter conditions in 2008, 2016, 2019, and 2023. Groundwater storage also showed a noticeable recovery during the 2023 water year.
However, those recovery periods did not fully replace the water lost during earlier droughts.
Across the entire Colorado River Basin, the researchers estimated that total water storage declined by approximately 15.4 trillion gallons between the 2004 and 2023 water years.
That figure includes water stored underground, in soil, in snow and in surface reservoirs.
Groundwater alone accounted for an estimated 6.7 trillion gallons of net losses over the full study period.
The researchers also found that groundwater losses during the shorter 2012–2023 period exceeded the net losses measured across the entire 2004–2023 study period. That difference reflects periods of groundwater recovery earlier in the record.
These estimates describe conditions through September 2023, not groundwater levels in October 2026.
Why Groundwater Matters to the Colorado River.
Groundwater does more than supply wells.
In parts of the Colorado River Basin, underground water slowly moves into streams and rivers. This helps maintain river flows during dry periods when rainfall and snowmelt are limited.
Scientists call this contribution baseflow.
When underground water supplies decline, less groundwater may be available to support those streams.
That creates another concern for the Upper Basin, where groundwater and surface water are closely connected.
“Continued groundwater depletion, especially in the Upper Basin, could further reduce groundwater contributions to rivers and intensify the feedback between groundwater loss and surface-water shortages,” said Susanna Werth, an assistant professor of hydrology and remote sensing at Virginia Tech.
The study found that prolonged drought, warmer temperatures and reduced snowpack were associated with declining groundwater supplies in the Upper Basin.
Researchers also noted that groundwater can continue draining into streams during dry conditions, further reducing underground storage.
What the Study Cannot Tell Scientists.
Although satellite observations provide a broad picture of groundwater conditions, the researchers identified several limitations.
The satellites measure water changes across large geographic areas, making it difficult to identify conditions in individual communities or smaller groundwater basins.
The estimates also depend partly on computer models of soil moisture and snow storage, which carry uncertainties.
Groundwater monitoring wells are particularly scarce in the Upper Basin, limiting opportunities to compare satellite estimates with measurements taken directly underground.
The researchers also did not separately account for every change in land use or water policy that could influence groundwater pumping.
As a result, the study does not establish exactly how much groundwater loss was caused by drought compared with human water use.
Additional monitoring and more detailed regional studies will be needed to determine whether the recent changes represent a lasting shift in groundwater conditions.
Managing Water Above and Below Ground.
The Colorado River Basin supplies water to roughly 40 million people and supports extensive agricultural operations.
As surface water supplies become less reliable, groundwater remains an important water source for communities, farms, and industries.
But the new research shows that underground reserves are also vulnerable to prolonged drought and continued water demand.
The findings suggest that the Upper and Lower Basins face different groundwater challenges.
In the Upper Basin, underground water supplies are strongly connected to drought-related surface water shortages. In the Lower Basin, sustained groundwater use may continue reducing underground reserves even when surface water conditions do not change substantially.
Werth said the findings demonstrate why monitoring rivers and reservoirs alone cannot provide a complete picture of the region’s water supplies.
The researchers emphasized the importance of considering groundwater and surface water together when evaluating the Colorado River Basin’s long-term water needs.
Citation.
Oyedele, E. O., Werth, S., & Carlson, G. (2026). Contrasting Groundwater Response to Surface Water Shortages in the Upper and Lower Colorado River Basins. Water Resources Research, 62, e2025WR041695. American Geophysical Union. DOI: 10.1029/2025WR041695.
