Virginia Tech research (published in Water Resources Research) shows that recent severe drought in the Colorado River Basin has driven an unprecedented shift in groundwater depletion, with the historically more resilient Upper Basin now losing groundwater faster than the Lower Basin.

Declining river flows and falling levels in reservoirs like Lake Powell and Lake Mead are the visible signs of the long- running Southwestern megadrought. Underground, groundwater reserves- which supply a critical buffer for about 40 million people, agriculture, and ecosystems- are depleting rapidly, and the pattern has changed.

Historically, groundwater declined more in the Lower Basin (Arizona, southern Nevada, southeastern California). The system has now reached a turning point: the Upper Basin (Colorado, Wyoming, Utah, New Mexico) is experiencing rapid losses, with surface- water shortages increasingly affecting groundwater.

Researchers led by geosciences graduate student Esther Oyedele, working with assistant professor Susanna Werth and colleagues, isolated groundwater changes by combining NASA GRACE (Gravity Recovery and Climate Experiment) satellite measurements of total water storage with estimates of surface water, soil moisture, and snow-water storage for 2004- 2023.⁠

Key findings include:

• Acceleration of groundwater depletion after 2019.
• During the severe 2020- 2022 drought, the Upper Basin lost about 4.6 trillion gallons of groundwater, nearly double the Lower Basin’s estimated 2.5 trillion gallons.⁠

Oyedele noted: “We generally think of the Upper Basin as more resilient because of its snowmelt- driven recharge. 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.”

Groundwater and surface water are often connected; declining aquifers can reduce base flows into rivers, creating a feedback loop that worsens surface- water shortages. Continued depletion, especially in the Upper Basin, risks intensifying this cycle. Werth emphasized that managing surface water alone is no longer sufficient, protecting the basin requires treating water above and below ground as a connected resource.

The original study is available via DOI 10.1029/2025WR041695. The Virginia Tech news release by Kelly Izlar was published on October 13, 2026.

NASA’s GRACE (Gravity Recovery and Climate Experiment) satellites measure tiny changes in Earth’s gravity field to track how mass, especially water, moves around the planet.

How GRACE works

GRACE does not take traditional “pictures” of Earth with cameras or radars pointed downward. Instead, the two nearly identical satellites themselves form the instrument.

• The satellites fly in the same near- polar, low- Earth orbit (initially around 450- 500 km / 280, 310 miles altitude), one trailing the other by about 220 km (137 miles).
• As they orbit, local variations in Earth’s gravity (caused by concentrations of mass below) pull on the lead satellite first, slightly speeding it up and increasing the distance to the trailing satellite. Moments later, the trailing satellite feels the same pull and the gap closes again.
• A highly precise microwave ranging system (K/Ka-band) continuously measures these minuscule distance changes, to within about 1 micron (the diameter of a red blood cell, or roughly 1/100th the thickness of a human hair).
• GPS receivers determine each satellite’s exact position relative to Earth. Onboard accelerometers measure non- gravitational forces (atmospheric drag, solar radiation pressure, etc.) so scientists can isolate only the gravitational effects.
• Star trackers keep track of the satellites’ orientation.

From these data, scientists produce monthly maps of Earth’s gravity field. Differences between months reveal where mass has been added or removed.

What the measurements reveal

Because water has mass, changes in the amount of water stored on or below the surface alter local gravity. GRACE (and its successor GRACE- FO) can therefore detect:

• Groundwater depletion or recharge in aquifers
• Changes in soil moisture
• Snow and ice mass loss or gain (glaciers, ice sheets)
• Surface- water variations in large lakes, reservoirs, and rivers
• Ocean mass changes related to sea- level rise and currents

The original GRACE mission flew from 2002 to 2017. GRACE Follow-On (GRACE-FO) launched in 2018 and continues the record (with an added experimental laser ranging interferometer for even higher precision). Together they provide a continuous multi- decade view of terrestrial water storage changes.

Relevance to the Colorado River Basin study

In the recent Virginia Tech research, scientists used GRACE/GRACE -FO total water storage data and subtracted independent estimates of surface water, soil moisture, and snow water. The remaining signal isolates groundwater storage changes. This is how they quantified the unexpected rapid groundwater losses in the Upper Basin during the 2020- 2022 drought.

GRACE’s strength is its ability to “weigh” large regions from space, capturing the total water mass change even where ground- based well networks are sparse.

Its spatial resolution is relatively coarse (typically hundreds of kilometers), so it is best for basin- or regional -scale studies rather than individual wells.

Contrasting Groundwater Response to Surface Water Shortages in the Upper and Lower Colorado River Basins

Key findings from the abstract and plain- language summary

• The study analyzes basin-scale terrestrial water storage and groundwater storage in the Colorado River Basin (CRB) from water year 2004- 2023 using GRACE/GRACE-FO satellite observations combined with hydrological datasets.
• There has been sustained basin- wide groundwater depletion over two decades, with accelerated losses since 2019, especially during the severe 2020- 2022 drought.
• During that recent drought:
  • Upper Colorado River Basin (UCRB) lost 17.5 ± 3.5 km³ of groundwater.
  • Lower Colorado River Basin (LCRB) lost 9.4 ± 6.5 km³.
  • (These volumes convert to roughly 4.6 trillion gallons in the Upper Basin and 2.5 trillion gallons in the Lower Basin, matching the press release figures.)
• A copula- based statistical framework was used to quantify probabilistic relationships between storage- based drought indicators and to characterize groundwater- surface water interactions.
  • In the Upper Basin, there is strong coupling between surface- water deficits and groundwater drought probability (groundwater losses closely track surface- water shortages).
  • In parts of the Lower Basin, the statistical dependence is weaker, groundwater declines can occur somewhat independently of surface-water conditions, consistent with chronic human extraction under limited supply.

Broader context and implications

Groundwater is a critical buffer in the CRB, which supplies water to ~40 million people and extensive agriculture. The Upper Basin has traditionally been viewed as more resilient due to snowmelt-driven recharge, but the recent drought revealed greater vulnerability there. Continued depletion risks reducing groundwater contributions to river baseflow and intensifying feedback loops between groundwater loss and surface-water shortages.

The authors emphasize the value of satellite monitoring for large managed river basins and call for integrated water- management strategies that address both climatic pressures and sustained anthropogenic demand (above and below ground).

The full paper includes maps of the hydrogeological setting, GRACE mascon analysis, well data, and detailed statistical results. You can access the open- access version (including PDF download) directly at the DOI link above.

Journal: Water Resources Research,  (Volume 62, Issue 10)

DOI: 10.1029/2025wr041695

Provided: Virginia Tech

First published: 13 October 2026 (Open Access)

Authors:  E. O. Oyedele, S. Werth, G. Carlson (and others)

Abstract

Groundwater plays a critical role in sustaining the water supply in the Colorado River Basin (CRB), a region experiencing persistent drought, population growth, and increasing water demand. Using GRACE/GRACE-FO satellite observations combined with hydrological data sets, we analyze basin-scale variations and long-term trends in terrestrial and groundwater storage across the CRB from water year 2004–2023. We further apply a joint framework integrating copula-based statistical modeling with statistical inference to quantify probabilistic relationships among storage-based drought indicators and to characterize groundwater–surface water interactions. Results reveal sustained basin-wide groundwater depletion over the past two decades, with accelerated losses since 2019 and during the water year 2020–2022 drought. Notably, during this recent drought period, groundwater losses were larger in the typically more resilient Upper CRB (17.5 ± 3.5 km³) than in the Lower CRB (9.4 ± 6.5 km³), indicating increasing vulnerability across both subbasins. Copula-based analysis shows strong coupling between surface-water deficits and groundwater drought probability in the Upper CRB, whereas parts of the Lower CRB exhibit weaker statistical dependence, consistent with groundwater extraction operating partly independently of surface water availability under chronically limited supply conditions. These findings provide a basin-wide observational synthesis of groundwater–surface water dynamics and demonstrate the value of satellite-based monitoring for diagnosing hydroclimatic stress in large managed river basins. The results highlight emerging shifts in groundwater vulnerability across the CRB and underscore the need for integrated water-management strategies that jointly consider climatic pressures and sustained anthropogenic demand.