The snow-fed Upper Colorado River Basin lost approximately 4.6 trillion gallons of groundwater during the 2020-2022 drought, according to a new satellite study led by Virginia Tech. This loss was nearly double the roughly 2.5 trillion gallons lost by the drier Lower Basin during the same period. The 2020-2022 drought was identified as the most intense in the basin since 2004, which is the start of the record for this study.
Esther Oyedele, a geosciences graduate student at Virginia Tech, served as the lead author of the study. Oyedele, who arrived in the United States in late 2020, expanded her doctoral work, conducted with assistant professor Susanna Werth, to cover the entire Colorado River Basin, which supplies water to more than 40 million people. G. Carlson of Indiana University Bloomington was the third author, and a NASA grant supported part of the work.
To conduct the research, the team utilized NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites and their follow-on pair, launched in 2018. These satellites detect subtle changes in Earth’s gravity field to track water volumes above and below ground, which was crucial given the scarcity of wells, particularly in the mountainous regions of the basin. The team analyzed GRACE data from 2004 through 2023, meticulously removing snow, soil moisture, and water from major reservoirs like Lake Powell, Lake Mead, Lake Mohave, and Lake Havasu to isolate groundwater measurements.
The study compared three distinct drought periods: 2006-2007, 2011-2015, and 2020-2022. By referencing the U.S. Drought Monitor, a drought index called SPEI, and rainfall records, all three indicators ranked the 2020-2022 drought as the most intense. While Virginia Tech stated that groundwater historically declined more in the Lower Basin, the study indicated that the Upper Basin is now experiencing rapid groundwater loss.
During the 2020-2022 drought, the Upper Basin saw an annual drainage of about 1.6 trillion gallons, a rate faster than either of the earlier drought periods examined. The entire basin lost approximately 7.2 trillion gallons during this timeframe. The Lower Basin experienced its most severe stretch between 2011-2015, losing about 4.3 trillion gallons. Since 2004, the satellites have recorded a net loss of approximately 15.4 trillion gallons of water across the basin, with groundwater accounting for about half of this total.
The recent drought coincided with prolonged snowpack deficits, warmer temperatures, and reduced natural recharge, factors contributing to the observed groundwater depletion. The study indicated that many headwater streams in the Upper Basin depend on groundwater to sustain their flow, and a separate study determined that vegetation in the Colorado headwaters also taps into shallow groundwater during hot summers. The authors concluded that while the Upper Basin had historically rebounded better than the Lower Basin, basins that typically refill between dry spells may show larger declines when drought conditions return.
Assistant professor Susanna Werth stated in the Virginia Tech news release that the study serves as a clear signal that focusing solely on surface water is no longer sufficient. Continued losses, especially in the Upper Basin, could further diminish groundwater flow to rivers and exacerbate surface-water shortages. The Virginia Tech news release also stated that while groundwater can buffer shortages, “that buffer is not unlimited.”
The study also examined the timing of groundwater versus surface-water declines. In the Lower Basin, wells began dropping about four months before reservoirs saw declines, whereas in the Upper Basin, groundwater levels trailed reservoir drops by approximately five months. A plain-language summary of the study indicated that these findings point to human water use as the primary stressor in the drier southern portion of the basin.
In Arizona, where pumping is managed in zones known as Active Management Areas, the largest groundwater losses between 2020 and 2022 occurred in Phoenix and Pinal, losing approximately 650,000 acre-feet and 730,000 acre-feet, respectively. Both areas experienced significant reductions in surface-water supply. Conversely, Tucson recorded a slight gain, attributed to Colorado River water from the Central Arizona Project, conservation efforts, and managed recharge, which involves deliberately replenishing underground water supplies. Outside these managed zones, the southern Lower Basin lost approximately 4 million acre-feet.
In an essay for the Natural Hazards Center at the University of Colorado Boulder, Oyedele characterized groundwater depletion as “a slow-motion disaster that is hard to see.” She highlighted challenges in Pinal County, where small farmers' wells have run dry while larger agricultural producers drill deeper. Oyedele also noted that monitoring wells tend to cluster around cities and large farm districts, leaving rural, tribal, and marginalized communities with insufficient data. Satellite studies elsewhere have found that 6 billion people live in countries experiencing freshwater loss.
The authors noted some limitations, including error bars that indicated potential variations in the totals (Upper Basin by 3.5 cubic kilometers either way, and Lower Basin by 6.5). The GRACE satellites observe the land in broad patches, approximately 186 miles wide. With few wells in higher elevations, the study's largest groundwater estimates could not be directly verified against ground data, and snow and soil models can be less accurate in steep terrain. The study also did not account for changes in land use or water policy, and the authors noted that while statistics show which droughts tend to occur together, they “do not establish directionality or causation.” Oyedele observed in her essay that satellite data represent only one component of the complete picture.
For additional context, NASA’s Earth Observatory reported that a meager snowpack in winter 2025-2026 led to record-low levels for Lake Powell, the second-largest reservoir in the U.S., in late August and early September. According to the U.S. Bureau of Reclamation, the lake first surpassed its previous record low in August 2023. Lake Mead, further downstream, also reached record lows in August 2026, prompting the federal government to agree to compensate Southern California’s largest water supplier to leave part of its water share in Lake Mead. The Virginia Tech study's data concludes in 2023, so it does not directly address groundwater levels for the current year. The full study was published in Water Resources Research.