Researchers estimated that the Upper Basin lost about 4.6 trillion gallons during the drought, compared with 2.5 trillion gallons in the Lower Basin. Across the basin, groundwater storage fell by an estimated 27.1 cubic kilometers, or about 7.2 trillion gallons, with an uncertainty range of plus or minus 9.3 cubic kilometers.

The study examined water years 2004 through 2023, using measurements from NASA’s GRACE satellites and their successor mission to track changes in gravity. The researchers subtracted estimates of surface water, soil moisture and snow to isolate groundwater storage. A water year runs from Oct. 1 through Sept. 30 and is named for the calendar year in which it ends; the drought period studied ran from Oct. 1, 2019, through Sept. 30, 2022.

Over the full 20-year study period, total water storage in the basin declined by an estimated 58.3 cubic kilometers. Groundwater accounted for 25.5 cubic kilometers of that loss, and researchers said depletion accelerated after 2019. By three drought measures, including the U.S. Drought Monitor, 2020-22 was the basin’s most intense drought since 2004.

The pattern differed between the two halves of the basin. In the Upper Basin, groundwater losses closely tracked declines in surface water. In the Lower Basin, groundwater storage began falling before surface-water levels dropped, a pattern the researchers said may reflect the influence of human water use. They cautioned that their analysis identified statistical relationships but could not establish which factor caused the other.

Modeling found that the likelihood of groundwater drought in the Upper Basin rose to about 95% when surface-water storage was far below normal. In the Lower Basin, the probability peaked near 65%. The researchers said groundwater pumping may continue in some Lower Basin areas even when surface supplies change, where water is chronically limited.

The study also compared well measurements with surface-water changes in five Arizona areas where groundwater use is regulated. During the 2020-22 drought, the Phoenix and Pinal areas lost about 0.8 and 0.9 cubic kilometers of groundwater, respectively, while also experiencing substantial surface-water supply reductions. The Tucson area, which imports Colorado River water and replenishes its aquifers, gained about 0.2 cubic kilometers.

Lead author Esther Oyedele, a Virginia Tech geosciences graduate student, said she had expected the Upper Basin’s snowmelt-driven recharge to make it more resilient. Susanna Werth, an assistant professor of hydrology and remote sensing and a co-author, said continued groundwater depletion could reduce groundwater flowing into rivers and worsen the cycle of groundwater loss and surface-water shortages.

The findings matter to Southern Nevada, which obtains about 90% of its water from the Colorado River and about 10% from Las Vegas Valley groundwater, according to the Southern Nevada Water Authority. The study does not provide a separate estimate for Nevada or assess the federal river operating plan or the state’s lawsuit over its water allocation.

The Colorado River supplies about 40 million people. Lake Mead reached 1,040.4 feet above sea level on Aug. 7, its lowest level since the reservoir filled about 90 years ago. Later that month, federal officials set operating guidelines for 2027 and 2028 that cut deliveries to Arizona, California and Nevada by a combined 1.25 million acre-feet annually. Nevada’s proposed share of the reductions is 50,000 acre-feet a year.

Nevada officials and the Southern Nevada Water Authority have challenged the state’s reduction in federal court. Their lawsuit remains pending.

The study has important limits: the satellites have a coarse spatial resolution of about 186 miles, and monitoring wells are sparse across the basin, particularly in the Upper Basin. Researchers therefore could not validate the Upper Basin estimates against well data. They also said their analysis did not account for land-use or policy changes that could independently affect groundwater use.

Werth said the results point to the need to manage water both above and below ground, rather than relying on surface-water measurements alone.