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Seven Western states that rely on the Colorado River Basin for valuable water are drawing more heavily from groundwater supplies than previously believed, a new study finds, the latest indication that an historic drought is threatening the region’s future access to water.

In the past nine years, the basin — which covers Wyoming, Colorado, Utah, New Mexico, Nevada, Arizona and California — has lost about 65 cubic kilometers of fresh water, nearly double the volume of the country’s largest reservoir, Lake Mead. That figure surprised the study’s authors, who used data from a NASA weather satellite to investigate groundwater supplies.

About two-thirds of the water lost over the past nine years came from underground water supplies, rather than surface water.