THE CONTINENTAL DIVIDE, Colorado — Even in mid-summer, the air is cold and restless atop the dividing line that separates the Colorado River watershed to the west from the drainage basins that eventually feed into the Mississippi River to the east.

Above the tree line in Rocky Mountain National Park, flowers have bloomed but are stunted, clinging to the surface to anchor themselves against the constant, battering winds. Pikas, which look like a cross between a chipmunk and a marmot, thrive at these cold, high altitudes, darting in and out of rock formations to the delight of summer tourists.

See also: California Drought Causes Largest Water Loss in State History

There are still a few snowdrifts in mid-July, leftovers from last winter, and soon the snows will begin again. Lately, though, the winter snows and late summer rainfall has not been nearly enough to put a dent in the drought that has gripped the Southwest since 2004.

The wetness, or water content, is a depiction of the amount of groundwater on July 7, 2014, compared to the average from 1948 to 2009. Image: NASA Earth Observatory

Something fundamental is out of balance on the western side of the divide, where spring snowmelt and summer monsoon rains are barely slaking the thirst of the growing Southwest region, which has been in the grips of a prolonged, severe drought since 2004 at the same time that the population has been growing.

The Colorado River Basin supplies water to farmers that grow 15% of the country’s food, and about 40 million people in seven states rely on it for all or part of their water supply. Once a mighty river, the Colorado has been dwindling to a trickle in recent years, just as demand for water from sprawling cities such as Las Vegas, Nevada and Phoenix, Arizona has skyrocketed.

The declining surface water supply is most vividly illustrated by the record low level of Lake Mead in Nevada. The lake, which was built in 1936 to help meet the water needs of the Southwest states, feeds into Hoover Dam. In July, it hit an all-time record low level, raising the prospect of downstream water rationing that could affect urban areas from Las Vegas to Los Angeles.

Water management officials are acutely aware of the declining surface water supplies in the Southwest, which is especially worrisome given climate change projections that show this area is likely to become even drier with time. But until Thursday, they had no idea about what was happening with the region's groundwater reserves.

A new study, published Thursday in the journal Geophysical Research Letters, provides reason for increased concern, since it reveals that due to the drought, water users throughout the Colorado River Basin are tapping into hard-to-replace groundwater supplies to make up for lost water.

This, the study’s authors, from NASA and the University of California at Irvine, say, is bound to worsen the water challenges the region faces.

Based on measurements from a satellite system that can detect subtle changes in land mass that are related to water above and below the surface, scientists found that the Colorado River Basin has been losing staggering amounts of groundwater in recent years. Unlike surface water supplies, groundwater cannot be easily recharged. Groundwater is analogous to a pension built up over many years — in the case of groundwater, we're talking hundreds of years — rather than a simple checking account.

In other words, water users in Western states are drawing down their long-term water savings at alarming rates, since the study found that more than 75% of the water loss in the Colorado River Basin since 2004 came from groundwater.

The data, from NASA’s Gravity Recovery and Climate Experiment (GRACE) satellite system, shows that between December 2004 and November 2013, the Colorado River basin lost nearly 53 million acre feet of freshwater, which is double the total volume of the country’s largest reservoir, Lake Mead, Arizona. More than three-quarters of the total — or about 41 million acre feet — was from groundwater.

"This is a lot of water to lose. We thought that the picture could be pretty bad, but this was shocking,” said Stephanie Castle, a water resources specialist at the University of California, Irvine, and the study's lead author, in a press release.

GRACE, twin satellites launched in March 2002, are making detailed measurements of Earth's gravity field that helps measure changes in groundwater supplies and ice sheets. Image: NASA and Cal-Tech

Groundwater falls into a regulatory black hole, as it is governed by individual states and local governments. For example, in drought-stricken California, which is one of the states tapping into the Colorado River, there are no regulations requiring water users to limit their groundwater withdrawals, or even to quantify how much groundwater there is. A recent study, also by NASA and UC Irvine, found that farmers in California have been tapping into groundwater supplies there to sustain their operations during the state's severe drought.

"With Lake Mead at its lowest level ever, we wanted to explore whether the basin, like most other regions around the world, was relying on groundwater to make up for the limited surface-water supply,” said Jay Famiglietti, senior water cycle scientist at JPL and a professor at UC Irvine.

“We found a surprisingly high and long-term reliance on groundwater to bridge the gap between supply and demand."

“Combined with declining snowpack and population growth, this will likely threaten the long-term ability of the basin to meet its water allocation commitments to the seven basin states and to Mexico," Famiglietti said.