According to recent NASA satellite studies, the western United States has been using up its underground groundwater supply over the last 10 years in light of the drought.

According to the studies, 75 percent of all the water from the Colorado River Basin has been lost through underground sources between 2004 and 2013. That amounts to 41 million acre feet of freshwater. The Weather Channel reports that 40 million people in seven southwestern states rely on the Colorado River as their primary source of drinking water.

"We don't know exactly how much groundwater we have left, so we don't know when we're going to run out," said lead author of the study and University of California, Irvine water specialist Stephanie Castle. "This is a lot of water to lose. We thought that the picture could be pretty bad, but this was shocking."

The data from the study was collected from NASA's Gravity Recovery and Climate Experiment (GRACE) satellite. The satellite measures changes in groundwater above and below the surface.

"There's only one way to put together a very large-area study like this, and that is with satellites," said Jay Famiglietti, senior water cycle scientist at Jet Propulsion Laboratory who is currently on leave from University of California, Irvine.

Gizmodo reports that the GRACE satellite is like a large scale in the sky. The amount of water in a specific area changes the overall mass of that region, which affects its gravitational attraction. GRACE measures the area's gravity and can show if an area is gaining or losing its mass in groundwater.

The loss in groundwater reveals that states are pumping more water from the Colorado River Basin in order to compensate for the lack of rain and surface water.

The Los Angeles Times reports that the loss of groundwater "reflects long-term trends in groundwater overdraft as well as stepped-up reliance on wells during a stubborn drought that settled over the Colorado River Basin in 2000."

NASA notes that while surface water use is largely regulated and monitored the U.S. Bureau of Reclamation, the water that is pumped from underground is largely undocumented due to being regulated by individual states. According to Famiglietti, reports NASA, "the rapid depletion rate will compound the problem of short supply by leading to further declines in streamflow in the Colorado River."