A recent study conducted by NASA scientists in collaboration with researchers from the University of California, Irvine has found that more than 75% of the water lost in the parched Colorado River Basin originated from groundwater resources. The researchers are concerned that the loss of such a large percentage of the groundwater resources could pose a far graver threat to water supplies in the western states than initially thought.

This study, which was recently published online in the journal Geophysical Research Letters, is the first scientific study to quantify the contribution of groundwater in satisfying the water requirements of the western USA. The Colorado River Basin has been severely water-stressed for a prolonged period, having suffered the driest 14 year spell over the past hundred years.

Using data gathered by NASA's Gravity Recovery and Climate Experiment (GRACE) satellite project, the scientists recorded monthly changes in water volume of both surface-water and groundwater resources. Data recorded between 2004 to 2013 have shown that the approximately 65km3 of freshwater reserves equates to nearly twice the capacity of the largest freshwater reservoir in the US -- Lake Mead in Nevada. More than 75% of this total (50 km3) has been attributed to loss of groundwater.

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

The US Bureau of Reclamation is responsible for managing surface water resources in the Colorado River Basin, and this management body records these water losses. However, water extraction from underground sources is controlled by individual states, and in many cases losses are rarely if ever recorded.

According to Jay Famiglietti, a senior water cycle scientist at JPL and senior author of the study, satellites provide the only solution for observations of such a wide scale, as is the case in this study. GRACE acts like a massive airborne scale that is able to measure changes in mass in the land below. Fluctuations in water levels affect the strength of local gravitational forces, so by regularly recording these gravitational measurements GRACE provides information on regional water fluctuations over any given period.

As the single major river supplying water to the 40 million residents and 4 million acres of agricultural land across 7 states in the southwest, the Colorado River is essential for the survival of millions of people -- both for drinking water and for food.

Considering that water levels in Lake Mead are currently at record low levels, the research team wanted to assess whether the Colorado River Basin, lake many other regions across the world, is relying on its underground reserves to supplement the surface-water shortfalls. This study shows that there is indeed a strong and long-term dependence on groundwater to meet the shortfall between water supply and water demand.

According to Famiglietti, this rapid rate of water depletion will exacerbate the water woes of the nation by further reducing the rate of stream flow of the Colorado River. "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 notes.