Satellite data suggests more than 75 percent of water loss in drought-stricken basin is from groundwater pumping
By Summit Voice
FRISCO — Groundwater pumping is a huge factor in the Colorado River Basin water equation, California-based researchers said this week, announcing the results of satellite study that for the first time quantifies how groundwater contributes to the water needs of western states.
Along with surface diversions and pipelines, water users in the basin are also unsustainably depleting underground aquifers. For example, mountain resort towns in Colorado tap underground water from headwaters streams like Tenmile Creek and the Blue River for municipal use.
The new study found that more than 75 percent of the water loss in the drought-stricken Colorado River Basin since late 2004 came from underground resources. The extent of groundwater loss may pose a greater threat to the water supply of the western United States than previously thought, the researchers concluded.
While precipitation in the Colorado River Basin always varies from year to year, federal water managers say the last 14-year period has been the driest in 100 years. The study has been accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union, which posted the manuscript online today.
Monthly measurements of the change in water mass from December 2004 to November 2013 revealed the basin lost nearly 53 million acre feet (65 cubic kilometers) of freshwater, almost double the volume of the nation’s largest reservoir, Nevada’s Lake Mead. More than three-quarters of the total — about 41 million acre feet (50 cubic kilometers) — was from 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.”
Water above ground in the basin’s rivers and lakes is managed by the U.S. Bureau of Reclamation, and its losses are documented. Pumping from underground aquifers is regulated by individual states and is often not well documented.
“There’s only one way to put together a very large-area study like this, and that is with satellites,” said senior author Jay Famiglietti, senior water cycle scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, on leave from UC Irvine, where he is an Earth system science professor. “There’s just not enough information available from well data to put together a consistent, basin-wide picture.”
Famiglietti said GRACE is like having a giant scale in the sky. Within a given region, the change in mass due to rising or falling water reserves influences the strength of the local gravitational attraction. By periodically measuring gravity regionally, GRACE reveals how much a region’s water storage changes over time.
“The Colorado River Basin is the water lifeline of the western United States,” said Famiglietti. “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. We found a surprisingly high and long-term reliance on groundwater to bridge the gap between supply and demand.”
Famiglietti noted that the rapid depletion rate will compound the problem of short supply by leading to further declines in streamflow in 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 said.
Coauthors included other scientists from NASA’s Goddard Space Flight Center, Greenbelt, Maryland, and the National Center for Atmospheric Research, Boulder,