Massive groundwater losses detected in the Colorado River Basin

Water from the Colorado River Basin is disappearing. The Colorado River runs from headwaters in Colorado though Utah, Nevada, Arizona, California and Mexico and has tributaries that flow into Wyoming and New Mexico. Not only are water demands high in these states, but a severe drought that has lasted more than 14 years has also struck the Basin. The drought — which began in 2000 and remains in effect today — has caused steep declines in surface water levels due to the inability of reservoirs to refill from surplus inflows generated from normal precipitation and snow pack levels. Two of the Colorado River Basin's largest reservoirs, Lake Powell and Lake Mead, have declined to dangerously low levels.
 
Colorado River Basin managers have known for many years that groundwater resources are crucial in supplementing water demands that surface water supplies cannot meet during times of drought. Recognizing the problem of shrinking freshwater resources, the United States Bureau of Reclamation in 2012 highlighted the need for Basin states to use local groundwater supplies to meet water requirements. Groundwater reserves, however, are not well regulated within the Colorado River Basin. The resulting lack of understanding raises questions as to the amount of groundwater currently consumed in the Basin and the extent to which states can rely on groundwater for future water security.
 
In a recent study, a team of researchers from the University of California at Irvine, the National Center for Atmospheric Research, and NASA examined the state of the Colorado River Basin's water supply to assess whether groundwater reserves will be able to meet current and future demands. The researchers specifically assessed how groundwater stocks in the Basin have changed over time relative to the total freshwater supply. Published in Geophysical Research Letters in August 2014, the team looked at nine years (Dec., 2004 – Nov., 2013) of data gathered by the NASA Gravity Recovery and Climate Experiment (GRACE) mission. GRACE uses satellites in outer space to calculate how Earth's mass — and consequently the amount of water storage — has changed over time. The researchers verified their data by comparing it to known observations of groundwater levels, provided by the United States Geological Survey and the Arizona Department of Water Resources. 
 
Results from this study indicate that the Colorado River Basin has lost a massive 53 million acre-feet of freshwater. Of this number, 41 million acre-feet of water — nearly 77 percent of this loss — is credited to groundwater depletion alone.
 
The findings suggest that groundwater resources contribute to a larger portion of freshwater consumption in the Colorado River Basin than previously thought. The Southwestern drought prevents surface water reservoirs from replenishing each year, while population growth and increased irrigation in the Colorado River Basin has boosted water consumption. Interestingly, however, strict surface water regulations during times of drought have successfully held reservoir levels fairly constant, even if these levels are strikingly low. The vast groundwater loss found in this study therefore implies that water users have switched to relying on underground water resources to meet public water demands. The authors propose that the lack of groundwater management in the Colorado River Basin can explain the imbalanced loss between groundwater and surface water resources.
 
Scenarios evaluating the future of the Colorado River Basin imply that the region will experience increased threats to water security. Climate change models suggest that decreases in future snowpack and precipitation levels will continue to produce insufficient replenishment of surface water and groundwater sources. At the same time, projected population growth will place additional stress on an already constrained water supply.  
           
The authors suggest that decision-makers in the Colorado River Basin must take immediate action to meet future water demands under these pressured circumstances. The study advocates for water management plans and policy that integrate surface and groundwater allocation in order to ensure sustainable water supplies that will overcome changing environmental conditions.