The study's coauthors looked at the effects of a range of reductions in Colorado River stream flow on future reservoir levels and at the implications of different management strategies.

Even under the harshest drying caused by climate change, the large storage capacity of reservoirs on the Colorado might help sustain water supply for a few decades. However, new water management approaches are critical to minimize the chances of fully depleting reservoir storage by mid-century.

"This study, along with others that predict future flow reductions in the Colorado River Basin, suggests that water managers should begin to re-think current water management practices during the next few years, before the more serious effects of climate change appear," says lead study author Balaji Rajagopalan of the University of Colorado in Boulder (CU-Boulder).

The findings by Rajagopalan and his colleagues have been accepted by the journal Water Resources Research, published by the American Geophysical Union (AGU).

The Colorado River system is enduring its 10th year of a drought. Fortunately, the river system entered the drought in 2000, with the reservoirs at approximately 95 percent of capacity. The reservoir system is currently at 59 percent of capacity, about the same as this time last year, says Rajagopalan. Roughly 30 million people depend on the Colorado River for drinking and irrigation water.

The research team examined the future vulnerability of the system to water supply variability coupled with projected changes in water demand. They found that through 2026, the risk of fully depleting reservoir storage in any given year remains below 10 percent under any scenario of climate fluctuation or management alternative. During this period, the reservoir storage could even recover from its current low level, according to the researchers.

But if climate change results in a 10 percent reduction in the Colorado River's average stream flow as some recent studies predict, the chances of fully depleting reservoir storage will exceed 25 percent by 2057, according to the study. If climate change results in a 20 percent flow reduction, the chances of fully depleting reservoir storage will exceed one in two by 2057, Rajagopalan says.

"On average, drying caused by climate change would increase the risk of fully depleting reservoir storage by nearly ten times more than the risk we expect from population pressures alone," Rajagopalan says.

"By mid-century this risk translates into a 50 percent chance in any given year of empty reservoirs, an enormous risk and huge water management challenge," he says.

The river hosts more than a dozen dams along its 2,330-kilometer (1,450-mile) journey from Colorado's Rocky Mountains to the Gulf of California.

Total storage capacity of reservoirs on the Colorado exceeds 60 million acre feet, almost 4 times the average annual flow on the river, and the two largest reservoirs - Lake Mead and Lake Powell - can store up to 50 million acre feet of water. As a result, the risk of full reservoir depletion will remain low through 2026, even is stream flow drops 20 percent stream flow as a result of climate change, Rajagopalan says.

Between 2026 and 2057, the risks of fully depleting reservoir storage will increase seven-fold under the current management practices when compared with risks expected from population pressures alone. Implementing more aggressive management practices - in which downstream releases are reduced during periods of reservoir shortages - could lead to only a two-fold increase in risk of depleting all reservoir storage during this period, according to the study.

The magnitude of the risk will ultimately depend on the extent of climate drying and on the types of water management and conservation strategies established.

"Water conservation and relatively small pre-planned delivery shortages tied to declining reservoir levels can play a big part in reducing our risk," says Ken Nowak, a graduate student with CU- Boulder's Center for Advanced Decision Support for Water and Environmental Systems, or CADSWES, and a study co-author.

"But the more severe the drying with climate change, the more likely we will see shortages and perhaps empty reservoirs despite our best efforts." Nowak says. "The important thing is not to get lulled into a sense of safety or security with the near-term resiliency of the Colorado River basin water supply. If we do, we're in for a rude awakening."

The study was conducted with support from the Western Water Assessment, a joint venture of CU- Boulder and the National Oceanic and Atmospheric Administration (NOAA), as well as CADSWES and the Bureau of Reclamation.

Other study authors included James Prairie of the Bureau of Reclamation, Martin Hoerling and Andrea Ray of NOAA, Joseph Barsugli and Bradley Udall of the Cooperative Institute for Research in Environmental Sciences (CIRES) at CU-Boulder, and Benjamin Harding of AMEC Earth and Environmental Inc. of Boulder.

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