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Future of Western supplies grim, but not unmanageable

Rajagopalan. A 50 percent chance in any given year is an enormous risk and huge water management challenge, he said. But even under the most extensive drying scenario, threats to water supplies won’t be felt immediately, he said.

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 with a 20 percent stream flow reduction induced by climate change.

Between 2026 and 2057, if current management practices continue, the risks of fully depleting reservoir storage would be about 7 times the risk expected otherwise. Implementing more aggressive management practices—reducing downstream releases during shortages, for example—would lessen the additional depletion risk in the future, to just twice what would be expected from pouplation increases. The magnitude of the risk willultimately depend on the extent of climate drying and on the types of water management and conservation strategies established, according to the research.

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

“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 said. “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 End

Streamflows and reservoirs

Reservoirs could have too little water to draw from in any given year as streamflows decrease:

Flow Reduction

Chance of depletion per year after 2057







streamflow, the chances of fully depleting reservoir storage will exceed 25 percent by 2057. If climate change results in a 20 percent reduction, the chances of fully depleting reservoir storage will exceed 50 percent by 2057, Rajagopalan said.

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

As the West warms, a drier Colorado River system could see as much as a 1-in-2 chance of fully depleting all of its reservoir storage by mid-century, assuming current management practices continue on course. That’s grim news for the roughly 30 million people who depend on the Colorado River for drinking and irrigation water.

A research team, led by CIRES Fellow Balaji Rajagopalan, examined how vulnerable the Colorado River system is to water supply variability and to projected changes in water demand. The team found that through 2026, the risk of fully depleting reservoir storage in any given year remains less than 10 percent under any scenario of climate fluctuation or management alternative. During this period, reservoir storage could even recover from its current low level, about 65 percent of capacity in the summer of 2009.

But if climate change results in a 10 percent reduction in the Colorado River’s average