ESRL Quarterly Newsletter - Fall 2009

Western Water Outlook: Grim

But good management can lower the risk of reservoir depletion, study shows

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. That’s grim news for the roughly 30 million people who depend on the Colorado for drinking and irrigation water.

Lake Powel in 2005.

Lake Powell in 2005.

A research team—including PSD’s Marty Hoerling, Andrea Ray, Joseph Barsugli (CIRES), and Bradley Udall (CIRES), and led by CIRES’ Balaji Rajagopalan— examined how vulnerable the Colorado River system is to water supply variability and to projected changes in water demand. The scientists 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.)

But if climate change results in a 10-percent reduction in the Colorado River’s average 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 Rajagopalan. “A 50-percent chance in any given year is an enormous risk and huge water management challenge,” he said.

The results were published in the American Geophysical Union journal Water Resources Research.

Even under the most extensive drying scenario, threats to water supplies won’t be felt immediately, the researchers found. Total storage capacity of reservoirs on the Colorado (including lakes Mead and Powell) exceeds 60 million acre feet, almost four times the longterm average annual flow of about 16 million acre feet. As a result, the risk of reservoir depletion will remain low through 2026, even if climate change induces a 20-percent reduction in streamflow. However, after 2026, the risk of drying increases to 26-51 percent, depending on the effects of climate change and management, with lower risk associated with aggressive management to reduce demand.

The Colorado’s flow has been very low in the last 10 years, Hoerling said, averaging only about 10 million acre feet. Reservoirs have dropped to a little more than half capacity, but managers still delivered water where it needed to go. “So the system is working, from a gross point of view,” Hoerling said. But climate models and modelers are still struggling to understand the future of the system in a warmer world; some models don’t include the high-elevation snowpack critical to the Colorado River System, for example.

“Our models are not yet good enough to inform, with the accuracy desired by most decision makers,” Hoerling said.

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