Climate Change Puts Western Water Supply at Risk

Under the most severe climate change scenario, the study shows a 50 percent chance of empty reservoirs if current management practices continue while the West warms and the Colorado River dries up. The study is published in the American Geophysical Union journal "Water Resources Research."

This is the 10th year of drought for the Colorado River system, said lead author Balaji Rajagopalan, a CU-Boulder associate professor of civil, environmental and architectural engineering.

The reservoir system is now at 59 percent of capacity, about the same as this time last year, said Rajagopalan, also a fellow at CU-Boulder's Cooperative Institute for Research in Environmental Sciences, CIRES.

The study suggests that water managers should begin to rethink current water management practices during the next few years before the more serious effects of climate change appear, Rajagopalan warns.

Water levels in Utah's Lake Powell reservoir are way below earlier water marks in white on the cliffs. (Photo courtesy Bureau of Reclamation)

The research team, which included scientists from the Bureau of Reclamation and the National Oceanic and Atmospheric Administration, 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, said 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, the study shows.

And if climate change results in a 20 percent reduction in average stream flow, the chances of fully depleting reservoir storage will exceed 50 percent by 2057, the scientists found.

"On average, drying caused by climate change would increase the risk of fully depleting reservoir storage by nearly 10 times more than the risk we expect from population pressures alone," said Rajagopalan. "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."

But even under the most extensive drying scenario, threats to water supplies would not be felt immediately. "There's a tremendous storage capacity on the Colorado River that helps with the reliability of supply over periods of a just few years," said Rajagopalan.

Total storage capacity of reservoirs on the Colorado exceeds 60 million acre feet, almost four 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 would remain low through 2026, even if climate change causes a 20 percent stream flow reduction, said Rajagopalan.

Between 2026 and 2057, the risks of fully depleting reservoir storage would increase seven-fold under the current management practices when compared with risks expected from population pressures alone, the researchers calculate.

If downstream releases are reduced during periods of reservoir shortages, there would be only a two-fold increase in risk of depleting all reservoir storage during this period, the study finds.

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

Entitled "Water Supply Risk on the Colorado River: Can Management Mitigate?" the study was conducted with support from the Western Water Assessment – a joint venture of CU-Boulder and the National Oceanic and Atmospheric Administration, as well as CADSWES and the Bureau of Reclamation. It will be published in the American Geophysical Union journal "Water Resources Research."

Other study authors include James Prairie of the Bureau of Reclamation, Martin Hoerling and Andrea Ray of NOAA, Joseph Barsugli and Bradley Udall of CIRES and Benjamin Harding of AMEC Earth & Environmental Inc. of Boulder.

Copyright Environment News Service, ENS, 2009. All rights reserved.