2. WATER: Climate, population growth could dry up Colorado River by 2057 (ClimateWire, 07/21/2009)

Lauren Morello, E&E reporter

Climate change could leave the Colorado River's reservoirs running dry by midcentury, without significant changes in how the river's flow is managed, according to a new study.

The Colorado, now in its 10th straight year of drought, supplies water to 27 million users in seven Western states and Mexico, under a system of allotments negotiated decades ago. But that system could fail in coming decades thanks to a combination of climate change and population growth, said researchers at the University of Colorado, Boulder.

"Until 2026 or 2027, the risk is pretty low ... something water managers can plan for," said the study's lead author, Balaji Rajagopalan, a hydroclimatologist. "But after 2027, through 2050 -- that's when the risk pretty much explodes."

Rajagopalan and his colleagues examined how climate change, combined with population growth and management practices, would affect the river's ability to meet demand for water through midcentury.

Under the harshest scenario -- in which climate change reduces the runoff that feeds the river by 20 percent and water managers don't change how they mete out stored water -- there's a 51 percent chance the Colorado's reservoirs could run dry by 2057.

But if managers take more aggressive steps, reducing water releases from reservoirs in times of drought, that risk falls to just 33 percent.

If climate change cuts runoff by 10 percent, there's a 25 percent chance reservoirs will be depleted in 2057 under "business as usual" practices, and just an 11 percent chance under a strict "adaptive management" scheme.

Risk 'substantial' after 2 more decades

"The risk [of not satisfying user demand] is real, and it's substantial," said Rajagopalan. "The good news is there is relatively low risk in the first one to two decades, and that should be seized upon. The framework that is currently there, the management and legal framework, can rise to the occasion."

One factor working in water managers' favor is the Colorado River system's massive reservoirs, which can store more than 60 million acre-feet of water -- nearly four times the river's annual flow. That means that water managers may be able to draw on stored water to make up for decreasing water runoff for 10 or even 20 years.

Still, that's not a long-term solution to the West's changing water patterns, the new study and similar research suggest.

An analysis published in April by researchers at the Scripps Institution of Oceanography predicts that climate change could cause intermittent water shortages along the Colorado River Basin by midcentury. Reduced runoff could leave the Colorado unable to meet its scheduled water deliveries 60 to 90 percent of the time by 2050, that research found (ClimateWire, April 21).

The best climate models predict a drop of 5 to 25 percent in mountain runoff into the Colorado over that time, making it harder for water managers to plan for the future. But Rajagopalan said he hopes his study and other recent work will kick-start discussion.

"The question we get asked by water managers is, 'OK, the climate is changing, inflow is decreasing. Tell us, what can we do?'" he said. "We wanted to take that question on and sort of sidestep that climate change debate per se, by discussing a broad range of future outcomes."

The study will be published in an upcoming issue of the American Geophysical Union journal Water Resources Research.

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