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Researchers explore climate, river links

By Scripps Institution of Oceanography, UCSD

Scripps researchers reported last week that currently scheduled water deliveries from the Colorado River are unlikely to be met if human-caused climate change reduces runoff in the region.

“All water-use planning is based on the idea that the next 100 years will be like the last 100,” said Scripps research marine physicist Tim Barnett, a co-author of the report. “We considered the question: Can the river deliver water at the levels currently scheduled if the climate changes as we expect it to? The answer is no.”

Even under conservative climate change scenarios, Barnett and Scripps climate researcher David Pierce found that reductions in the runoff that feeds the Colorado River mean that it could short the Southwest of a half-billion cubic meters (400,000 acre-feet) of water per year 40 percent of the time by 2025.

(An acre-foot of water is typically considered adequate to meet the annual water needs of two households.)

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Could double

By the later part of this century, those numbers double.

The paper, “Sustainable water deliveries from the Colorado River in a changing climate,” appeared in the April 20 edition of the journal Proceedings of the National Academy of Sciences.

The analysis follows a 2008 study in which Barnett and Pierce found that Lake Mead, the reservoir on the Colorado River created by Hoover Dam, stood a 50 percent chance of going dry in the next 20 years if the climate changed and no effort was made to preserve a minimum amount of water in the reservoir. The new study assumes instead that enough water would be retained in the reservoir to supply the city of Las Vegas, and examines what delivery cuts would be required to maintain that level.

Providing data

“People have talked for at least 30 years about the Colorado being oversubscribed, but no one ever put a date on it or an amount. That’s what we’ve done,” Barnett said. “Without numbers like this, it’s pretty hard for resource managers to know what to do.”
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Barnett and Pierce also point out that lakes Mead and Powell were built during and calibrated to the 20th century, which was one of the wettest in the last 1,200 years.

Their research shows that the biggest effects of human-induced climate change will probably be seen during dry, low-delivery years. In most years, delivery shortfalls will be small enough to be manageable through conservation and water transfers, they estimate. But during dry years, there is an increasing chance of substantial shortages.

“Fortunately, we can avoid such big shortfalls if the river’s users agree on a way to reduce their average water use,” Pierce said. “If we could do that, the system could stay sustainable further into the future than we estimate currently, even if the climate changes.”