Earlier this year, Seattle water managers wrung their hands over the dismal snowpack in the mountains. The city urged residents to start...

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Earlier this year, Seattle water managers wrung their hands over the dismal snowpack in the mountains.

The city urged residents to start conserving water and warned of mandatory water restrictions if the situation didn’t improve.

Expect that to happen more in the future because of global warming, according to the findings of University of Washington scientists.

While late-spring rains helped pull Seattle from the brink this year, the city could expect droughts to come more frequently in the future, the scientists predicted.

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Today’s “average” snow year would happen only about once every four years by 2040. And it would be more difficult for salmon in the Cedar River, where the city gets much of its water.

The impact probably would be the equivalent of another 170,000 people moving to the city and using its water, the scientists found.

“The impacts are significant. They’re in the range of what can be dealt with. And we need to plan for them,” said professor Richard Palmer, a civil engineer and member of the university’s Climate Impacts Group.

He is crafting a report about global warming for the city’s water department. The basic findings were reported in a student’s thesis published last year.

A top city of Seattle water official, Guillemette Regan, questioned the accuracy of the projections on the decline in water supply. She said the computer models aren’t perfect and don’t account for how the city can run its system to capture more water.

But city officials already are starting to make adjustments in case global warming affects water supplies or how the system operates.

“Climate variability will become more extreme, and we have to be able to adapt to that now,” Regan said.

While scientists have warned for years that global warming could pose problems for Washington water supplies, this is one of the first studies to attempt to quantify the impact on a specific water system. Seattle provides water for 1.3 million customers, including residents and businesses in Seattle and numerous other King County cities.

Seattle water managers haven’t calculated how global warming would affect the amount of water available for future growth, Regan said. But they are devising a computer model that will take global warming into account in future plans.

The city also is collecting more detailed data on the water in the Cedar River and Tolt River watersheds that feed the city system, and is adjusting when it releases water from reservoirs to respond to low snow years such as 2005, Regan said.

The UW study concluded the city’s water supply would probably fall roughly 14 percent by 2040. But the computer models found the decrease could be as small as 4 percent or as great as 22 percent.

The projected decline in overall water supply, and the increased frequency of drought conditions, is driven primarily by higher temperatures, particularly in winter, said Matthew Wiley, the graduate student who wrote the thesis. The models showed local average temperatures rising 1.5 to nearly 2.8 degrees by 2040.

With higher temperatures, winter precipitation is more likely to come as rain that immediately washes into rivers feeding Seattle’s water system, Wiley said, and not all of that can be stored for future use.

Snow accumulates and then gradually melts in the warmer months, providing a more steady supply of water that can be metered out from dams on city reservoirs.

Chuck Clarke, director of Seattle Public Utilities, said it is unclear exactly how global warming will affect the city water supply. He said he had seen predictions that it would actually increase the water supply because of more precipitation.

But Wiley said four independent computer models showed a decline in water supply over time. The question, he said, is how much and how quickly.

The findings could give a boost to claims of the Cascade Water Alliance, a Bellevue-based utility consortium trying to tap Pierce County’s Lake Tapps as a source of drinking water.

Alliance officials have said regional growth will probably create water shortages without a new supply of water. But Seattle officials have said they don’t see a pressing need for additional water sources.

“If the Cascade Water Alliance develops Lake Tapps as a new source of water, we’re really going to have a chance to rethink how we want to use our water resources,” said Palmer, the professor. “If they don’t get Lake Tapps on line, the region will really have to redevote itself to conservation.”

Warren Cornwall: 206-464-2311 or wcornwall@seattletimes.com