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Report: Last 50 Years
Northwest Water Supply Shrinking

by CBB Staff
Columbia Basin Bulletin - February 14, 2002

A warming climate the past 50 years has, through early melting, relentlessly reduced the water content of the Pacific Northwest's springtime snowpack, straining the supply of water for drinking, irrigation and other uses during the region's typically dry summers, according to new research at the University of Washington.

"I was surprised at the size of the result," said Philip Mote, a research scientist with the Pacific Northwest Climate Impacts Group at the UW. "There's already a clearer regional signal of warming in the mountains than we expected."

The research shows that from 1950 through 1992, the amount of water contained in snowpack declined steadily throughout the region, as much as 60 percent in some places, Mote said.

In measurements taken on April 1 each year at 145 sites throughout the region, nearly all in the mountains, 141 sites registered decreases in the water content of the snowpack, and 90 of them had declines of at least 25 percent. Nine of the sites -- eight in Oregon and one at Hurricane Ridge on Washington's Olympic Peninsula -- had at least a 60 percent decline.

The data came from 127 sites in Washington, Oregon, Idaho and Montana west of the continental divide, plus 18 sites in the Columbia River basin in British Columbia. The sites ranged from 3,000 feet above sea level to 9,000 feet.

Measurements at the Canadian sites ended after 1992, but readings were taken for another five years at the U.S. sites and those supported the previous data, Mote said. In addition, a computer model that simulated snowpack for part of the region by using actual weather readings from 1915 through 1998 confirms the results.

Most of the decline in water content of the snowpack -- called snow-water equivalent -- is directly attributable to higher temperatures that cause the snowpack to melt earlier, he said. But at the nine sites with the sharpest decreases, the cause was a combination of rising temperatures and declining precipitation.

The warming climate has the tendency to reduce snowpack accumulations as well as "accelerate the onset of the melt," Mote said.

The current winter is perhaps an extreme example of what appears to be a long-term trend toward warmer weather and lesser amounts of snowpack on April 1 -- and decreased summer runoff.

"If you have a warm winter, more of your storms are not going to be depositing snow," Mote said. That leaves lower elevation, particularly, with reduced snowpack.

"The losses generally decrease with elevation, especially in the Cascades, which is consistent with a temperature effect," Mote said.

Mote presented the findings Wednesday at the American Meteorological Society's annual meeting in Long Beach, Calif., where more than 2,000 atmospheric scientists gathered to discuss a broad range of weather and climate-related issues.

The springtime snow-water equivalent measurements are a practical means of predicting summer water supply in the Northwest, because "snow on the ground in the spring is the precursor to the streamflows when the weather warms and the snow melts," Mote said.

The region's relatively dry summers create a heavy reliance on stored water from spring runoff for drinking, irrigation and other uses, but warming temperatures have meant an increasingly early spring runoff.

He said the research demonstrates a clear need to keep monitoring the same sites very closely, to continue accumulating data that will help understand future changes in the region's climate.

"Predictions of continued increases in temperature associated with the buildup of greenhouse gases indicate that the warming trend of the 20th century will continue, and even accelerate, causing even earlier spring melting," Mote said.

"These trends have profound and disturbing implications for water resources in the region, where conflicts over water have already drawn national attention."

For more information, contact Mote at (206) 616-5346, (206) 601-0668 or

CBB Staff
Report: Last 50 Years Northwest Water Supply Shrinking
Columbia Basin Bulletin, February 14, 2003

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