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Runoff Forecast Drops Dalles Dam to 70 Percent of Normal

by Barry Espenson
Columbia Basin Bulletin - February 7, 2003

The days are getting longer, but the time is getting shorter for the mountains that ring the Columbia River basin to accumulate enough snowpack to please hydropower producers, farmers and fish managers.

The "early bird" runoff forecast released Jan. 31 by the Northwest River Forecast Center in Portland predicts that the runoff through July as measured at The Dalles Dam would total 74.8 million acre feet or 70 percent of normal. That would be the 11th lowest total on record (since 1929). That means the runoff would be at a low ebb during the late summer when water is most critical for fish and farms, said Tom Fero, NWRFC senior hydrologist.

The snowpack deficit comes as the region struggles to catch up from the drought-plagued winter of 2000-2001. The snowpack two years ago yielded the second lowest runoff total on record -- 52.8 million acre feet runoff or 55 percent of average. Last winter was slightly below normal.

Despite a relatively wet January in parts of the basin, the runoff prediction slumped from a forecast of 75 percent of average made in early January. Unusually warm weather last month virtually sapped mid and low elevation snowpacks.

The early bird forecast is based on observed precipitation through Jan. 27. It assumes precipitation will be 80 percent of normal through mid-February, 90 percent of average precipitation from mid-February to the end of March, and normal precipitation thereafter.

Forecasters say that the climatic phenomenon "El Nino," which tips the odds toward warmer, drier winters in the Northwest, is loosening its grip on the region but will not likely fade in time to boost runoff totals.

"You're going to have to get an ungodly amount of precipitation (between now and summer) to reach normal," Fero said of snowpack accumulation, which normally peaks around April 1. The months have passed in which the largest accumulations normally take place. A mid-January calculation estimates that precipitation at 125 percent of normal from that point forward would only boost that runoff volume at The Dalles to about 87 percent of average. Conversely, precipitation at 75 percent of average would drop the runoff forecast to only 59 percent of normal.

The NWRFC, in conjunction with the Natural Resources Conservation Service and other cooperating agencies, puts together seasonal water supply forecasts for the Pacific Northwest. The monthly "final" forecasts are based on precipitation reports from over 400 sites. Also included in these procedures are snow-water equivalent and observed runoff values from all available sites in Oregon; Washington; Idaho; Western Montana; Western Wyoming; Northern Nevada and British Columbia, Canada. The early bird is a preliminary look at available date.

"There's been a pretty significant El Nino that began in June," said Nathan Mantua, an atmospheric science at the University of Washington. The weather phenomenon peaked in early November or early December, leaving in its wake below average precipitation totals. October-November precipitation was 51 percent of normal on the Columbia above Grand Coulee Dam and at the Snake River above Ice Harbor Dam. Precipitation in the basin above The Dalles Dam was only 49 percent of the average for the period 1971-2000, according to the National Weather Service's NWRFC. The past two months produced average precipitation.

Mantua said El Nino is waning but its influence is expected to linger through March. The near-term forecast is for dry weather.

If the spring/summer meltdown proceeds as normal, the water supply would be tapped out with less water available for fish and crops during late summer heat.

"The runoff that comes later in the year will not be there," Fero said. That water shortage would be most noticeable in the smaller river basins.

The Columbia River Basin's snowpack totals as of this past Tuesday ranged from 95 percent of normal in Idaho's Big and Little Wood basins to a paltry 15 percent for streams that feed into Oregon's Hood River and then the lower Columbia. The Willamette River snowpack was only 26 percent of normal and the Deschutes subbasin snowpack was at 53 percent of normal. All three of those Oregon drainages that flow into the lower Columbia River held less snow than they did on the same date two years ago.

Virtually all of the other subbasins held snowpacks of greater water content on Feb. 3 than existed on that date during 2001's drought year.

The mountains above the Kootenai River in Montana on Tuesday had 77 percent of average snowpack (compared to 45 percent on Feb. 3, 2001) and the Flathead River Basin was at 73 percent of normal (48 percent in 2001). The upper Clark Fork subbasin has 83 percent of its average snowpack and the Bitterrroot drainage was at 85 percent of normal.

Idaho's panhandle region has only 66 percent of its average snowpack water content and the Raft, Goose, Salmon Falls, Bruneau at 62 percent. But most of the rest of the state has fared better. The Snake River above Palisades was at 85 percent of normal, Henrys Fork, Teton, Willow, Blackfoot, Portneuf at 78 percent, the Big and Little Lost basins at 90 percent and the Weiser, Payette, Boise region at 89 percent of average. The Clearwater and Salmon basins of central Idaho were at 82 percent of average.

In Washington, the Chelan, Entiat, Wenatchee region had 80 percent of its normal snowpack as of Tuesday and the Yakima, Antanum had 79 percent. The Grande Ronde, Power, Burnt, Imnaha snowpacks in Oregon were 66 percent of average.

Fero said drainages in Canada that normally provide 28 percent of the Columbia's flows were from 60 to 70 percent of average.

Related Pages:

Barry Espenson
Runoff Forecast Drops Dalles Dam to 70 Percent of Normal
Columbia Basin Bulletin, February 7, 2003

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