Late December and early January storms brought above-average precipitation to Idaho, but snowpacks in southern Idaho river basins are still below normal due to long-term drought.
The Idaho Water Supply Outlook Report, released last week by the USDA Natural Resources Conservation Service, shows streamflow, snowpack, precipitation and reservoir storage information throughout Idaho as of Jan. 1. However, heavy storms in early January improved snowpacks statewide.
"As of January 10, some of our snowpacks have increased nearly 20 percentage points since Jan. 1," said Julie Koeberle, an NRCS hydrologist. "Snowpacks range from near normal to slightly above normal except in southern Idaho."
She said snowpacks in southern Idaho are approximately 85 percent of average.
"Our above-average fall precipitation will help prime the soils to allow for efficient spring runoff delivery to our streams and reservoirs," Koeberle said. "But we still need more snow."
The beginning of the 2008 water year, starting Oct. 1, initially brought above-average precipitation to all of Idaho's basins. Then, the precipitation subsided in November and resumed in mid-December, lowering and then bringing most basins back to near normal levels.
Koeberle said January is a critical snow-accumulation month, and the amount of snow received will be crucial for adequate summer water supplies.
Snowpack conditions vary statewide. Snowpacks range from above normal in the northern and central parts of the state to below average south of the Snake River. Snowpacks are the highest in the Weiser and Payette basins, with 126 and 114 percent of average respectively, and lowest in the Bear River basin, with 83 percent of average.
What's more, reservoir carryover from last summer's irrigation season is not robust. Record high heat and below-average precipitation in 2007 created a high irrigation demand that taxed reservoirs. With the exception of a few northern Idaho basins, reservoir storage across Idaho is significantly lower than one year ago.
"It is just too early in the season and there is not enough information to be certain about our summer's water supply," Koeberle said. "However, we are on our way to a good start if this storm track continues as predicted in the long-range climate forecast."
- North Idaho rivers range from 99 percent of average in the Panhandle Region to 104 percent in the Clearwater Basin. The soils are primed for efficient spring runoff, and the weather shows no signs of stopping due to La Niña conditions in the Pacific Northwest. If the storm track continues, NRCS expects normal runoff this spring.
- Central Idaho rivers, which include the Big Wood, Salmon, Payette and Boise rivers, have near-average to above-average snowpacks. This past summer was one of the driest on record, leaving the snowpack for this winter critical for summer 2008 water supplies. The La Niña conditions are forecast to keep the winter storms building up the snowpacks, which should yield adequate water supplies.
- Southern Idaho rivers are the driest in the state. Snowpacks there range from 85 percent of normal in the Bear River basin to 91 percent of normal in the Oakley basin. Reservoirs along the south side of the Snake River are storing water at 15 percent of capacity. Last year's storage was 76 percent of capacity.
- Eastern Idaho rivers vary. The Henry's Fork drainage has the best snowpack in this region at 98 percent of average, while the upper Snake above Palisades has only 88 percent of normal snow. Fall precipitation was above average, which will allow efficient spring runoff. However, upper Snake reservoirs are only at 37 percent of capacity. Last year they were at 66 percent. If the current storm track continues, streams are expected to flow near normal this summer.