This time of year, Idaho skiers keep an eye on the mountains - and so do water experts.

This winter's snow will determine whether streams and reservoirs are replenished next spring and dictate whether farmers and river recreationists will have a good year.

Southwest Idaho has been in the parched grip of a drought for about 10 years, putting a strain on area waterways, reservoir storage volumes and groundwater supplies.

So lately, even the lightest mountain snow has been under close observation.

In a region that has averaged about a foot of rain every year, only about 7.4 inches of precipitation have fallen so far in 2008.

And since 1999, we have hit that average just twice, and just barely - 12.1 inches in 2006 and 12.04 inches in 2000.

During the same time, the Valley recorded two of the 10 driest years in the past 100 years: 8.09 inches in 2008 and 6.96 inches in 2002.

The last year the area recorded above-average precipitation was 1998, when the Valley was soaked by 16.75 inches.

That was the same year the mercury last dipped below zero. The Valley has gone a decade without feeling the sting of zero-degree temperatures - a feat that happened on average three days each winter from 1940 to 1990.

QUALITY, QUANTITY AND TIMING

Starting in late December, Ron Abramovich, a Natural Resources Conservation Service water supply specialist, will make monthly forays into the mountains to measure the snowpack.

"We need to be at 80 percent of average by April to have adequate irrigation supply," he said.

This winter's forecast calls for average precipitation.

"The dice are not loaded this year. We cannot put too much weight into El Nio or La Nia, so it is looking like a normal year," Abramovich said.

Though Eagle riverside homeowners were worried about floods this spring, the high flows of rivers like the Boise this year didn't signal an end to the regional drought.

Unusually hot spring days caused a surge in snowmelt, and the reservoirs were already full and ready for the irrigation demands.

Under these conditions, a heavy spring rain can burst river banks.

But one or two moisture-laden winters are not be enough to fully pull us out of a nearly decade-long drought.

"There is not a one-to-one relationship between snowpack and streamflow because of lingering drought effects, like the decrease in groundwater and the wetlands and springs that are drier than normal," he said.

Not all precipitation is the same. How it falls, when it falls and how it melts affect the water supply, Abramovich said.

Last winter, skiers reveled in the deep, powdery snow that cloaked Idaho's mountains, but looks can be deceiving.

"Last year it looked like we had a lot of snow in the mountains, but light fluffy powder doesn't have as much water content," said Abramovich. The amount of water in the snowpack is more important than the depth of the snowpack.

For the calendar year, the Treasure Valley is about 40 percent behind average in precipitation, but for the water year, which runs from October through September, Southwest Idaho finished the year near average.

"It is the timing of when it fell. Last winter we had moisture, but the summer was a very dry summer," he said. Conversely, a dry winter but a summer of cloudbursts and thunderstorms may bring a lot of precipitation but not at a time when it can boost reservoir storage.

How the snow melts also affects water supply. A long, slow gradual melting means more of the water is absorbed into the ground. A rapid melting fills the streams, which in turn fill the reservoirs and replenish groundwater.

Soil condition also affects how much of the snowmelt makes it into the streams.

Early winter and late spring rains keep the soil saturated, which helps keep the melting snow moving into creeks and streams.

"Soils take 2 to 5 inches of all rains or spring snowmelt," Abramovich said. "You've got to pay the soil moisture now or next spring."

Cynthia Sewell: 377-6428