GIN FLAT -- In deep winter, water scientist Frank Gehrke straps on his cross country skis and trudges uphill in the thin, cold air to one of the continent's most closely monitored frozen meadows, 7,200 feet above sea level in the Sierra Nevada.
To understand why his arduous, breath-sucking hike is important, stand still and listen to the snow. In the pale morning sun, the forest of pine and cedar comes alive with sound. Clumps of fresh powder fall with a thud or drip-drop from treetops, quickening with the staccato of popping corn.
This place is like a Rosetta Stone for California's water supply.
It's where the convergence of snow, sun and temperature enables scientists to predict floods or drought. It's where they have installed sophisticated equipment to help understand how climate change is altering snowmelt in the Sierra, a source of water for millions of Californians.
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"Gin Flat's always been the place where we try things and invest first," said Michael D. Dettinger, a U.S. Geological Survey hydrologist based at the Scripps Institution of Oceanography in La Jolla.
Although the state oversees more than 300 survey sites, what makes Gin Flat unique is its location, an elevation in Yosemite National Park just above the point where rain commonly turns to snow. That makes it an ideal spot to test the premise that a warming climate will produce more rain at higher elevations -- a shift that would bring more flooding and less snowpack to fill California reservoirs in midsummer.
Readings at Gin Flat will help determine how much it will cost to drink a glass of water, take a shower or water the lawn.
That is why Gehrke, 60, didn't hesitate to ski three miles up the mountain last week, hauling a sled loaded with 60 pounds of fuel cells and tools. He is California's snow survey chief, a man respected as the don of the Si-erra snowpack. As caretaker of Gin Flat, he needed more power to fuel all the equipment at the site he helped develop.
Last week, his agency, the state Department of Water Resources, reported that the Sierra snowpack was at 118 percent of normal for this date, compared with 63 percent of normal at this time last year.
But Gehrke is a cautious man, and never more so than when explaining snowpack surveys.
"We could slide back to below average. A March without snow could do it," he said. "A lot of the reservoirs are pretty low from last year."
Worried state and local officials look to data from locations such as Gin Flat for assurances the Sierra will continue to provide a steady stream of water to keep the economy growing and water rates low.
Gehrke cannot offer the certainty they crave.
"When it's wet or dry, people go out and get into hyperbole about how much snow they've seen," he said "We always think it's good to keep things grounded and not get carried away by the moment."
He is more cautious about climate change predictions. Yet, he is concerned about how snowpack levels have become erratic during his 27 years of measuring. It was thick in 2005-06, extra-thin last year and is slightly better than normal this winter.
"A lot of people I really respect say you're going to see a lot more of this," he said.
Gehrke and other scientists have equipped 11 sites in Yosemite with high-tech monitors, turning one of the nation's most famous parks into an electronic snow laboratory.
Gin Flat, east of the park's Big Oat Flat entrance, provides a snowpack record dating to 1930 that makes today's data even more valuable.
In this outpost, devices measure the weight of snow, the temperature of snow, the strength of the rays of sunshine heating the snow and the moisture in soil under the snow. Sonarlike sensors test the depth of the snow. Results from Gin Flat are reported hourly, and are transmitted every three hours to Virginia and then back to Sacramento.
Those results are reshaping how scientists look at snow.
Research scientist Bob Rice is preparing a paper based on Gin Flat data that shows conventional measuring methods are over-estimating the water content of snow by 20 percent.
"That 20 percent can make you or break you in a given year," said Rice, of the Sierra Nevada Research Institute, part of the University of California at Merced. "If we overestimate by 20 percent, then we have given out more water than we actually have."
But last week, all this science came down to a man and sled.
Gehrke stripped off his fleece jacket as he trudged with the sled harness around his waist, breaking trail in fresh powder for more than three hours.
With each step, pine shadows grew longer. Gehrke decided to stash the generator and fuel cells at midafternoon, then skied the rest of the way to Gin Flat.
He'll be back to retrieve the generator and install it. He'll return in early April to help UC Merced students use probes to make hundreds of depth measurements of the snowpack.
And when the snow disappears, he will hike up to mend a wood-shingled instrument hut mangled by bears.