Today, state water officials will trek into the Sierra for one last snow survey of the season. All of the late-season storms -- there was snow falling in Tahoe as recently as this week--would seem to portend a shot in the arm for California's water supply. But there is still much we don't understand about how and when the mountain runoff offers up its annual gift of water for reservoirs and irrigation.
MEASURING THE SNOWPACK
At a spot near Lake Tahoe known to snow surveyors as Phillips Station, Frank Gehrke of the Department of Water Resources is measuring the water content of the snow. Gehrke and his team check the depth of the snow, and then weigh it. In addition to doing these monthly snow surveys in the winter, a co-op of state and federal agencies collects data from more than a hundred-thirty stations set up around the Sierra, outfitted with instruments that measure things like temperature, humidity, and snow depth. But one thing none of these gadgets can capture is the energy balance: how much of the sun's radiation is hitting the snow and causing it to melt.
"If we can measure that, the difference between the incoming radiation and the outgoing radiation, we can more accurately determine when the snow's going to start to melt, and that's very critical for reservoir management," says Gehrke.
THE ALBEDO EFFECT
This energy balance changes during the season, which is why it's an important thing to measure. What makes it change is what scientists call "albedo."
"'Albedo' is a Latin way of saying 'whiteness,'" says Hans Moosmuller, a professor at the University of Nevada's Desert Research Institute in Reno.
It's a measurement of how much of the sun's energy a surface reflects, he said. Something really bright white, like freshly fallen snow, reflects almost all of the energy that hits it. Older or dirtier snow absorbs more energy.
"Think about getting a black layer on white snow. The snow will look black, and just like you would be warmer in a black sweater on a sunny day, the snow will get warmer," says Moosmuller. This will make the snow melt faster, which is why being able to measure albedo could help improve melt predictions.
About a hundred miles south of Gehrke's survey site, hydrologist Jeff Dozier is breaking a trail to an instrument tower halfway up Mammoth Mountain, a ski resort in the Eastern Sierra Nevada.
Snow fell the day before our visit, making it blindingly bright. That means this snow will be around for a while because even if the air temperature rises above freezing during the day, the snow's albedo is high enough that it won't melt.
"So what we're measuring there is the amount of radiation that the snow is reflecting," says Dozier. Sensors on the tower called radiometers are trained on the sky, as well as the snow below.
"We not only want to measure how the albedo is changing but we also want to have an idea of why it's changing," he says.
It varies depending on a couple factors. Snowflakes change shape as they age, and the shape of older snowflakes absorbs more energy than freshly-fallen snow. Older snow has a lower albedo.
The other factor, one that concerns scientists worldwide, is pollution. When dark-colored particles of dust and soot settle on snow, they act like that black sweater. Scientists are just beginning to study how those deposits affect snow melt in California.
GATHERING DATA FOR THE SHORT-TERM AND THE LONG HAUL
At Mammoth Mountain, Dozier isn't measuring dust or soot, but the data he does collect helps create a more complete picture of melt rates not only here, but throughout the entire watershed. The data is relayed to a lab hidden beneath the snow. Dozier dusts the snow off a metal lid the size of a manhole cover, then climbs down through the opening.
"This is technically called a Santa Claus entrance because it's the same way Santa Claus gets into your house. So we just go down the chimney," says Dozier.
Data loggers forward information to a website every fifteen minutes. That's important in the short-term because Dozier and his colleagues, and really anyone with Internet access, can see the status of the snow pack in nearly real-time.
It'll be valuable in the long-term, too. Climate models project a shrinking snowpack and altered precipitation patterns in the Sierra Nevada. Even if we don't end up with less precipitation, we're likely to have less snow and an earlier runoff in the spring.
"The climate is changing," says Dozier, "and what that means is these statistical methods we've been using for estimating runoff are going to get less reliable in the future."
Those statistical methods depend on historical records to project water supply. They can't account for changes in the climate or in pollution. Since albedo describes on-the-ground conditions, it will be all the more valuable in a future that looks less and less like the past.