Tom Painter strapped $85,000 worth of scientific equipment to his back and scrambled up the steep, smooth face of Pothole Dome in California's Yosemite National Park. Ravens squawked overhead, and the muffled voices of tourists murmured from the easier route up the 250-foot-high (76 meters) granite dome.

"We're taking the sporting route," said Painter, as he jogged up the pinkish rock. Tan and fit, Painter wasn't slowed by the thin air at 8,700 feet (2,650 meters) or by his 40-pound (18-kilogram) load.

"My two sons like to slide down parts of these granite domes," Painter said. "It rips their pants to shreds."

Painter is a snow scientist with NASA's Jet Propulsion Laboratory in Pasadena, California, and he had come to Yosemite in late July to make some measurements that will help him and colleagues better measure the amount of snow that collects there in the winter.

As California suffers through the worst drought in the state's recorded history, a finer understanding of the snowpack will help people plan for how much water they can expect. Already, the new data available have allowed for more accurate and more frequent water predictions, and that has helped city officials, farmers, power producers, industry managers, and dam operators make the most of a limited resource. As the drought continues, knowing how much water remains will be increasingly critical.

"The work is going to revolutionize our understanding of mountain hydrology," said Frank Gehrke, who arrived a few minutes later at the top of Pothole Dome.

"It's going to dramatically improve our ability to manage our water supply by boosting our ability to measure and forecast the amount of water available in the snowpack," said Gehrke, who serves as the chief of snow surveys for California's Department of Water Resources in Sacramento.

That's important because 75 to 80 percent of the state's water comes from the snowpack in the Sierra Nevada. That's water that serves tens of millions of people, grows about half of the fruits and nuts in the United States, produces hydropower, drives industry, supports recreation, and nourishes wildlife. And California is currently in the midst of the worst drought in state history. (See "California Snowpack Measure Shows No End in Sight for Drought.")

"Regardless of what happens with climate change, even in a good year we don't have enough water," Gehrke said. (Watch video of Gehrke taking a snow survey.)

The Science of Snowmelt

Near the dome's summit, Painter unpacked his gear. He took a reading with a high-accuracy, $12,000 GPS device. Then he calibrated his handheld spectrometer by beaming it at a piece of white Teflon, which has a reflectivity of nearly 100 percent.

Next, Painter walked in a straight line over the granite, for about the length of a football field. As he did so, he trained the spectrometer across the ground.

The device measures how much radiation is reflected by a surface, both in the visible spectrum of light and in the invisible infrared spectrum. That's important, because "to us, a surface might look highly reflective, but it may not actually be so across the whole range of the spectrum," Gehrke said.

To demonstrate how it works, Painter trained his beam on a blue shirt. The curve on his laptop's screen registered in the wavelength that human eyes perceive as the color blue. He aimed the plastic "gun" at green pants, and the curve shifted slightly to that color range.

"That's the same range we see for green vegetation," Painter said. "Different surfaces all reflect the sun's rays differently."

When it comes to snow, the amount of radiation reflected by a sample is a good indicator of how fast the material is going to melt, Painter said. That's because 90 to 95 percent of the energy that melts snow comes from solar radiation.

"People think it's the temperature that causes snow to melt, but it's not," Painter said. "Almost nothing would happen to a pile of snow if you suddenly raised the temperature to 90 degrees [Fahrenheit, or 32 degrees Celsius]."

Fresher snow tends to have smaller crystals, which reflect more of the sun's energy. Therefore, they stay cooler longer and melt slower.

But as snow ages, the crystals tend to expand. At that point, they absorb more energy and melt faster. That process is speeded up even further by dust and black carbon residue from natural and man-made sources, such as plowing and smokestacks, settling over the snow's surface. Such impurities decrease the reflectance and increase absorption of energy.

In a feedback loop, the faster the snowpack melts, the more farmers and others have to pump groundwater from the valleys below. That, in turn, lowers the water table and dries out the overlying soil. That creates more dust, which then blows up onto the snow, causing it to melt even faster.

Aerial Measurements

Standing on the granite dome in warm, sunny weather, it is hard to imagine snow and ice. But the reason Painter and Gehrke began summer spectrometer tests this season is to improve the accuracy of measurements they will make when the area is covered with eight feet (two and a half meters) of powder. They want to see how reflective the terrain in Yosemite is without snow so they can better compare it to winter readings.

In 2012, NASA's Jet Propulsion Laboratory and the California Department of Water Resources launched the Airborne Snow Observatory (ASO). In winter, fixed-wing aircraft fly over the California mountains, equipped with a spectrometer, to measure reflectance, and LIDAR, to measure snow depth. LIDAR emits a laser and then observes how the beam is reflected, in a way similar to radar and sonar.

Reflectance and snow depth help Gehrke forecast how much water is going to run off the snowpack over the ensuing days, weeks, or months. He combines that information with data from a series of electronic pressure sensors that have been placed in the mountains, which indicate snow depth at their specific locations. He also consults tests that have been done continuously for decades, in which scientists or rangers walk predetermined transects and take core samples of the snow, to measure how much is there and how much water it contains.

The ASO program has vastly expanded the amount of data available on the California snowpack, from just a few select locations to whole mountaintops, Gehrke said. It has also provided much faster results. In less than a day from a surveillance flight, JPL technicians can have the one terabyte or so of data analyzed and sent to Gehrke's office.

Anne Nolin, a geosciences professor at Oregon State University in Corvallis, says the ASO program uses a "powerful combination" of technologies that is "very exciting and the first of its kind." The methods should work well in other snow-covered watersheds, "where we just don't have the information that we need for monitoring snow," says Nolin, who was not involved with the team's work but who also studies snowpacks through remote sensing.

Nolin added that the process requires local flights, because satellites don't yet have LIDAR with enough resolution.

Optimizing a Critical Water Supply

Painter said Yosemite is a good place to measure snowpack because the national park has highly varied terrain. Mapping the reflectance of all those surfaces will therefore improve the resolution of water system models. It doesn't hurt that working there allows him to follow in the footsteps of John Muir, America's most famous and influential naturalist and conservationist, he noted.

Yosemite's Tuolumne River Basin is also important because it provides water to the San Francisco Bay area, the Central Valley, and even Los Angeles through a long system of reservoirs and conveyances.

Like much of the state, the Yosemite area gets most of its precipitation for the year during the winter and early spring months. When that moisture falls as rain, it tends to run off the land quickly, and much of it ends up in the ocean. But when it falls as snow, it tends to stick to the mountains. It then melts slowly, so the water becomes available gradually and can recharge reservoirs and groundwater aquifers.

The past few years have seen shrinking snowpacks in Yosemite, as in most of the Sierra. "We kept thinking something was wrong with our equipment, but then we went and checked and saw that the numbers were right," Painter said of last winter's measurements, which were some of the lowest on record. In February, the Sierra snowpack was 14 percent of what's considered normal.

The drought is also readily apparent by the scent of a wildfire in the southeastern part of the park and in the charred remains of last year's Rim fire, which was the largest in recorded history in the Sierra.

As it stands, California can store only enough water in reservoirs to cover the state's needs for about a year and a half. The nearby Colorado River Basin, in contrast, can store water that will last for up to four and a half years, thanks to the massive Mead and Powell reservoirs.

All this means Gehrke's monthly and, increasingly, daily forecasts of water available from the Sierra snowpack are highly valued by city officials, farmers, hydropower producers, and flood-control managers. When they know how much water will be coming down from the mountains, city officials can order rationing and dam operators can decide how much water to release downstream.

Those users shouldn't expect to get any more surface water from the mountains this year, Gehrke said. They'll have to rely on reservoirs, which are currently only half full across the state, and groundwater, which is getting increasingly tapped. (See "California Snowpack Measure Could Reveal Future of Drought.")

Mitigating Climate Change

As much of the western U.S. gets even hotter and drier in the coming years, thanks to climate change, spectrometry studies will help scientists better measure the water that is left, Gehrke said.

The data will also help scientists refine and test climate models. Such models have proved not to be very accurate in predicting the amount of water in mountain systems because they are so complex, Gehrke said. But that could change soon. (See "Study Finds 'Extreme' Climate Change in National Parks.")

"The main icon of climate change is melting glaciers, and understanding the snowpack is key to understanding both our frozen world and our shrinking water supply," Painter said.

Snow science may also help governments lessen climate change itself. Over the past few years, scientists have suggested that reducing the amount of dust and pollution that falls on snow could not only slow snowmelt, but also cool the climate. That's because snow reflects the sun's heat back toward space while bare ground tends to absorb it.

"Our work will determine to what extent that's true," Painter said. "If it does work, it could provide a regional solution to help fight global warming, as opposed to dealing with the atmosphere directly, which is a global problem."

Earlier, Painter and Gehrke walked through a meadow to check on a snow observation station in a remote part of northern Yosemite. There had been a rare summer rain recently, so the ground was covered with wildflowers. It wasn't nearly enough moisture to alleviate the drought, but it was enough for the colorful blossoms to inspire a sense of hope.

Gehrke stood on the flexible metal "snow pillow," which undulated like a stainless steel waterbed. When flakes pile up on the metal surface, sensors underneath measure their weight. Gehrke then donned a helmet and harness and climbed up a tower about two stories tall. He checked the instruments that measure temperature, pressure, humidity, and wind speed and direction, and he installed a radio transmitter to improve data sharing.

On the way down the tower, Gehrke skinned his shin. A trickle of blood ran down to his sock. But he waved it off, saying it happens all the time.

After the snows fall, Gehrke will board a refurbished Twin Otter plane and fly back and forth over Yosemite's granite peaks, the same craggy features scaled by John Muir and photographed by Ansel Adams. Painter and colleagues will then analyze the data on the size and reflectivity of the snowpack, and that will inform Gehrke's reports on how much water Californians can expect to receive.

"Understanding the Sierra snowpack is critical to California's future," Painter said as he took in the spectacular view. "I hope my children and their children will be able to rely on the snow, too."

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