In the middle of a clearing, beneath the bright blue bowl of the western Colorado sky, two scientists stood chest-deep in a pit dug into the snow. Crisp morning sunlight glinted off the blocky metal trowel in Andrew Hedrick's hand. Hedrick, a hydrology technician, stuck the instrument into the solid white wall before him, withdrew it — now loaded with snow — and delicately skimmed off the extra flecks. Amid the muffled whine of snowmobiles ferrying researchers and equipment around the field site, he weighed the snow, then emptied his trowel, ready for another sample. Researchers will use the information Hedrick collected to validate new ways to measure snow and calculate the density of the snowpack — which, together with its depth, indicates how much water it contains.
A handful of other scientists, from agencies and universities across the globe, swirled around the clearing and among nearby spruce trees. The researchers measured the snow with poles and rulers, radar and microwave sensors, and even packed a cooler with snow samples destined for micro-CT scanning at a lab in New Hampshire. Overhead, sensors affixed to airplanes made similar measurements throughout the day.
Snow delivers about 60 to 70 per cent of the western U.S.'s water supply. The snowpack is an icy natural reservoir that swells throughout the winter, then melts during the summer, providing rivers, agricultural fields, and communities with water. But the amount of moisture in snow varies, and keeping track of how wet snow is across an entire landscape — information essential to the resource managers, farmers and scientists who forecast water supplies and flood potential — has proven difficult. To figure out the best way to do it, Hedrick and about 100 other researchers converged on snowy, flat-topped Grand Mesa, in western Colorado, for an ambitious scientific treasure hunt in February. For three weeks, they took measurements and tested dozens of instruments and methods, looking for the optimum suite of sensors to survey what one scientist calls "the holy grail" of snow-sensing research: the amount of water held within the snow.
Government agencies monitor the Western snowpack at hundreds of locations across the region. But, useful as those point-measurements are, they don't tell the whole story, said Kelly Elder, a research hydrologist with the U.S. Forest Service in Fort Collins, Colo., and the leader of the team that managed the ground campaign at Grand Mesa. Aerial measurements from a satellite or a plane that cover the entire landscape — not just a series of individual sites — are also necessary. "If we can measure from space, or from the air, then there's hope," Elder said.
Satellites are already doing some of that work: For decades, they have provided information about how much of the globe is covered by wintertime snow. But that's not enough, says hydrologist Jessica Lundquist at the University of Washington in Seattle. "What that doesn't tell you is, OK, do you have a thin little bit of snow, or do you have a really deep pile of snow?" Without corresponding knowledge of how deep and dense the snow is, researchers don't know how much water is stowed within it.
Years of research suggest that there isn't a "silver bullet" sensor that can measure snow's water content on its own from a satellite, said Jeffrey Deems, a scientist at the National Snow and Ice Data Center in Boulder, Colo. One goal of the Grand Mesa research — part of a planned five-year project, the first year of which is supported by a US$4.5 million NASA grant — is to find the ideal combination of instruments that might someday be launched on a satellite to monitor how much water is in snow worldwide. "We have all of these different techniques," he said. "They all have shortcomings, but they all have advantages as well — and if we can put the right advantages together, we can solve the problem."
The project's aerial sensors include LIDAR, a "really fancy range-finder," Deems said, that uses lasers to measure distances; by comparing scans taken before and after a snow dump, researchers can calculate snow depth. Measurements of the microwave radiation naturally emitted by the Earth are also part of the project: Different densities of snow modify the microwave signal as it moves skyward, so monitoring it from above can give an estimate of snowpack density. Additional airborne sensors measure the temperature of the snow and the albedo, or how much sunlight it reflects — factors that can affect how quickly it melts.
One of the difficulties of measuring snow from planes or satellites is that trees can get in the way. About half of the snow-covered landscape in the Western U.S. is vegetated, so for an aerial sensor to work in the West, it must be able to measure the snow beneath the canopy cover. Grand Mesa, with its 85-square-kilometre flat top and range of forest types — from open shrub-land to dense stands of spruce and fir — is an ideal location to test how well different instruments deal with trees in the absence of other complicating factors, like large changes in topography. To ground-truth the aerial assessments, researchers conducted manual measurements — such as weighing trowels of snow — at about 100 locations across the mesa.
The researchers also visited another area, a steep alpine basin near Silverton, Colo., to expose their instruments to a different set of conditions. That will help them find a suite of sensors that can work equally well over open Alaskan tundra and a forested Colorado hillside, Deems said. "Can we make the same system work and be resilient to these very different environments?"
And even in the same place, conditions can change, sometimes within just a few hours. As the temperature on Grand Mesa climbed through the early afternoon, scientists shed their gloves and hats, dripping water puddled beneath a snowmobile trailer, and the snow along the footpath from the researchers' basecamp to the nearest field site took on the consistency of a heavy, jacket-soaking snowball. NASA scientist Ludovic Brucker crouched down to scoop loose snow from underneath a surface crust, demonstrating the layers of differing density that often form within a snowpack. When he stood, a white research plane bearing light reflection and temperature sensors and other snow-sensing instruments flew overhead, its faint roar mingling with the chatter of a nearby creek.
Emily Benson is an editorial intern at High Country News.