Surveying snowpack from the air
The ASO is a repurposed de Havilland Twin Otter plane, equipped with LiDAR (a remote sensing method that uses light in the form of a pulsed laser to generate three-dimensional information on surface characteristics), spectrometer, GPS, and an inertial measurement unit that tracks the pitch and yaw of the aircraft.
Together, the LiDAR and spectrometer, which measure both topography and reflected sunlight, create a three-dimensional map of the snowpack.
Incredibly, LiDAR data gathered from 20,000 feet in the air can measure snowpack topography to within 10 centimeters, Painter said.
The purpose of this data is to represent snowpack, particularly its water equivalent, “in a more meaningful way.”
Until now, snowpack data was typically measured using snow courses or snow pillows. Both snow courses and pillows offer limited information, as neither one covers a large amount of terrain. “The greater purpose of them is as an indicator of percentage [of average],” Painter said. “But water isn’t allocated in percentages; it’s allocated in acre-feet.”
“Remote sensing is key to understanding the whole of the mountain snowpack,” he concluded.
Considering about 75 percent of the freshwater supply for the Western U.S. comes from snowmelt, understanding mountain snowpack and calculating the snow water equivalent with greater precision is crucial for more refined water management.
Painter, who completed his PhD at SNARL in 2002, said his interest in the remote sensing of snow covered areas began when he was a graduate student. He moved to Colorado after receiving his PhD to study the controls on snowmelt in the mountains of the Colorado River Basin.
“In particular,” he said, “we discovered that the primary control on snowmelt rates in those mountains is increased absorption of sunlight by dust deposition from deserts of the Western U.S.”
This impact was heretofore unrealized.
The ASO project allowed Painter to return to the Eastern Sierra, which he said was never far from his mind. With funding from both NASA and the California Department of Water Resources, ASO lifted off last snowpack season.
Painter said that the ASO data has revealed many surprising features of the Eastern Sierra snowpack, including a greater dust discoloration at Tuolumne’s Mt. Lyell than anticipated.
“That was a big surprise, last year,” he said.
Dust decreases the snow’s reflectivity from 80 percent near the top of the mountain to 50 percent lower on the mountain. The decrease in reflectivity increases the snow’s sunlight absorption, “and accelerates that melt,” Painter said. “It has a huge impact on that snowpack.”
Some of that dust may come from far away deserts, like the Gobi, but Painter has another theory: “This year and last year, we saw so much dust in the snowpack that I’m starting to wonder if it isn’t from the Central Valley,” he said. According to satellite imagery, dust is not coming up from Owens Lake, but “distinctly from over the mountains.”
This and other ASO findings have shown that snowpack is “a hell of a lot more heterogeneous than we thought, and than our models projected,” Painter said. “It’s a fascinating time for looking at data.”
The more detailed snowpack data available through the ASO is already being put to use at Yosemite’s Hetch Hetchy Reservoir, Painter said. Last year, Hetch Hetchy relied on ASO data after their snow pillows melted out, leaving them with no direct snowpack information to inform their operations.
“With ASO, they had complete basin knowledge,” Painter said. This allowed Hetch Hetchy to more carefully budget water use in energy generation, among other things.
Painter said the ASO data “could be transformative in this [water management] business.”
Not only the Los Angeles Department of Water and Power, but also Southern California Edison, Pacific Gas & Electric, Metropolitan Water District, Mammoth Community Water District, the National Weather Service, and many other agencies and organizations have expressed interest in the ASO data.
As for this year’s data, unsurprisingly, Painter said the forecast “is grim. As of May 2, we can see that we’re really starting to lose the snow water equivalent in the basin … Most likely, we’re going to see our snowpack just plunge.”
This year, the ASO found basin wide snow water equivalents of 73-78 percent of last year.
“In terms of volumes,” Painter said, “that is 30,000-50,000 acre-feet less.”
Painter said the ASO project “appears headed for long haul adoption … We’re working on a plan to fly the entire Sierra Nevada and the mountains of the Colorado River Basin.”
Given California’s three-year drought, “Demand has reached up hard against supply … We need to have that data now.”
