Even as a changing climate appears to be carving away California’s vital mountain snowpack, scientists have begun to take its measurements with unheard-of precision – using aircraft-mounted lasers to gauge snow depth within a few inches, and timing out the melt by the snow’s reflected light.
While the technology isn’t new, NASA’s Airborne Snow Observatory is applying it on a scale of stunning breadth.
The scientists have begun by using a river basin north of the Yosemite Valley, along with a Colorado watershed, as test cases. But their plans are much bigger: capturing the entire Sierra Nevada range as well as the Upper Colorado River basin.
“This is a new era,” said Tom Painter of the Jet Propulsion Laboratory, the principal investigator on the project. “This is the way it’s going to be.”
The measurements of snowpack are essential for water-supply managers, especially in the western United States, where snowmelt accounts for about 75 percent of the freshwater supply.
Some managers, for example, must try to determine how much water to hold in reservoirs, and how much to release, by anticipating the amount of water that will flow down mountainsides as the snow melts.
But until now, many of those measures have been made with surprisingly primitive tools – often, a specialist walking into a snow field to push a long tube deep into the snow.
Mountain stations with snow-sensing equipment also have their limits. In some cases, none at all exist at the highest elevations, where winter snow lingers into spring.
“In the Southern Sierra, our highest measurement is probably around 10,000 feet, yet fully one third of the watershed is above that elevation,” said Frank Gehrke, a co-investigator on the project and chief of the California Cooperative Snow Surveys Program at the state Department of Water Resources.
That leaves large gaps in knowledge about the snowpack, and even larger uncertainties for water managers.
“It’s like looking at your TV screen or your computer monitor,” Painter said. “Up until now, it has been that essentially four of the pixels on the computer screen are active, and all the rest of them are black.”
The three-year, $4 million demonstration project, a collaboration of NASA’s Jet Propulsion Laboratory in Pasadena and the state Department of Water Resources in Sacramento, has already begun to prove the value of the new approach.
The scientists are turning around their data within 24 hours and providing it to water managers.
“The snow observatory turns on all of the pixels,” Painter said. “All of a sudden, it’s a complete picture.”
Since April, the scientists have been making weekly flights over the Tuolumne River basin – north of the Yosemite Valley – shooting lasers into the snowpack with one device, and marking how much light the snow reflects with another.
Bolted to a Twin Otter plane, a scanning lidar system, for Light Detection and Ranging, bounces lasers off the surface of the snow.
Comparing those readings to scans taken of the same terrain before the snow fell yields precision measurements of snow depth, within an accuracy of four inches.
A second instrument called an imaging spectrometer captures how much light the snow is reflecting. The instrument sees the reflected light in nearly 100 different colors, far more sensitive than the human eye.
New, fine-grained snow has high reflectivity, and also melts more slowly. Older snow has bigger grains and bigger gaps, absorbs more sunlight and melts much more rapidly.
And both can be covered in dust and soot, accelerating melting.
The laser measurements allow scientists to calculate how much water the snow holds, known as the snow-water equivalent.
And knowing how much light the snow reflects reveals how fast it will melt.
That can lead to far better predictions in a given season of when the snowmelt will occur and how fast it will fill up reservoirs, or whether areas downstream are likely to be flooded.
“The driving force – especially once you get into spring – pushing snowmelt is the sun’s energy,” Gehrke said. “How much is going to be absorbed by the snowpack can vary quite dramatically.”
Last month, the scientists measured 375 million cubic meters of water contained in snow in the Tuolumne basin, enough to fill the Rose Bowl 1,180 times.
The measurements will continue into July, the traditional end of the snowmelt season.
In some ways, the scientists are in a high-flying race against time. Climate researchers say a warming world is already having a measurable effect on California’s snowpack.
“It’s something that has been going on for awhile now,” UC Irvine Earth System Science Professor Jay Famiglietti said. “We’re seeing snow seasons shortening, we’re seeing the snowpack getting smaller, and those long-term trends are definitely pointing to the impacts of climate change.”
That, along with groundwater depletion, could mean California – like many places across the world – is rushing headlong toward a water crisis, said Famiglietti, who uses satellites to measure groundwater depletion around the planet.
“The prediction for the end of the 21st century is for as much as a 90 percent reduction in the snowpack, relative to the previous century,” he said. “That may be an extreme case, but on the other hand, sometimes these things are underestimated.”
The darkening effects of dust on snow, for instance, and its effects on snowmelt, are only now being factored into computer models of Earth’s climate, Famiglietti said.
In any case, he said, time is of the essence.
“What might be happening is that some of these things that are human driven are speeding up the process,” Famiglietti said. “Dry areas are getting drier, and that accelerates the use of groundwater.”
Our immediate future is no more comforting.
California is heading into a dry period in an already dry year, the second in a row.
Normally, the snowpack supplies about 30 percent of the water used in California.
But the snowpack was only 3 percent of normal as of Wednesday, Department of Water Resources spokesman Ted Thomas said.
Reservoirs remain at fairly high levels, however, thanks to a burst of winter precipitation.
That is allowing Water Resources officials to estimate delivery of 35 percent of the requested water supply to the State Water Project, despite the extremely thin snowpack.
A recent report on the season ahead by the National Oceanic and Atmospheric Administration warned of the likely spread of drought conditions in the western United States, including in California.
And a May-through-August assessment by the interagency Fire Predictive Services group calls for above-normal wildfire danger, above-normal temperatures and below-normal rainfall for much of Southern California.
In Orange County, the fire danger is forecast to rise above normal by July.
Painter says he is greatly concerned about the prospect of a megadrought, lasting decades. Such droughts have occurred in the past, the paleontological record shows, and could bring drastic changes to the western United States.
“That is the thing I most worry about,” he said. “We’re now arguably in the 13th year of drought in the Southwest. And do we know for sure that it’s not going to go on for another 17 years? What will happen to Lake Mead? What will happen to Lake Powell? What will happen to Mexico, what will happen to Southern California? What is going to happen to the climate? Because there’s that much less snow-covered area that can be reflecting the sunlight back.”