How much water in that snowpack? Scientists seek a better gauge.
More accurate, more frequent measurements of mountain snowpacks will allow water managers to mete out reservoirs with greater confidence. Two watersheds in the western US are testing grounds for a new aerial approach.
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"Once we start nudging that with climate change, we go into a very ugly scenario," he continues. With warmer temperatures, winter snows come later at lower elevations, replaced by rain early on – water that immediately flows down the slopes. This reduces the amount of water stored as snow. Rising temperatures also increase evaporation off of reservoirs and increase the rate at which snow changes into water vapor without melting into a liquid first, a process know as sublimation.Skip to next paragraph
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The federal government maintains a network of snow gauges throughout the US, and forecasters augment that information with measurements taken from aircraft. The data are used to build maps of snow cover and the snow's water content, as well as to feed forecast and other weather and climate-related models. For much of the country the system works well, says Andrew Rost, director of the National Weather Service's National Operational Hydrologic Remote Sensing Center in Chanhassen, Minn.
But the mountain West is a challenge. Mountain terrain makes it difficult to set up and maintain a dense network of snow-measuring stations on the ground. The sensors the center uses to measure the snow's water content from aircraft require the planes to fly no more than about 500 feet above the surface – out of the question in mountainous terrain. The sensors can't accurately measure water content with the snow when it gets especially deep, as it does in the mountains. And the aircraft fly a narrow path over set locations, largely to support forecasting needs in areas threatened by spring floods – much as federal hurricane-hunter aircraft are sent aloft only when a hurricane threatens.
All this means that "we don't know the snowpack well," Painter says. "Given how little information there actually is about it, it's remarkable" that forecasters and water managers "do as well as they do."
There is plenty of room for improvement. Citing California's American River as an example, Painter says the forecasts for total run-off expected between April and July, when the melt season typically ends, are off by at least 20 percent more than half the time. Twenty-five percent of the time, the forecasts are off by as much as 40 percent.
That's where Painter's project comes in. It uses NASA's Airborne Snow Observatory to gather key information about the snowpack over a wider area and in more detail than current approaches can deliver.
The aircraft, which fly at altitudes of between 17,000 and 19,000 feet, use a radar-like device called lidar, which sends rapid pulses of light, rather than radio waves, and receives the pulses reflected off the snow to measure snow depth. Focusing on the Tuolumne watershed in the Sierra Nevadas, which feeds the Hetch Hetchy reservoir, the team first took measurements of the land height when the ground was bare. Now they are measuring the height of the snow surface. By subtracting the two, they get the snow depth and can estimate its water content.
Another instrument measures how much light the snow's surface reflects. Past research in the Rockies has shown how dust can accelerate snowmelt by giving the dust a darker hue.