Satellites can map the snow’s area, but they provide huge snapshots, compared with the aerial surveys, which can show a patch of snow a yard or two square, and a patch of bare ground next to it, Painter said. And the satellite images can take weeks to arrive.
A sparse network of underground sensors, known as “snow pillows,” provides some idea every few hours of the snow’s depth, but they weigh everything that lands on them, including tree branches and the occasional bear, Painter said. They are small and there are far too few of them, none in the higher elevations of a mountain range that reaches more than 13,000 feet. There is no method to calculate snow albedo.
“We are massively undersampling the snowpack,” Painter said.
Climate change has begun to make the historic data that snow scientists use less reliable, McGurk said. “Our predictive tools get noisy,” he said. “What that means is you have to hedge more.”
Attempts at aerial snow measurement have been made for the past five or 10 years, said Ethan Gutmann, associate scientist at the National Center for Atmospheric Research, who has mounted a single, fixed laser in the Rocky Mountains near Boulder, Colo., to measure the snowpack there.
The aerial survey can cover a huge area once a week, while Gutmann’s laser provides continuous, precise information but for a much smaller swath. It is much less expensive than the $1 million aerial project and not subject to days when planes can’t fly, he said.
But the ASO, he said, is “the future of snow science and resource management for mountainous areas” and probably beyond.
Cost of moving water
Hetch Hetchy is a small reservoir by California standards; it holds just 360,000 acre-feet of water. (An acre-foot is the volume of water that would cover an acre at a depth of one foot, or about 326,000 gallons.). When water is plentiful, the challenge is to channel it to hydroelectric generators without allowing it to collect too quickly in the reservoir and spill over the dam.
The water needed to generate electricity at the Kirkwood Powerhouse 11 miles away is worth $66,000 per day, McGurk said. Any mistake that results in the use of gas or coal to generate power is therefore quite expensive and releases greenhouse gases into the atmosphere. Heavy spills also can affect downstream ecosystems, he said.
In times of scarcity, like the approaching summer, the trick is to keep the reservoirs as full as possible, while supplying consumers and farmers with the water they need. Moving water is an extremely expensive proposition in California. About 15 percent of the power used in the state goes to pumping water through its vast network of reservoirs and pipelines, McGurk said.
“There are very large dollar figures attached to this water and knowing how much is there and when it’s going to come out,” Gutmann said.
In droughts, water managers may have to shortchange power generators to ensure that reservoirs are full when the snowmelt stops flowing in the spring. “There is a great penalty, both political and personal,” for missing that goal, McGurk said.
“We’ve had reasonably good tools, and we were doing okay,” he said. “And what we’re trying to do is get to the next level.”