Airborne Lasers Yield Better Measure of California's Water Supply

California has had one weird winter this year: lots of snow and rain early, and almost none since January. It’s in years like this that it’s especially crucial to know just how much water to expect from melting Sierra snows — runoff that provides about a third of the state’s water supply. Current estimates combine patchy measurements with a kind of sophisticated guesswork. But that may be about to change with new technology that’s currently being tested.

The Limits of Statistics

A funny thing happened when I accompanied Frank Gehrke out on the regular Sierra snow survey this month. He and a group of reporters made their way out to one of the meadows near South Lake Tahoe where he conducts the monthly manual survey in the winter and spring. But this time, there was nothing to do.

“There’s not even a patch of snow we can seek out and measure,” he said. It wasn’t a surprise — he knew it had all melted already.

Normally he’d stick an aluminum tube into the snow and weigh it to calculate the amount of water in the snow, or the snow-water equivalent.

“That’s been the procedure really since about 1910,” said Gehrke, who’s been doing it for years as the snow survey chief for the California Department of Water Resources (DWR). But that’s a little difficult when, as on this day, spring had sprung at the survey site.

DWR uses a combination of manual surveys and remote electronic sensors that feed a statistical model to project how much water there will be for cities and farms throughout the state when the snow melts each summer. But they never really know, exactly.

That can get nerve-wracking for water managers like Bruce McGurk, who used to run San Francisco’s Hetch Hetchy Reservoir.

“I wouldn’t quite call it chicken,” McGurk said. “But you sure are watching. You’re hedging; you’re always being aware. You can’t be too full too early.” That would put people downstream at risk if there were a big late-season storm. On the other hand, managers need to keep enough in storage, to get through the dry summer. “A reservoir operator has to be nimble and take action now,” he added.

Which is why McGurk is excited about the project he’s working on with NASA, DWR and the San Francisco Public Utilities Commission, which runs Hetch Hetchy.

Going Airborne

It’s called the Airborne Snow Observatory, or ASO: an airplane outfitted with instruments that give scientists a better understanding of the snowpack than they’ve ever had before.

“It’s kind of like looking at your TV screen,” said Tom Painter of NASA’s Jet Propulsion Laboratory at CalTech, the principal investigator on the project. “Your TV screen is the mountain basin,” he explained. And the way we’re currently measuring the snow in the mountains is by just looking at a few points — ike a screen that’s blank except for a few illuminated pixels — and using statistics to complete the picture.

“What we’re doing,” said Painter, “is turning on the entire TV set. We’re allowing you to see every one of the pixels.”

The end result is a set of more comprehensive maps showing how much water is really up there, in the mountains. This is the first month of the three-year, four million-dollar project, funded by NASA and DWR. Right now, they’re focusing on a watershed in the Rockies and the Tuolumne River in Yosemite, which feeds into Hetch Hetchy. It’s a test phase, to work out the kinks.

The Plane

The maps are created with two high-tech tools mounted in a DeHavilland Twin Otter, a Canadian-built aircraft frequently used for research flights. Painter offered a tour of the gear mounted inside.

“The lidar lets us know the snow depth,” he said. Lidar is a laser that measures distance — really accurately, and really quickly. (If you’ve ever been pulled over by a police officer using a “radar” gun, you’re familiar with the technology.) By multiplying snow depth and snow density — the density calculation comes from manual surveys — you get the all-important snow water equivalent.

“The spectrometer here, the black box, this allows us to know how much sunlight is being absorbed by the snow surface,” he said. That tells scientists when the snow will melt.

“(The maps) are really beautiful,” Painter says. “The spatial distribution of snow water equivalent is — maybe it’s an acquired taste — but I think it’s fascinating and quite beautiful.”

The scenery from the plane isn’t bad, either. On a sample flight in the Lake Tahoe area, flight operator Cate Heneghan, who usually works on NASA space missions, marveled at the famously clear water.

“It’s greens and blues,” she said over the crackly headset. “You know, you see pictures, but you always wonder: is it color enhanced? But then you come here and see it in real life and you go, ‘Whoop, no, I guess it’s not color enhanced.'”

Heneghan operates the scientific instruments and coordinates between the plane’s pilots and Gehrke and Painter, who work in the back. The plane crisscrosses the watershed, measuring the depth of the snow and how much light it’s reflecting, down to the half-meter.

Big Snow Data

After the flight, a team from NASA works around the clock to process all that data. The goal is to run weekly flights, with a 24-hour turnaround time, so it’s relevant to reservoir managers. McGurk is a consultant on the project, coordinating between NASA and Hetch Hetchy.

“Yeah we all get real excited when the images start to come back and everybody goes ‘Woah, cool look at that,’” he laughed.

These tools are becoming increasingly important. Painter says that statistical models rely on historical data. And with a changing climate, the future is going to look less and less like the past.

“With these physical measurements, you don’t have to rely on what happened last year,” he explained. “What happened over last 10 years, what happened over the last 20 years, has no bearing on your ability to model the snow-melt in this year.”

Gehrke, elder statesman of California snow surveys, welcomed the change. He said this program, along with new stations that measure atmospheric rivers and a program that supports better coordination between water managers is bringing in a next generation of water management in California.

“You know, it’s sort of like the internet,” he said. “Initially, it’s like, ‘Yeah, whatever, who cares.’ And all of a sudden, it’s now like, people can hardly go anywhere without it. And I think that’s what we’re going to find, as all of this starts to build out, people are going to start to think, ‘Wow, I don’t know how they operated before this.'”

With this new data, water managers will be better able to plan for water supply, flood control, hydropower and the environment. And scientists will be able to use this deluge of data, too, to learn more about the alpine environment, one of the places most threatened by climate change.