Shortly before 5 P.M. Pacific Daylight Time on Friday, a small, nondescript Beechcraft Air King climbed from the runway at Mammoth Yosemite Airport, reached 15,300 feet and headed over the snow-starved Sierra Nevada.

To anyone driving past the airport, nestled on the backside of the mountain range, the airplane's departure might have seemed like just another private plane taking off from a rural airfield.

Instead, the 1960s-vintage, twin-engine turboprop NASA's Airborne Snow Observatory (ASO) is playing a key role in helping water managers in drought-ravaged California track the amount of water stored in the state's paltry Sierra snow pack.

The aircraft carries two instruments whose data combine to provide the most comprehensive estimates yet of snow's water content critical information for forecasting the mount of water that the mountains hold in reserve for what traditionally has been the state's dry season.

The observatory began flying in 2013 as a three-year demonstration project, starting with one watershed. On Friday, the ASO would fly two sorties, traveling along tightly spaced, back-and-forth tracks over four watersheds.

The observatory's progress during its first two years has transformed from a let's-see-if-this-works effort to a must-have data source that has caught the attention of other western states.

Until now, water managers have never known the true distribution of snow water equivalent across a watershed, says Thomas Painter, a hydrologist at NASA's Jet Propulsion Laboratory in Pasadena and the project's lead scientist.

"You can't manage what you don't measure," he says.

Yet mountain snows provide about 75 percent of the West's water. Population growth, a relentless draw-down of water stored in aquifers, and global warming's projected impact on precipitation and soil moisture pose significant challenges for managing water resources.

As if to underscore the point, researchers at NASA's Goddard Institute for Space Studies and Columbia University's Lamont-Doherty Earth Observatory published a study in February that yielded projections for "a remarkably drier future that falls outside the contemporary experience" of people and ecosystems in western North America.

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Eyes in the sky: How NASA helps gauge drought impact