Mammoth Lakes, Calif. — The majestic beauty of California’s Sierra Nevada never fails to impress. But the mountain range, which stretches hundreds of miles, is much more than a stunning vista. It’s a linchpin that helps make living in an arid state possible.

That’s because one of California’s most important water supplies is melted snow. Each spring and summer, the Sierra sends runoff down its slopes that recharges rivers and reservoirs, allowing crops to be irrigated and drinking glasses to be filled.

Knowing with precision how much snow has accumulated is crucial for farmers and water managers.

That’s where a mapping project at NASA's Jet Propulsion Laboratory known as the Airborne Snow Observatory comes in. Using measurements gathered by specialized instruments on a plane, scientists have been able to gain an unprecedented understanding of the amount of water present in the Sierra’s snow.

This year, after California’s very wet winter, the totals have been remarkably big.

Using the NASA data, we compared this year's snowpack with that of 2015, when the state was in the grip of drought (which, at least officially, is still ongoing). In the interactive maps below, the white areas had a meter, or 3.3 feet, or more of snow on the ground in March.

High in the mountains, this year's snow blankets the ground in layers tens of feet deep in many places. In 2015, almost none of this area had snow that thick:

2017 2015 Burro Pass The Slide Finger Peaks 11,498’ PIUTE CANYON Arndt Lake Rock Island Lake SUICIDE RIDGE Whorl Mountain 12,033’ SPILLER CREEK CANYON Crazy Mule Gulch 23.8 feet 15.1 feet deep Bath Mountain Doghead Peak ROCK CANYON Quarry Peak 11,161’ Slide Mountain Doe Lake 14.6 feet Tallulah Lake MATTERHORN CANYON Surprise Lake Sister Lake 1/4 MILE Smedberg Lake Benson Pass Crazy Mule Gulch 15.1 feet deep Bath Mountain Doghead Peak ROCK CANYON Quarry Peak 11,161’ Slide Mountain Doe Lake 14.6 feet Tallulah Lake Surprise Lake Sister Lake 1/4 MILE Burro Pass The Slide Finger Peaks 11,498’ PIUTE CANYON Arndt Lake Rock Island Lake SUICIDE RIDGE Whorl Mountain 12,033’ SPILLER CREEK CANYON Crazy Mule Gulch 12.6 feet 3.9 feet deep Bath Mountain Doghead Peak ROCK CANYON Quarry Peak 11,161’ Slide Mountain Doe Lake 3.4 feet Tallulah Lake MATTERHORN CANYON Surprise Lake Sister Lake 1/4 MILE Smedberg Lake Benson Pass Crazy Mule Gulch 3.9 feet deep Bath Mountain Doghead Peak ROCK CANYON Quarry Peak 11,161’ Slide Mountain Doe Lake 3.4 feet Tallulah Lake Surprise Lake Sister Lake 1/4 MILE Tap to see 2017 Click to see 2017 Tap to see 2015 Click to see 2015

At the lower elevations around the Hetch Hetchy reservoir, which collects most of the melting snow runoff in this area and supplies water to millions, there was almost no snow to speak of in 2015. This year, the snowpack reached down to within a few hundred feet of the reservoir's edge:

2017 2015 TILTILL MOUNTAIN Mount Gibson TILTILL VALLEY 7.2 feet Rancheria Falls Hetch Hetchy Reservoir Le Conte Point RANCHERIA MOUNTAIN 1/4 MILE PATE VALLEY TILTILL VALLEY Rancheria Falls Hetch Hetchy Reservoir Le Conte Point 1/4 MILE TILTILL MOUNTAIN Mount Gibson TILTILL VALLEY Rancheria Falls Hetch Hetchy Reservoir Le Conte Point RANCHERIA MOUNTAIN 1/4 MILE PATE VALLEY TILTILL VALLEY Rancheria Falls Hetch Hetchy Reservoir Le Conte Point 1/4 MILE Tap to see 2017 Click to see 2017 Tap to see 2015 Click to see 2015

These maps show parts of the Tuolumne Basin, which in late February was blanketed by 1.2 million acre-feet of snow-water equivalent, or the amount of water that would result if the snow were instantly melted.

That's about 10 times the amount as the same time in 2015, said Thomas Painter, a snow hydrologist at the NASA Jet Propulsion Laboratory/California Institute of Technology, who leads the NASA program.

He added, “And it keeps on coming.”

The pattern has held for the central Sierra region as a whole:

200% of average 1983 Maximum Snowpack this year 100 Average 2016 2015 Minimum 0 January February March April May June July 1983 Maximum 200% of April 1 Average Snowpack this year Average 100 2016 2015 Minimum 0 Jan. F M A M June Source: California Department of Water Resources | Note: Chart shows the water content of snow in the central Sierra region as a percentage of the long-term April 1 average.

The airborne observatory has been detecting snow depths in the mountains ranging from a few feet at lower elevations to more than 70 feet in avalanche areas.

“Some of the snowdrifts have faces of 25 to 40 feet,” said Jeffrey Payne, a water resources manager at the Friant Water Authority who has analyzed the NASA data. “So we’ve got some pretty serious snow.”

Strong winds created huge snowdrifts near the western cliffs of the Minarets in the central Sierra Nevada. Photo by Jim Wilson/The New York Times

Trees on a slope in the snow-covered eastern Sierra Nevada. Photo by Jim Wilson/The New York Times

Ski resorts that typically close in the spring are so deeply blanketed that they have been making plans to extend their seasons.

Officials at Squaw Valley, in the Lake Tahoe area, and Mammoth Mountain, below, in the eastern Sierra, said they anticipated staying open well into summer.

Skiers at Mammoth Mountain. Photo by Jim Wilson/The New York Times

A snow-covered ridge on the western slope of the central Sierras. Photo by Jim Wilson/The New York Times

The snow observatory project, which began flights over the Sierra in 2013, is a groundbreaking initiative in California, where aging infrastructure, a warming climate and rapid population growth have made water management a high-stakes job.

For decades, state officials have estimated snowpack levels by extrapolating from ground-based data gathered at points across the range.

The margin of error, unsurprisingly, has been huge.

“It’s like turning on your TV screen and four of the pixels turn on, and you can only use those four every single time you watch ‘Breaking Bad,’” Dr. Painter said.

Every one to four weeks, the NASA crew circles above the Sierra Nevada in an airplane that fires laser pulses toward the ground. By measuring how fast the pulses bounce back, the scientists are able to create detailed topographical maps.

Compare those with maps of the mountains when bare and factor in the snow’s density, and they can tell how much water is present.

With the view from the sky, Dr. Painter said, “we turn on the whole screen, every pixel.”

Thomas Painter on the steps of the plane NASA uses to gauge snow depth. Photo by Jim Wilson/The New York Times Dr. Painter with a device that uses laser beams to precisely measure topography. Photo by Jim Wilson/The New York Times

The observatory also measures the reflection of sunlight off the snow, which is critical to understanding how much energy the snow absorbs and how fast it could melt.

Mr. Payne of the Friant Water Authority, which manages water for agricultural land in the San Joaquin Valley, said the new snow data was game-changing for farmers, who will be able to plan their crops with greater confidence.

“We need to be smarter about how we approach water resource management,” Mr. Payne said. “And this new technology is sort of a beacon of hope.”

For now, the observatory is taking measurements for most of the central Sierra Nevada. The hope is to get more buy-in from state officials and expand to the whole range, Dr. Painter said.

It has been an increasingly easy sell.