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To better measure the water in our snow, California is sending sharper eyes up into the sky.

Two sensors peer out from a turboprop aircraft, soaring from Mammoth Yosemite Airport over the white Sierra Nevada – collecting data that tells us almost exactly how much water we’ll have this summer. It’s far more precise than the manual snowpack readings that state water officials took Thursday in an anxiously watched rite of spring. Surveyors found nearly double the average snowpack for this time of year when they plunged aluminum tubes into the snow Thursday to sample for water content.

Last week’s findings from the sky measured 1.1 million acre-feet, or 350 billion gallons of water in the mountain snow of Yosemite’s Tuolumne River basin, which flows into Hetch Hetchy Reservoir and provides water to major Central Valley irrigation districts, San Francisco and several other Bay Area communities.

That’s more than 2.5 times the amount as the same time last year, and nearly as much as the record-breaking snow year of 2017. Because our winter was cold, its pattern of distribution is different than in recent years, with more snow at lower elevations and less snow at higher elevations. And there’s been less melting than expected, because recent flurries have kept snow so fresh and bright that it reflects sunlight.

That’s critical information for water managers, because they need to make room in reservoirs — and keep promises to farms and cities, who rely on the steady supply of water to irrigate our farms and fill our faucets.

“The later this big snowpack waits into the spring, the more powerful the likely snowmelt will be,” boosting risk of flooding, said Tom Painter, principal investigator of the Airborne Snow Observatory, designed by NASA’s Jet Propulsion Laboratory in Pasadena.

With the Snow Observatory’s aerial survey, “we can understand the magnitude and timing of snowmelt runoff,” he said. “Without this information, there’s a lot of guessing going on. Huge mistakes can be made.”

The survey, which began in 2013, covers the central Sierra, from Yosemite to Sequoia National Park, including the San Joaquin, Tuolumne, and Kings River basins. It depends on support from a coalition of local and regional water agencies, plus state funds from 2014’s Proposition 1, which authorized $7 billion in water infrastructure projects, because federal support has ceased.

But a $150 million funding package now under consideration in Sacramento could expand the project, supporting flights over the entire Sierra Nevada as well as the Trinity Alps.

Once a month in the winter and every two weeks during spring’s snowmelt, a pilot climbs into the aircraft and crests the eastern Sierra, climbing 20,000 feet in the air.

Like a lawnmower, the plane travels in long and perfectly straight lines, using GPS to traverse the sky.

“The pilot sees the flight line on a screen right in front of him, like a video game,” said Painter.

Two tools are pointed through glass in the plane’s belly.

One is LIDAR, which shoots 400,000 pulses per second of laser light toward the snow. It compares this to measurements taken in the summer. The difference between those two is snow depth.

The other is a spectrometer, which measures reflectivity, a sign of how quickly the snow will melt and reach downstream reservoirs.

Scientists combine this data with ground-based measurements and modeling to calculate water content.

For decades, more traditional tools — like Thursday’s final trek to Phillips Station — have helped us calibrate the water levels in our major reservoirs to meet the needs of the economy, environment and public safety.

The state also reports daily snowpack data from more than 90 remote electronic sensors, called “snow pillows,” that weigh the snow atop them. The April 1 measurement — considered the most important for predicting summer water supply — showed California’s snowpack at 161 percent of normal for that date.

But the tubes surveyors use measure just a cylinder of snow. And the pillows, located on flat spots at middle to lower elevations, don’t measure snow on high steep slopes with wind-blown drifts or avalanches. They can’t differentiate the melting rates of south and north-facing slopes. They cover only about 270 square feet — an infinitesimally small fraction of the vast landscape.

So their tally is just an extrapolation — with a margin of error that ranges from 20 to 40 percent. For a major reservoir, that’s a difference of billions of gallons of water.

In 2017, “our snow sensors were at zero and we still had inflow into the reservoir of 20,000 cubic feet per second,” said Kings River Watermaster Steve Haugen at an Association of California Water Agencies conference in Sacramento last year. “We were flying blind at that point.”

To hedge imperfections in those surveys, water managers have long over or under-estimated water forecasts to avoid flood damage or shorting deliveries.

But as demand increases, this once-acceptable practice is outmoded.

In contrast, the Snow Observatory is capable of measuring depths at several points in every square meter of a watershed. Its tools can see through hazy atmospheres and between trees. It measures avalanche depths and bare ground.

As a result, it is far more accurate, with runoff forecasts that are 96 to 98 percent accurate.

While satellites will someday work well for snow mapping, the technology isn’t ready yet, said Painter. And snow-specific satellites would be expensive, costing an estimated $1 billion. Moreover, satellites have a fixed schedule, while airplanes can fly whenever weather is good.

“The Observatory provides key information about snowpack, especially above the levels where our sensor network are deployed,” said state climatologist Michael Anderson. “It provides more certainty for water managers’ decisions.”

The Turlock Irrigation District, which co-owns Don Pedro Reservoir on the Tuolumne River, has taken advantage of the program for the past six years. The San Francisco Public Utilities Commission and Modesto Irrigation District are also partners in the project.

“By having more of an exact number on the state of the watershed, the operations can be made more efficient since they do not have to take into account the uncertainty that currently exists,” said Turlock Irrigation District’s Brandon McMillan.

If the Snow Observatory project gets state funding, it seeks to add two more planes and new facilities, covering every major watershed in the state.

Senate Bill 487, introduced by Sen. Anna Caballero, D-Salinas, is now awaiting action by the Senate Appropriations Committee. The bill would require the department to collect and share the aerial survey data up to 10 times per year in each hydrologic area of the state.

Proponents of the bill say it’s crucial, because as California’s population grows, more people depend on a reliable water supply. At the same time, our warming planet is expected to create “climate whiplash,” with volatile swings between wet and dry years.

With more extremes between dry and wet, accurate water management becomes essential, said Painter.

“Having that understanding of snowpack,” he said, “is really critical to protect our infrastructure, protect our water resources, and in general to stay safe.”