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Why is it difficult to figure out how much snow will make it to the Colorado River?

MICHEL MARTIN, HOST:

Snow in the Rocky Mountains is vital to the farmers, cities, and electric utilities that rely on the Colorado River. But it's surprisingly hard to say exactly how much water trapped in that snow will actually reach the river in a given year. So scientists are looking for ways to let people downstream know how much water to expect. Alex Hager with KUNC has this report.

ALEX HAGER, BYLINE: Here's the problem with the Colorado River. There's more demand than there is supply.

JESSICA LUNDQUIST: We know the Colorado River is oversubscribed. Every drop of water is wanted by multiple people.

HAGER: That's Jessica Lundquist, an engineering professor with the University of Washington.

LUNDQUIST: And so if you're off by the number of drops of water you're promising to somebody, they're not happy.

HAGER: But it's really hard to figure out exactly how much snow will make it into streams, rivers and reservoirs. Some of it is vanishing, evaporating or getting soaked up by the ground.

LUNDQUIST: The amounts that disappear range in models from 10% to 90% of the snowpack, so the uncertainty is huge.

HAGER: But the region's water managers need certainty, so Lundquist's grad students are in the mountains looking for clues almost 10,000 feet above sea level.

(SOUNDBITE OF SNOW CRUNCHING)

HAGER: The journey through these Colorado mountains feels like a polar expedition. This line of bundled-up skiers is breathing heavy and trudging alongside flags that mark the route into the whiteout up ahead. Eli Schwat is one of the researchers.

ELI SCHWAT: What's the snow moon in "Star Wars Chapter V" (ph)? It looks like Hoth...

DANNY HOGAN: Yeah.

SCHWAT: ...Is what I would say.

HOGAN: That's a good description.

SCHWAT: It's like the rebel base on Hoth.

HAGER: At this site, full of towering scientific measuring devices, we're getting pelted by heavy, wet flakes. But that's a necessary sacrifice for this team of snow detectives. The clues they find have big consequences. The snow that does melt off joins the Colorado River, which supplies people from Wyoming to Mexico. Danny Hogan is one of the researchers.

HOGAN: Essentially, it's a loss of water that is difficult to measure because you're kind of going from something you see - something you don't see.

HAGER: They're talking about a process called sublimation. That's when snow evaporates into the air even before it has a chance to melt. Jessica Lundquist says it's the same kind of process that makes dry ice give off that spooky fog. Scientists also think a lot of the region's snow is getting soaked up by dry soil, which gets baked by hot summers. University of Colorado professor Edie Zagona says that makes it hard to accurately predict how much water will be in the river each year.

EDIE ZAGONA: There are so many variables, and it's so difficult to get them all right.

HAGER: But there is one constant.

ZAGONA: Climate change is a major factor in all aspects of the problem with forecasting.

HAGER: And good predictions get more important every year in a growing region with a shrinking water supply. And up in the mountains, data to help inform those predictions comes from underneath our feet.

Is it time to get digging?

HOGAN: Yeah, it's time to get digging.

HAGER: Hogan and Schwat are shoveling out a snow pit. Inside, they're using thermometers, scales and rulers to gather readings.

HOGAN: From surface at 152 to 143 - fist (ph), new snow.

HAGER: Almost all the tools they're using are on the shelves at your local hardware store. But with the right context, the numbers from this niche study, carried out by two guys with a shovel, can make a big difference in understanding where our water comes from and where it might be going.

For NPR News, I'm Alex Hager in Gothic, Colo.

(SOUNDBITE OF HI PRYCE'S "THE WIND CAN BE STILL") Transcript provided by NPR, Copyright NPR.

Alex Hager