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Determining how much water Colorado’s snowpack will yield is an inexact science, but researchers persist

The specter of climate change underscores the importance of gauging how well Colorado’s mountains can wring moisture from those enigmatic flakes

Snow covered Pyramid Peak and Maroon Bells near Aspen in winter of 2019, when snowpack statewide exceeded 100% of normal. (Provided by EcoFlight)
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On a sunny April day at the tail end of last winter, Jeff Deems, a researcher at the Boulder-based National Snow and Ice Data Center, and Nathan Elder, the water supply manager for Denver Water, used snowshoes to reach a point high above the Blue River in Summit County.

There they dug a pit, which looked a bit like a grave, into the 5½-foot-deep snowpack and took measurements of the snow’s temperature profile, weight and density in order to estimate its water equivalent. They did measurements from the top to the bottom of the pit.

Overhead a small plane – the Airborne Snow Observatory – equipped with a mass spectrometer and a laser-radar technology, called lidar, scanned the Blue River Basin assaying its snowpack.

As the world’s climate warms, forced by the buildup of man-made greenhouse gases in the atmosphere, the key questions for the arid West will be how much snow falls on the region’s mountains and how much usable water it will yield.

A lidar scan, used to measure snowpack on Maroon Bells near Aspen. One of the problems water forecasting is that snow doesn’t fall uniformly across the mountains. To better plan for the implications of climate change, researchers are trying to be more precise in their measurements of how much snow there is.

“As climate changes, we need a more accurate understanding of the water in the snowpack,” said Deems, 46, who got hooked on studying snow as a University of Colorado undergraduate, in part because, as a backcountry skier, he was interested in avalanches. “So far it has been difficult to measure snowpack and get good information on how much water is going to come out of it.”

Snow is elusive. It comes in storms that mix and mingle with mountains and forest. Even after the storms leave, winds continue to move it around the landscape. “Pretty rapidly it becomes a difficult problem to simulate the snow accumulation process,” Deems said.

“Snow is close to its melting point, so it can change its character over a short distance or time,” he said. “It’s weird stuff.”

The Future of Snow

Climate is changing, Colorado researchers agree. But how will it change snow and water in the West?

  • Day 1: The “breath” of tiny flowers growing in a mountain meadow provide critical clues to climate computer data models used around the world.
  • Day 2: The “Eagle” supercomputer is doing the math to calculate how climate change will affect Western water, but researchers worry that modeling still shows the extreme, not what is probable.
  • Day 3: Measuring the water in snowpack used to be a boots on the ground enterprise, but as the climate changes, more reliable methods like lidar are needed.

The big Earth system models (ESMs) used to plot global climate change have challenges simulating snow. Deems said he hopes the data generated from remote-sensing exercises, like the airborne observatory, will eventually “nudge the ESMs in the right direction.”

The airborne observatory is the result of a collaboration between the snow and ice data center, which is affiliated with NASA and CU Boulder. Deems has been with the project since it began in 2012.

Lidar – light detection and ranging radar – measures the snow-water equivalent and the mass spectrometer measures the albedo, the snowpack’s reflective capacity, an indicator of melting.

The airborne observatory is part of NASA’s larger SnowEx project, which aims to estimate how much water is stored in Earth’s snow-covered regions.

The snow-pit measurements were to help calibrate – “ground truthing” scientists call it – the remote sensing data from the airplane above.

Snow presents a water-gauging enigma 

What scientists know from on-the-ground observation and model simulations is that in the last few decades, there has been less snow in the West, and it is likely there will be even less in the future.

In ascertaining what is going to happen to the region’s water, understanding snow is the most bedeviling element.

When it rains, it rains. But snow only comes when both the precipitation and temperature are just right. At warmer temperatures, the air holds more water and the snowfall is bigger, but a few degrees more and the snow turns to rain.

Since 1980, the area covered by snowpack has declined 20% in the low to middle elevations across the basins of the Colorado, Missouri and Columbia rivers, according to a study led by U.S. Geological Service (USGS) researcher Gregory Pederson. This contributed to low stream flows and a more active fire season.

A tree-ring paleo study by Pederson found that in the last 1,000 years, there were only two periods of sustained low snowpack that compared to current levels — from 1300 to 1330 and from 1511 to 1530.

But in each of those cases, there was a regional split, tied to forces such as the El Niño, the periodic warming of Pacific Ocean waters, which led to drying in either the northern Rockies or the southern Rockies while the other area was wet. Now the dearth of snow stretches across the West.

“Over the past millennium, late 20th-century snowpack reductions are almost unprecedented in magnitude across the northern Rocky Mountains,” according to a research paper by the Pederson team.

An analysis by researchers at Oregon State University and the University of California, Los Angeles found that between 2005 and 2016, snowpack in the Sierra Nevada and Cascades declined at 90% of the snow-monitoring sites with long records, with a third posting drops that were “significant.”

To see what the future may hold, a team led by John Abatzoglou, a University of Idaho geography professor, regionalized temperature and precipitation projections from 20 of the global ESMs and added data from the U.S. Department of Agriculture’s Snow Telemetry System (SNOTEL).

Snow maps generated by University of Idaho Professor John Abtazoglou combine regional Earth system models with data from SNOTEL sites across the west to project likely changes to snowfall in the West.

There are about 730 SNOTEL sites across the West and Alaska. Each has a “snow pillow,” akin to a waterbed filled with antifreeze that measures the density of the snow and its water content.

In warmer areas, such as the Cascades, snowpack reductions of 35% to 70% were projected by mid-century, according to the Abatzoglou models. In the coldest locations, including the middle and southern Rockies, snowpack was seen declining 5% to 20%.

But the model also showed that in a few of the coldest, high-elevation stations, including some in Colorado, the impacts of warming would be offset by increased precipitation. Still, overall the loss of snow would lead to an 18% decline in the flow of the Upper Colorado River.

The added moisture in the air would lead to more of the snowpack coming from a handful of very heavy storms, while there would be fewer snowfalls over the snow season. That season will narrow to December through February.

The shoulder months of November and March, which have been in part of the snow season, will see a mix of rain and snow – except a few areas where snow will continue to dominate, such as central Colorado and the Unita and Bighorn ranges, according to the Abatzoglou analysis.

At “broad scales,” the models indicate a 30% decrease in areas across the West with temperatures favorable for snow, one Abatzoglou study calculated.

Still, figuring out what is going to happen in the mountains is tricky, Abatzoglou said in an interview. “We’ve under sampled our mountains. We don’t nearly have as good a record,” he said. “The models are coarse.”

“We’ve seen decreases in snowpack, but how is snow going to be moving forward?” he asked. One scenario sees years with large snowfalls and years with “snow droughts.”

Another University of Idaho study found that as temperatures increase, snow droughts will become six times more likely in the second half of the century.

“When water falls, it is so much more valuable when it falls over the mountains as snow,” Abatzoglou said. “It contributes substantially to ecosystems and regional water supplies.”

As the snowfall becomes more limited and variable, understanding the spatial patterns of the snowpack becomes all the more important. “Where are the refuges that will be able to hang on to snow?” Abatzoglou said. “If we get a little more on spatial estimates, we can get a better handle on water supply.”

Digging by hand for details

Getting a handle was precisely what Deems and Elder were doing with their avalanche shovels. There are four SNOTEL sites in the Blue River drainage of Lake Dillon, Denver Water’s largest reservoir. Deems and Elder were above those SNOTEL sites.

It turned out when the Airborne Snow Observatory scans were twinned with the on-the-ground measurements, more than half of the season’s inflow to Lake Dillon was in the upper reaches of the mountains, even after the SNOTEL sites had melted out.

Nathan Elder, manager of water supply for Denver Water, in a snow pit dug near the top of Hoosier Pass in the Blue River watershed in 2019. Measurements taken in the pit were used to help calibrate, or “ground truth,” remote sensing data collected from an airplane above. Elder is 5 feet, 8 inches tall. (Provided by Denver Water)

There was an estimated 114,000 acre-feet of water above the SNOTEL sites, enough water to supply nearly 300,000 families of four for a year.

“Those SNOTEL sites don’t measure anything above 10,000 feet in our watershed,” Elder said. “We have a large area above that, and we don’t get good information on it.” 

This was the first time Denver Water has used the airborne observatory, but in California, water utilities have used it for years. The project has been spun off into a commercial company to continue to work with water managers.

Denver Water has seen large swings in snowpack, Elder said. In 2011, there was a hefty snowpack, followed by a really dry year in 2012. The same thing happened in 2018, hot and dry, and 2019, with a snowpack more than 100% of average.

“Ninety percent of water management is dealing with the extremes, the wet years and the dry years, which are getting drier, that is where climate change plays havoc,” said Douglas Kenney, director of the Western Water Policy Program at the CU Law School.

While the climate models may not agree on the exact numbers — some even show an increase in precipitation in the West — they all tend to show wide swings. “There is going to be more variability. That is the starkest message coming out of the models,” said James Prairie, a hydrologist with the U.S. Bureau of Reclamation.

So, knowing how much snow there is in the mountains will be a key component in managing supplies. “The past is less of a guide to the future” Deems said. “We are more vulnerable to mistakes.”

In August, the U.S. Congress approved a plan, negotiated among seven Western states, to manage the Colorado River in the face of a 19-year drought, which had drained the river system’s two largest reservoirs – Lake Mead and Lake Powell – to 40% of capacity.

Under this drought contingency plan, the Upper Basin states – Colorado, Utah and Wyoming – will send more water to Lake Powell if its levels get critically low and in the long-term, they will try to reduce water use.

The Lower Basin states – Arizona, Nevada, California and New Mexico – will implement conservation efforts to reduce their withdrawals from Lake Mead. The Colorado River provides the water for about 40 million people in those seven states.

“The states over the last 20 years have been remarkably cooperative with each other,” Kenney said. “It is, I think, an acknowledgment that things are going to get worse.”

At the Center for Advanced Decision Support for Water and Environmental Systems, based in the CU College of Engineering and Applied Science, Prairie is working to see if the latest science – including climate-modeling forecasts of the future and paleo studies of the region’s past – can be brought into water-management decisions for the Colorado River.

“The management community is accepting you can’t hang your hat on what happened in the last hundred years,” he said.

Last November, Colorado voters approved a ballot measure to permit casino sports betting and to tax it to provide an estimated $16 million a year to finance the Colorado Water Plan, which includes 42 projects around the state to add 530,000-acre feet of water through conservation and new water projects.

The projects are relatively small. The day of the big dams and reservoirs in the West are over. Still, Kenney said, they could “significantly improve the state’s ability to deal with climate change.”

Improve it, but not solve the problem. “Ultimately, we need to realize that there isn’t new or unused water out there waiting for new users to come grab it,” he said. “That era is over. We’re tapped out.”

Denver Water is trying to prepare for some still-undefined climate impact. “We don’t know how it is going to change, but we have to be prepared,” said Laurna Kaatz, the utility’s climate program manager.

The plan to expand the Gross Reservoir in Boulder County, raising the height of the dam 131 feet and adding 77,000 acre-feet of capacity, is part of those preparations, although it is opposed by local officials and tied-up in court.

Upgrading the system bringing snowmelt to a new water-treatment plant under construction in Golden is another part of the plan.

The goal is a policy that is adaptive and flexible, Kaatz said, because there is no crystal ball with which to tell the future. “We are aware of the limitations of what climate science can tell us,” she said. “We just don’t know what is really going to happen. That’s why planning for a range of scenarios makes sense.”

There is one certainty, she said. “The future is going to be warmer, hotter. Climate change is appearing here and now, and it is not going away.”


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