Yearly snow storms form an enormous frozen reservoir in Colorado, but climate change is threatening its future.
Winter precipitation is, historically, unpredictable in Colorado. Cities, farmers and recreationists are used to varying supplies each year, but there was almost always enough to go around. A prolonged drought, rising temperatures and climate change have thrown a worrisome curveball. Now, experts are starting to see signs of a declining snow season.
“The variation year-to-year is pretty extreme. Just because in 2023 we had a substantial snowpack doesn’t mean we’re not on this long-term decline,” said Jeff Deems, co-founder of Airborne Snow Observatories. “There’s a lot more below-average years these days than above-average.”
With an uncertain future threatening the state’s snowpack — the water source for millions of people in Colorado and beyond — The Colorado Sun is explaining the basics from how the snowpack forms to how it reaches household faucets.
Colorado River Explained. We’re answering your questions about the Colorado River. Send them our way and stay tuned for more!
What is “snowpack”?
Snowpack refers to snow that accumulates on the ground in mountainous areas and stays there until the arrival of warmer weather.
The amount of snow that falls on Colorado fluctuates each year. Generally, most of Colorado’s snowpack falls high in the mountains between about 9,000 and 11,000 feet in elevation, although some snow will also build up in lower and higher elevations. As of Jan. 1, the snowpack was accumulating between 7,500 and 11,600 feet across Colorado.
How does the snowpack form?
The snowpack’s formation changes every year based on factors like wind, sunlight, annual precipitation and mountain topography.
Typically, Colorado’s snowpack accumulates over a four- to seven-month period beginning in late September or early October at higher elevations. During that time, winter storms bring snow to the Rocky Mountains and create new layers of snow, which can vary in density.
Topography can impact accumulation. For example, shady northern-facing slopes can accumulate more snow because they get less sun. When snow layers don’t bond well, top layers can slip off of lower layers more easily and create avalanche hazards.
After a snowstorm, some of the water may evaporate back into the atmosphere right away. Wind can also whip snow off of mountaintops above treeline, which increases sublimation — the transition from snow directly to water vapor. About 10-20% of the snowpack in the Rocky Mountain West is lost this way each season.
Colorado’s snowpack accumulation typically peaks around April 1, although some basins peak later than others. The peak in high-elevation areas sheltered from the wind typically yields about 15 to 50 inches of water, also called the snow-water equivalent. Subsequent winter storms might bring more snow, but they won’t boost the peak once melting starts.
This Fresh Water News story is a collaboration between The Colorado Sun and Water Education Colorado. It also appears at wateredco.org/fresh-water-news.
Why is Colorado’s snowpack so important?
Mainly, because of where it goes when it melts. Colorado’s snowpack feeds rivers that eventually deliver water to 19 states, Mexico, Native American tribes, industries, homes, businesses, endangered species, ecosystems and more.
That snow is a major resource for the drought-stressed Colorado River Basin, which provides water to about 40 million people. About 60%-85% of the basin’s water supply originates as snowpack in Colorado, Utah and Wyoming. This snow reservoir creates about 40 million acre-feet of water once it melts, or about 1.5 times the full capacity of Lake Powell, one of the basin’s two main storage reservoirs.
One acre-foot is enough water to supply two to three households for a year.
About 83% of the water that Coloradans drink, use and play in comes from the snowpack and spring runoff. This dependency comes with economic consequences.
Farmers and ranchers across the state watch the snowpack to gauge how much water will reach the ditches and canals that supply their farmland. The agricultural industry, which contributes $47 billion to the state’s economy each year, is the state’s largest water user.
Hydropower plants use water to produce electricity. Communities use it to drink and play. The winter snow supply and spring runoff boost both tourism and economies dependent on the ski and rafting industries. Between November 2022 and March 2023, ski-season spending in 18 of Colorado’s resort-anchored mountain towns reached $4.4 billion.
How does the snowpack reach my tap?
When snow melts, some of it reaches streams and rivers. From there, networks of canals, ditches, pumps, reservoirs, tunnels, pipes and more get it to homes and businesses.
Snowmelt typically begins in earnest in April or May and continues over a span of two or three months. The snowpack melts from the top down as its surface warms from more direct sunlight during longer days. Dust dirtying the surface of the snowpack sucks in more solar radiation and speeds up snowmelt. Higher air temperatures also prime the snowpack for faster melt.
The amount of water released depends on the density of the snow. Wet, heavy snow can release about 1.5 inches of water per foot of snow. A foot of light, drier snow might contain about 1 inch of water.
Some snowmelt soaks into thirsty soils, is lost to evaporation or seeps further down into groundwater aquifers. The rest flows into streams and rivers. From there, it can be captured and stored in reservoirs. Popular reservoirs for recreation, including Dillon, Blue Mesa, Chatfield and Grand Lake, are all part of this extensive water delivery system.
Some water is left in streams to support the environment, but most is diverted for human and farm consumption. Many cities, for example, capture water straight from nearby rivers, treat it and pipe it to the kitchen faucet.
Is climate change shrinking Colorado’s snowpack?
Experts aren’t certain yet, but there are some concerning trends in how long the snow season lasts, peak snowpack accumulations and increasing temperatures.
Some scientific studies of the Colorado River Basin, which includes six other Western states, 30 tribes and Mexico, have linked rising temperatures to snowpack declines. The basin has roughly 10% less water available today than in the 1880s because of rising temperatures. About 86% of that runoff decline was driven by water loss in snowpack regions, according to the UCLA study. Other estimates show more dramatic declines in water supply, particularly since a prolonged drought began in 2000.
Snow-telemetry, or SNOTEL, data is also showing a slight downward trend in the peak snowpack buildup. It’s more drastic in some places than others, and certain locations may even see slightly higher seasonal peaks. But some models project that the April peak snowpack could decline 10% to 20% by mid-century if temperatures keep rising.
That doesn’t mean there’s less snow overall. These peak measurements don’t capture snow that melts between storms and doesn’t contribute to the depth of the snowpack. There could also be snow in areas that are not closely monitored by current technology.
The snow season also seems to be getting shorter. Instead of snow starting to accumulate in the mountains in late September or early October, it’s starting around mid-October to late November in some years. Warmer storms and spring temperatures also prime the snowpack to melt earlier in the year, which means water users, like farmers, might get their supply cut off sooner in the summer.
What is the snow-water equivalent?
The snow-water equivalent, measured in inches, is a measure of the depth of liquid water that remains after melting a column of snowpack. It’s a measurement taken at one location and used to approximate the amount of water in the snowpack in a given watershed.
For decades, snow surveyors with the federal Natural Resources Conservation Service snowshoed and skinned on skis to remote measurement locations. Once there, they plunged a hollow metal tube into the snow until it reached the ground, capturing snow in the rod and measuring its weight to calculate the snow-water equivalent.
In the 1970s, the federal agency began using measurement devices at a network of SNOTEL stations to automatically gather this data, although snow surveyors still need to manually take measurements under some circumstances. Today, the NRCS runs about 115 SNOTEL sites in Colorado as part of its larger network.
If a NRCS snowpack map says the snowpack is 150% of normal in a certain river basin, it means the amount of water in the basin is 150% of normal. That number is based on adding individual site measurements within a basin, calculating a median value for the whole basin, and then comparing that median with 30 years of historical data.
What does — and doesn’t — snowpack data tell you?
Snowpack data offers a glimpse into the amount of water available, but it is limited by where, when and how the data is gathered. Scientists are working to fill these information gaps.
The NRCS network of SNOTEL sites provides hourly data but it is limited by where the stations are located, which is primarily between 9,000 and 11,600 feet in elevation. The sites miss snow that gathers in areas below 9,000 feet or above 12,000 feet.
Snow surveyors, water managers and reservoir operators are often reliant on estimates as they try to make day-to-day decisions about how to store, release and deliver water resources. But experts have a set of complementary tools that they can use to gather data. For example, scientists with Airborne Snow Observatories Inc. shoot lasers down from planes to get more detailed snowpack data across larger areas. They can gather basinwide data on a certain day and compare that to historical and continuous data from the NRCS to get a better idea of how much water is in the snowpack.