Unraveling the mystery of disappearing Colorado River water
There’s a mystery afoot in the upper Colorado River Basin: the amount of winter snowpack and the resulting spring and summer runoff haven’t added up in recent years. In 2021, for example, measurements of the snowpack looked close to normal (80% of average), but the streamflows in the spring and summer of that year were only 30% of average, much lower than what was expected. Where did that water go? In the megadrought-stricken Western US, understanding the causes of this mismatch is a question that the 40 million users of water sourced from the Colorado River urgently need answered.
One possible suspect — or at least a contributor to the problem — is snow sublimation. Snow has been studied for quite a while, yet many questions remain about how it behaves in certain conditions, especially in windy mountain environments. One area that’s not well understood is sublimation, the process by which snow transforms from a solid directly to a gas, without passing through a liquid state first (i.e., melting and then evaporating). Global studies estimate anywhere from 10% to 90% of snow that falls is lost to the atmosphere through sublimation, which generates more questions than answers. Why such a wide range? How do wind or temperature affect snow sublimation?
As it turns out, these aren’t easy questions to answer. Snow sublimation accounts for the largest source of uncertainty in snow modeling, and methods vary widely on the best way to quantify the process in a given location. As part of a research team led by the University of Washington (UW), along with partners from the National Center for Atmospheric Research (NCAR) and the WSL Institute for Snow and Avalanche Research SLF in Switzerland, AGCI is helping to fill in data gaps through the National Science Foundation-funded Sublimation of Snow (SOS) Project.
Over two weeks in October, SOS Project scientists and engineers set up shop at the Rocky Mountain Biological Laboratory campus in Gothic, Colorado, a long-term research site located in a former mining town near Crested Butte. There, in a mountain meadow in the East River watershed, the investigators worked together to install an impressive suite of instruments to quantify snow sublimation. Engineers from NCAR’s Earth Observing Laboratory installed instrumentation from their Integrated Surface Flux System (ISFS) on one 20-meter (66-foot) tower and three 10-meter (33-foot) towers arranged in a triangular pattern, to better understand how basin-scale wind fields interact with surface turbulence and fluxes at multiple levels. Sensors on the towers will measure wind speed, solar radiation, air temperature, and relative humidity, while snow pillows on the ground will weigh the amount of water held in the snowpack. A terrestrial lidar system will use laser beams to scan across the entire field plot, helping provide three-dimensional context for the measurements collected by the other instruments.
“Most of the instruments are measuring incredibly detailed information, but only at a point,” explains NCAR’s Ethan Gutmann. “The lidar systems will map out the snow depth over the entire area as well as mapping the blowing snow component so we can see when an individual gust carries a plume of snow past another instrument and be able to time exactly what we’re seeing with that instrument when the plume of snow goes by.”
It’s not typical to have so many types of instruments on one field site. “The field design is intentionally over-instrumented,” says Principal Investigator Jessica Lundquist, who leads UW’s Mountain Hydrology research group. “We decided to deploy as many different ways of measuring snow as we could together in one spot and use them to collectively provide the most complete picture possible of the snow sublimation process.” The goal: to track every snowflake that falls on (or sublimates away from) the field site this winter, no easy feat.
Measurements collected by the SOS Project will be complemented by data from two concurrent experiments operating in the same field site: the US Department of Energy’s Surface Atmosphere Integrated Field Laboratory (SAIL) and NOAA’s Study of Precipitation, the Lower Atmosphere and Surface for Hydrometeorology (SPLASH). And throughout the process, AGCI team members, led by AGCI Research Director and SOS Project Co-Principal Investigator Julie Vano, will document the project, producing videos and other outreach materials to facilitate discussion with stakeholders, educate students and other researchers, and enrich cross-disciplinary dialogue.
Now that snow has started falling, data collection has begun and preparations for winter field work are underway. In January, two UW students will move to a cabin at the field station where they’ll remain throughout the winter, visiting the field site regularly and taking additional manual snow measurements. NCAR’s engineers will also visit each month to maintain the large suite of instruments.
“This is a unique, and timely opportunity that’s been made possible because of strong partnerships among many amazing individuals, projects, and institutions,” says Co-PI Vano. “Now, more than ever, understanding how snow behaves in the mountains, particularly in the Colorado River Headwaters, is of great interest to both research and water management communities.”
If you’re interested in learning more, bookmark the project webpage, which will be updated regularly as the snow falls, field work continues, and the analysis gets underway. Let it snow!