AGCI Insight

New research in the Colorado River headwaters offers clues to mystery of where mountain snow goes

May 9, 2024

Snowpack lost to spring sublimation complicates estimates of future water supplies 

This is the time of year when Colorado River Basin water managers are deciding whether to hold or release reservoir water as the runoff season begins. They base their decisions on carefully modeled spring season estimates of how much water from last winter’s snowpack will melt and make it into rivers and reservoirs — but each winter, an unknown amount of water from mountain snowpack disappears into the atmosphere.

New research, conducted over eight months in the Colorado Rocky Mountains by Dr. Jessica Lundquist (University of Washington) and collaborators at the University of Washington, Aspen Global Change Institute, and the National Center for Atmospheric Research, provides the most comprehensive examination to date of how snow sublimates, or transforms directly from a solid (snow) to a gas (water vapor), in a mountain environment. The research is captured in AGCI’s new video, Vanishing Snow: The Hunt for the Missing Water.

The results, published in the Bulletin of the American Meteorological Society (BAMS), fill longstanding gaps in the poorly understood process of sublimation and provide a critical benchmark for understanding and modeling sublimation as it relates to temperatures, air movement, snow conditions, and more. This new research reveals that most water loss to sublimation occurs after the release of the widely used April 1st runoff forecasts, which are critical for water management decisions. If managers expect a large runoff, they’ll release water from reservoirs to make room for the spring influx; if they expect less runoff, they’ll hold onto all the water they can in order to meet summer and fall needs. In years when forecasts aren’t accurate, challenges arise. 

Spring is the dynamic season. For managers, knowing that more sublimation occurs in spring than in the dead of winter offers a piece of the puzzle as to why forecasts don’t always predict runoff accurately: a portion of the snow that was measured on April 1st is getting lost to the atmosphere in the coming months,” explains Lundquist. 

Steven Oncley (ground) and William Nicewonger (tower) of the National Center for Atmospheric Research (NCAR) check equipment at the Sublimation of Snow Project field site, while University of Washington graduate student Daniel Hogan (skis) observes site conditions, Jan 21, 2023. Image Credit: Emilio Mateo.

This finding emerged from the Sublimation of Snow (SOS) Project, an intensive study conducted by Lundquist and collaborators at the University of Washington, Aspen Global Change Institute, and the National Center for Atmospheric Research, and funded by the National Science Foundation. To better understand and quantify how sublimation happens, the team deployed over 100 instruments at a research site at the Rocky Mountain Biological Laboratory (RMBL), near Crested Butte, Colorado.

Through the SOS Project, a clearer picture of sublimation in mountain terrain has emerged. In addition to providing new insight about the timing and intensity of sublimation events, the SOS Project’s extensive instrument array deployed across multiple towers revealed that much of the winter’s blowing snow was re-distributed elsewhere and not actually lost to sublimation.

Distinguishing between snow that has sublimated versus snow that has blown to a new location is a challenge for snow measurements taken by a single tower as well as for the models currently used for runoff forecasting. This challenge may lead to discrepancies in estimations of sublimation over the course of a winter. From their measurements taken across four towers at the study location, Lundquist’s team found that around 10% of water in the peak snowpack was lost to sublimation over the 2022-2023 winter. These data have the potential to help advance other research as well, as all SOS Project data are publicly available to researchers and modelers for conducting their own further investigations. 

“There’s still a lot to learn. As our climate changes, we can no longer rely on past conditions as our only guide for anticipating water supplies,” says project co-PI Julie Vano, Aspen Change Global Institute’s research director. “Having a clearer understanding of how sublimation works is incredibly valuable for developing tools and improving the models needed to plan for a warmer future.” 

Funding for this work was provided by the National Science Foundation Division of Atmospheric and Geospace Sciences (PD 98-1522), award numbers 2139836 and 2139809.