From field to film, measuring “missing snow”

UW hydrology team quantifies snow lost to the air and shows the work behind the numbers in a short film.

Why does a healthy winter snowpack sometimes fail to become spring river flow? Researchers in the University of Washington’s Department of Civil & Environmental Engineering (CEE) are investigating that question through a multiyear study of snow sublimation, which occurs when snow turns directly into water vapor instead of melting.

In recent years across the West, some mountain basins have seen solid snow totals yet lower-than-expected streamflow, a mismatch that sublimation can help explain. When snow sublimates, it never reaches rivers and reservoirs, leaving less water for farms and communities.

In a new study in the Journal of Hydrometeorology, the team reports that at their Colorado site, roughly 10% of the season’s snowpack was lost to the air. Losses were highest during windy, snow-blowing events in midwinter and rose again in spring with stronger sunshine.

Instrument height also mattered: during some wind events, a sensor mounted 10 meters above the ground measured up to four times as much sublimation as a sensor mounted 3 meters above the ground. Because height changes what a sensor “sees,” standard setups can undercount midwinter sublimation.

"Our results provide guidance to scientists estimating total sublimation," says Eli Schwat, a CEE graduate student who led the study. "Since most standard instruments sit between 2 and 5 meters above the ground, researchers could miss midwinter sublimation during windy periods unless instruments are mounted higher."

Zooming out, the team’s finding that roughly 10% of the season’s snowpack was lost to the air complements earlier work led by CEE graduate student Danny Hogan, which points to spring weather as the main driver behind the "missing water." Hogan’s analysis of the mountain valleys feeding the Colorado River found that drier, sunnier springtime weather explains nearly 70% of the gap between snowpack and streamflow in recent decades. With little spring rain, freshly melted snow soaks into the ground and fuels plant growth, so less water makes it to nearby streams.

Together with Hogan’s findings, the new results point to spring processes, not midwinter sublimation, as the bigger factor in recent Colorado River streamflow declines.

"When we started this project, most people thought snow sublimation would explain the missing mountain water," says CEE Professor Jessica Lundquist. "Many models represented sublimation as increasing with warming temperatures, and some represented 30% to 40% of snow sublimating. However, our measurements show sublimation in Colorado is tied to wind speed, not air temperature, and that spring weather conditions explain much of the shortfall between snowpack and streamflow."

The research is part of the Sublimation of Snow (SOS) project led by Lundquist with collaborators at the Aspen Global Change Institute and the National Center for Atmospheric Research, and supported by the National Science Foundation.

The team worked out of the Rocky Mountain Biological Laboratory near Crested Butte, Colorado, where they recorded wind, solar radiation and water vapor to track when snow is most likely to vanish into the atmosphere. They skied roughly two miles most days to the site, where they dug dozens of snow pits to read the snow’s layered “record,” and paired those profiles with turbulence measurements from a nearby field station.

Alongside the paper, a short Aspen Global Change Institute documentary, "Snow Science: A Season of Fieldwork and Discovery," follows Schwat and Hogan through a winter of data collection, showing why snow that vanishes into vapor never reaches rivers and reservoirs that supply farms and communities. The documentary shows the day-to-day fieldwork behind collecting the data — ski approaches, snow pits and instrument checks — and the measurements that inform decisions by water managers across the Western United States.

"Everyone wants to know why there’s less water," says Julie Vano, the research director at Aspen Global Change Institute. "This effort tackles that question and, using beautiful visuals and the art of storytelling, invites you along with Danny and Eli to experience the challenges and joys of field research and scientific discovery."

In the next steps of their research, Schwat is studying how spring winds mix the air over patchy snow, and Hogan is modeling what those effects mean for streamflow.

Together, the studies show how much snow is lost directly to vapor and how spring weather shapes what’s left, giving forecasters, agencies and students both the numbers and the story behind the search for the “missing” snow.