Location, location, location: Snowpack storage and runoff timing in burn scars depend on site and terrain

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Increasingly severe wildfires at high elevations are impacting snowpack—an important reservoir for the U.S. West. The altered landscape makes it more challenging to predict when snow will melt and how much water will be available for use.

Colorado State University researchers studied the 2020 Cameron Peak Fire's effects on snowpack across mountainous terrain and found that location is key to melt rate and timing.

Snowmelt rates vary from slope to slope, based on how much sun the slope receives and vegetation. Snow generally melts faster on south-facing slopes, which get more sun. The researchers found that the amount of water in the snow on a burned south aspect peaks earlier in the season than other snowpack locations, and south aspect snow melts sooner.

The maximum water in the snowpack occurred 22 days earlier at burned south-facing locations than at burned north-facing sites at the same elevation, because of the increased solar radiation absorbed at the south-facing burned sites and lack of trees to intercept it. Snow on the burned south-facing slopes also melted completely 11 days sooner than snow on burned north-facing slopes.

"This is an advance from previous understanding, because we've captured this spatial variability across complex mountainous terrain in a way that hasn't been done before," said Dan McGrath, a co-author of the study and Geosciences associate professor in the Warner College of Natural Resources.

Energy balance—the long and short of it

The study, published in Water Resources Research, is the first to quantify the impact of the full energy balance—both long- and shortwave energy—on snowpack in burned areas.

The researchers used radiometers to measure incoming and outgoing long- and shortwave energy. Shortwave energy comes from solar radiation, and the long-wave form is thermal energy.

Thermal energy is emitted by anything that absorbs solar energy, including the Earth's surface, trees and even cold substances, like snow. Loss of the forest canopy lowers this source of energy in burned areas, so the snowpack stays colder in winter. In spring, however, the energy balance flips, and the tree canopy in unburned areas prolongs snow by shielding it from sunlight, while burned areas bake under the higher sun and longer days.

Following a wildfire, soot and ash from remaining tree trunks darken snow, reducing its reflectivity—called albedo. The darker snow absorbs more solar energy, making it melt faster. By examining the complete energy balance, the researchers were able to determine how much of the energy change was due to albedo lowered by ash and soot and how much was from losing the forest canopy.

"We found that the incoming shortwave energy is five to six times more impactful than the change in albedo," said lead author Wyatt Reis, who worked on the project as the thesis for his master's degree from CSU. Reis is now a research civil engineer for the Army Corps of Engineers, where he works on hydrology in the U.S. West.

Reis added that this improved understanding of the energy balance will guide development of models and tools that water managers can use to make decisions.

Lasting impact to the landscape

Four winters post-fire, the snowpack is markedly cleaner, McGrath said, but the trees have not come back, and he's not sure they ever will.

Because south-facing, burned and therefore unshaded slopes accumulate less snow and are snow free earlier than other aspects, they may be too dry for tree seedlings to get established.

"These south-facing slopes may be transformed for good and not recover to their pre-fire state," McGrath said.

As a result, the watershed may never be the same. Adding together all the wildfire-impacted slopes equals uncertainty for resources downstream.

"When you consider the amount of area that has burned across the western U.S., which is increasingly occurring at high elevations that accumulate deep snowpacks, this is a really critical issue that has and will continue to impact the region's water supply," McGrath said.

He continued, "These field observations across mountainous terrain give us the framework to improve how we model and manage this resource."

Collaborators on the study, "Quantifying Aspect-Dependent Snowpack Response to High-Elevation Wildfire in the Southern Rocky Mountains," are Kelly Elder with the U.S. Forest Service's Rocky Mountain Research Station, CSU Professor Stephanie Kampf and David Rey with the U.S. Geological Survey.

More information: Wyatt Reis et al, Quantifying Aspect‐Dependent Snowpack Response to High‐Elevation Wildfire in the Southern Rocky Mountains, Water Resources Research (2024). DOI: 10.1029/2023WR036539

Journal information: Water Resources Research

Provided by Colorado State University