Modelling postfire recovery of snow albedo and forest structure to understand drivers of decades of reduced snow water storage and advanced snowmelt timing

Forest fires darken snow albedo and degrade forest structure, ultimately reducing peak snow-water storage, and advancing snowmelt timing for up to 15 years following fire. To date, no volumetric estimates of watershed-scale postfire effects on snow-water storage and snowmelt timing have been quantified over decades of postfire recovery. Using postfire parameterizations in a spatially-distributed snow mass and energy balance model, SnowModel, we estimated postfire recovery of forest fire effects on snow-water equivalent (SWE) and snowmelt timing over decades following fire. Using this model, we quantified volumetric recovery of forest fire effects on snow hydrology across a chronosequence of eight sub-alpine forests burned between 2000 and 2019 in the Triple Divide of western Wyoming. We found that immediately following fire, forest fire effects reduced snow-water storage by 6.8% (SD = 11.2%) and advanced the snow disappearance date by 31 days (SD = 9 days). Across the 15-year recovery following fire, forest fire effects reduced snow-water storage by 4.5% (SD = 11.4%). Postfire effects on snow hydrology generally recovered over time, but still persisted beyond 15-years following fire due to the observed postfire shift from forest to open meadow. Estimates of postfire reductions on peak SWE summed over the entire 15-year postfire recovery period were 18 times greater than the immediate losses in the first winter following fire alone. These lasting effects of forest fires on snow hydrology decades following fire highlight the importance of postfire parameterizations for more accurate watershed-scale volumetric estimates of forest fire effects on snow-water resources. We simulated postfire forest structure and snow albedo changes in a spatially-distributed snow model to estimate associated effects on snowpack volume and snowmelt timing over decades following fire. Over 15 years of recovery following fire, darkened snowpack and forest degradation decreased modelled peak snow-water storage by nearly 1/20th and advanced snow disappearance date by more than a week highlighting the importance of incorporating postfire effects on snow-water storage in hydrological forecasts. image