Graduate Thesis Or Dissertation
 

Forest Fire Effects on Radiative and Turbulent Fluxes over Snow : Implications for Snow Hydrology

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/jm214r26h

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  • As a result of a warming climate, subsequent declining snowpack, and a century of fire suppression, forest fires are increasing across the western United States. However, we still do not fully understand how forest fire effects snowpack energy balance, nor the volume and availability of snow melt and associated water resources. This dissertation investigated the radiative and turbulent energy fluxes over snow in a burned and unburned forest site using a suite of experimental, modeling, and remote sensing methods to determine the overall impact of forest fire disturbance to snowpack energy balance and snow hydrology. For three years following the Shadow Lake Fire, which occurred in September 2011 at the crest of the Oregon Cascades, a suite of field experiments were maintained, including snow water equivalent and snow spectral albedo measurement transects, snow surface sampling, snow depth and basic micro-meteorological monitoring and eddy covariance measurements of turbulent heat fluxes. These data were used to empirically characterize forest fire effects to the radiative and turbulent fluxes over snow, to parameterize key drivers of snowpack energy balance and to model forest fire effects to snow hydrology using a physically-based spatially distributed snowpack energy and mass balance model for both the burned and unburned forest sites. This resulted in three papers summarizing forest fire effects to snowpack energy balance and implications for snow hydrology. This dissertation documented forest fire effects to the radiative and turbulent fluxes over snow and evaluated implications for snow hydrology. These results showed a 40% reduction in snow albedo in the burned forest during the ablation period in the first year following fire, while 60% more solar radiation reached the snow surface, driving a 200% increase in net shortwave radiation. This dissertation documented that both sensible and latent heat fluxes were double the magnitude and variability in the burned forest compared to the nearby unburned forest. These results showed that the turbulent fluxes over snow can be periodically large and substantial over time. The contribution of sensible heat flux and loss of energy by the latent heat flux is responsible for a loss of snow mass of approximately 2% that measured snowmelt in the burned forest site during the clear-sky snowmelt period. Overall, the radiative fluxes dominate the overall snowpack energy balance in burned and unburned forests. An empirically-based parameterization was developed to represent the temporal and spatial variability of snow albedo relative to days-since-snowfall in the burned and unburned forests, which was employed in a physically based spatially distributed snowpack energy and mass balance model. Using this variable snow albedo parameterization improved model performance in both burned and unburned forest sites, and better captured the temporal and spatial variability of snow albedo and snow water equivalent than a fixed albedo parameterization. Overall this evaluation demonstrated that even though more snow may accumulate in burned areas than unburned forests, the combined effect of the increased postfire radiative forcing to snow and increased turbulent fluxes over snow accelerates snow melt, shortens the duration of snow cover, and advances the date of snow disappearance across the extent of the burned forest. Although this research focused on a relatively small burned area in the western Oregon Cascades, it has broad applications from regional to global scales particularly in forested maritime snow-dominated watersheds. Eighty percent of forest fires in the western United States occur in the seasonal snow zone, and those fires are 4.4 times larger than outside the seasonal snow zone. As forest fires increase and snowpacks decrease across forested montane headwater regions of the western US and beyond, it is critical that we incorporate forest fire disturbance effects to snow hydrology in our hydrologic modeling applications and our natural resource management decisions.
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file details GleasonKellyE2015.pdf 2017-08-07 Público Descargar