The rate at which a snow pack melts depends on the amount of energy available. The magnitude and direction of the components of the energy balance vary through the day, and change substantially after canopy removal by forest harvesting. Snowmelt rates are assumed to increase after harvest, resulting in increased spring peak streamflows. Regulatory limits to forest harvesting in British Columbia are intended to reduce such potentially negative hydrologic impacts. The regulations assume that a 9m tall juvenile forest has the same accumulation and melt rate as a mature forest and, as a result, no longer contributes to peak flow increases. These experiments are attempting to refine this index.
Measurements of the energy balance components were made in a clearcut, an adjacent mature Engelmann spruce - subalpine fir forest, and an adjacent juvenile stand of lodgepole pine (10 years old). Net irradiance, soil heat flux density, and heat storage within the pack, were combined with eddy correlation measurements of sensible and latent heat flux density to estimate the energy available for snowmelt. A 20 m long tram was used to determine average values of the radiation balance components under the forest canopies. Daily ablation of the snowpack was also calculated from snow density and changes in pack depth measured by a sonic depth gauge.
Latent and soil heat flux density, and heat storage were negligible when the ripe snowpacks were melting. Net irradiance provided most of the energy for snowmelt, reaching 200 to 300 W m-2 during the daytime in the clearcut, and 50 to 100 W m-2 in the forest. Due to the stability of the lower atmosphere, sensible heat flux densities were about 50 W m-2 in the clearcut and negligible in the forest. Calculated snowmelt rates for warm sunny days in the clearcut were over 2 mm h-1with daily totals often exceeding 10 mm. Those in the forest were about half this value. Daily melt from the energy balance was often greater than 10 mm, consistent with that calculated from snow density and changes in the depth of snow pack.