A statistical model of spatially distributed snowmelt rates in a boreal forest basin

Spatial variation in snowmelt rates in the boreal forest can be explained by diAerences in canopy density. Canopy density, represented as gap fractions (GF), controls both the amount of short-wave radiation reaching the snowpack surface and wind speed over the snow surface, which in turn regulates sensible and latent heat fluxes. Reductions in shortwave, sensible and latent heatfluxes outweigh any increased contributions from longwave radiation from the canopy. DiAerences in the total energy available for melt do not translate to equally proportional changes in melt rates under diAerent canopy densities. As available energy increases, the melt rate increases with decreasing canopy density and the form of the relationship can vary depending on climatic conditions. A good relationship between ground-based GF measurements and a canopy closure index derived from Landsat TM provides the spatial fabric for the distribution of snowmelt rates that show comparable patterns of snow ablation during years of very diAerent climatological conditions. This physically meaningful method of determining the spatial variability of snow ablation and subsequent meltwater delivery to the soil interface is particularly useful for providing insight to the heterogeneous active layer development in permafrost regions of the boreal forest and the implications for runoA processes. #1998 John Wiley & Sons, Ltd.