Quantifying the Effects of Forest Canopy Cover on Snow Accumulation and Ablation at a Continental, Mid-latitude Site, Valles Caldera National Preserve, NM

Basin scale estimates of water resource quantity and quality in snow-dominated systems, typically based on point measurements such as snow courses and SNOTEL sites, are complicated by the interrelated forcings of topography and vegetation on snowpack accumulation and ablation. Although the effects of forest density on snowpack are relatively well characterized in northern systems, the interactions in the higher solar-radiation environments found at mid-latitude sites are not well understood. In this study we quantify the effects of forest density and geometry on snow accumulation and demonstrate how this information can be used with remote sensing to improve estimates of snow distribution. We measured snow depth and density through a continuum of forest canopy densities in the Valles Caldera National Preserve of New Mexico during the early season, peak accumulation, and melt season of spring 2007. We measured snowpack properties in six snow pits, representing a variety of aspects and vegetation densities, and took a total of 1,350 measurements of snow depth in 20 plots of various canopy densities corresponding to specific Landsat Enhanced Thematic Mapper pixels. By comparing depth data to canopy cover as measured in the field and by satellite, we quantify the interactions between snow and forest vegetation at the 30-m scale. Relationships developed at the pixel scale are used to develop algorithms for basin-scale estimation of snow distribution using the newly available National Land Cover Data - Forest Cover dataset. Initial results show significant correlations between snow depth and both field-measured and remotely sensed forest canopy density. To determine the physical processes underlying these correlations we used statistical models to separate and evaluate the effects of canopy interception, solar radiation shading, enhanced longwave radiation, and wind redistribution on snowpack depth. We also found significant differences in snow depth corresponding to the density of vegetation in neighboring areas, indicating that vegetation influences snow depth beyond the canopy and canopy-fringe. This study is part of a larger effort to quantify the water balance in the Valles Caldera, which also comprises efforts to determine the isotopic signature and chemical load of winter precipitation, and to determine the residence time of water in the Redondo Peak massif using these chemical tracers in conjunction with the depth estimates of this study.