Within-Stand Boundary Effects of Snow Water Equivalent Distribution in Forested Areas

Accurately quantifying the spatial distribution of seasonal snow across a landscape is essential for water resource management. Forested areas exhibit high spatial variability in the distribution of snow due to canopy interception, forest-wind interactions, and heterogeneous distributions in radiation. Previous work has primarily focused on forested areas with respect to snow accumulation in adjacent clearings, with fewer investigations exploring forest effects on spatial accumulation patterns within forest stands and the effects of the boundary between forested areas and adjacent clearings. There will generally be less snow in forested areas with higher variability relative to open areas due to the influence of tree canopies. However, the length scale of the transition from open areas to forested conditions is uncertain. Hence, the goal of this study is to determine the length scales associated with forest boundary effects on snow water equivalent (SWE) accumulation within forest stands. To accomplish this, we utilize a unique ground penetrating radar dataset collected during the NASA SnowEx campaign on Grand Mesa (Colorado) in February 2017 to determine spatial SWE distribution patterns of areas under canopy and in clearing, and the length scales of transitioning between these patterns. The largest within-stand boundary effect occurred on the leeward side of forest stands with a mean extent of 48 m, or a ratio of 4.4:1 relative to mean stand canopy height. In contrast, windward within-stand boundary effects showed a mean extent of 18 m, a ratio of 2.2:1 relative to mean stand canopy height. We present a conceptual framework of the complex wind dynamics that occur in a forest stand to explain the within-stand boundary effects on SWE distribution. Modeling experiments using SnowModel captured the general pattern for windward within-stand boundary effects but did not capture the observed leeward within-stand boundary effects on SWE distribution patterns. Future investigations could improve understanding of this complex process and associated driving variables. Additionally, future survey designs and instrument deployments will benefit from considering within-stand boundary effects to ensure observations of the intended conditions at the required resolutions and extents.