Measured and Predicted Solar Transmission Through Conifer Canopies

Snow dynamics under forest canopies are strongly influenced by the large spatial variability of energy transfers in this environment. Transmission of solar radiation through a canopy is highly variable and depends on tree species, as well as canopy properties such as height, density, and leaf area. Modeling snow processes at the stand scale has proven challenging due to the highly variable structure of forest canopies controlling solar radiation incident at the snow surface. This study aims to describe and simulate the solar irradiance variability on the snow surface beneath two stands: an open, discontinuous conifer canopy, and a relatively uniform conifer canopy. The objectives are 1) to compare measured and predicted solar transmissivities based on field data and analysis of hemispherical photographs and 2) to evaluate the magnitude of the predicted solar fluxes and the timing of snow ablation using the snow model, SNOBAL, driven separately with both measured and modeled solar transmissivities. Field measurements were made during winters of 2002 and 2003 at the Local Scale Observation Site (LSOS) in Fraser, Colorado USA as part of the Cold Land Processes Experiment. The canopy structure of the trees in a 0.8 ha plot was measured in detail (species, tree location, height, crown height, diameter at breast height). We measured incoming global solar radiation at the snow surface, beneath uniform and discontinuous lodgepole pine canopies, using arrays of 10 upward looking pyranometers at each site. Incoming global solar radiation was measured above the canopy and used to calculate transmitted values. Hemispherical photographs taken, with a Nikon CoolPix995 digital camera equipped with a Nikon Fisheye Converter (183° FOV), at each pyranometer location (n=20) were analyzed with Gap Light Analyzer (GLA) software (Frazer, et al. 1999) to determine total solar transmissivity. Mean measured and predicted solar transmissivities compared well (r²=0.86) in the discontinuous canopy site (0.27 measured vs. 0.29 GLA-predicted) and in the denser, uniform site (0.47 measured vs. 0.48 GLA-predicted). Our snow ablation modeling results suggest that using digital hemispherical images along with GLA software to determine a solar transmission factor can adequately represent the sub-canopy solar radiation incident on the snow surface.