چکیده (انگلیسی):

Solar irradiance in and around forest gaps plays a key role in determining the snowmelt patterns that shape how forest gaps influence spring snow cover retention relative to open or continuously forested landscapes. To reduce uncertainty in forest gap snowmelt estimation, a new canopy solar transmittance model was developed to include the effects of shading and enhanced transmittance. The model couples a new a ray tracing approach with an existing canopy solar radiation extinction and transmission model. The new model analytically solves for the path length of a solar ray (beam) through a forest canopy and potential intersection(s) with a forest gap conceptualized as an upright circular cylinder surrounded by homogeneous forest. The model was tested alongside simpler transmittance models that ignored shading and treated the canopy as either a binary (open or forested) classification or that used proximal canopy descriptors. The models were evaluated using measurements of shortwave radiation from 20 pyranometers in and around a 56 m diameter gap in a coniferous forest. Errors from ignoring shading were greatest in the gap near the forest edge and in the forest nearest the northern gap edge. The new model reduced these errors; at the 20 pyranometer locations, individual biases up to +356 W m−2 and average biases up to +101 W m−2 were reduced to better than ±5 W m−2. Results suggest that an accurate description of spatial variability of solar irradiance in and around a forest gap requires explicit treatment of the effects of shading into a gap and enhanced transmittance beyond the gap extent; this is dependent upon solar position, gap and forest geometry, and location relative to the gap. A sensitivity experiment with the forest gap – ray trace model in which gap size, latitude, and day of year were varied under clear-sky conditions showed that cumulative daily solar irradiance has high spatiotemporal variability. The maximum of the spatial coefficient of variation (CV) of cumulative daily irradiance was a function of gap size and solar angles; smaller (larger) gaps and lower (higher) solar elevation angle ranges were more diffuse- (direct beam-) dominated with reduced (increased) spatial variability relative to the mean. It is found that there is no standard impact of a forest gap on shortwave irradiance – the impact of gap size on clear sky solar irradiance depends on forest structure, date, and latitude.