Topographic effects on bidirectional and hemispherical reflectances calculated with a geometric-optical canopy model

The effects of topography on both the bidirectional reflectance distribution function (BRDF) and the hemispherical reflectance (surface albedo) of a forested scene are investigated with the Li-Strahler geometric-optical model. The Li-Strahler geometric-optical model treats a vegetation canopy as an assemblage of partially illuminated tree crowns of spheroidal shape, and through geometric optics and Boolean set theory, models the proportion of sunlit or shadowed canopy and background as functions of view angle, illumination angle, and crown geometry. The model has been modified to accommodate a sloping surface in its computation of bidirectional and hemispherical reflectance. When the BRDF of a flat vegetated surface is compared to the BRDF of a sloping surface that is similarly vegetated, the interaction of the illumination angle and the slope distort the shape of the BRDF. A hemispherical integration of this distorted BRDF provides an albedo for the sloping surface