TOPOGRAPHIC EFFECTS ON BIDIRECTIONAL AND HEMISPHERICAL REFLECTANCES CALCULATED WITH A GEOMETRIC-OPTICAL CANOPY MODEL

The effects of topography on both the Bidirectional Reflectance Distri bution Function (BRDF) and the hemispherical reflectance (surface albe do) of a forested scene are investigated with the Li-Strahler geometri c-optical model. The Li-Strahler geometric-optical model treats a vege tation 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 comput ation 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 angl e and the slope distort the shape of the BRDF. A hemispherical integra tion of this distorted BRDF provides an albedo for the sloping surface .