SOLAR ZENITH ANGLE EFFECTS ON FOREST CANOPY HEMISPHERICAL REFLECTANCES CALCULATED WITH A GEOMETRIC-OPTICAL BIDIRECTIONAL REFLECTANCE MODEL

The Bidirectional Reflectance Distribution Function (BRDF) provided by the Li-Strahler geometric-optical forest canopy model has been integrated to provide spectral instantaneous hemispherical reflectances of sparsely vegetated surfaces. Further integration over the sun's zenith angles can yield daily or longer interval hemispherical reflectances as well. A variety of simulated canopies (conifer, savanna, and shrub) were modeled with varying solar angles. In all cases, as the geometric-optical model introduced increased shadowing of the surface with increased solar zenith angle, the direct-beam hemispherical surface reflectance gradually decreased. The hemispherical reflectance values are direct beam calculations and do not directly include canopy multiple scattering, leaf specularity or consideration of the impact of diffuse irradiance. These limitations are acceptable for sparse canopies, in which three-dimensional shadowing effects are large. However, radiative transfer calculations have shown that these phenomena (all of which are, to a greater or lesser extent, solar zenith-angle-dependent) must be incorporated before truly realistic modeling of hemispherical surface reflectances can be achieved for dense canopies.