Effect of crown architecture on the radiative transmittance and reflectance within canopy: modeling and validation

In our Geometric-Optical BRDF modeling study for forest canopy, we notice that the reflectance signature of sunlit crown is not uniform, a sub-model for modeling its feature is needed to evaluate the contribution of this component to the BRDF of surface. To modeling the effect of canopy structure on the radiation transmittance and reflectance within canopy, Li's path-scattering model for homogeneous layers is applied to calculate the transmittance in different layers within crown. In the positions where the foliage volume density are lower, such as near the surface of crown, the contribution of incident radiation is modeled by the openness distribution within tree crown, based on our directional gap probability model. For the model validation, field measurement was made in an apple orchard. The geometric size and foliage area volume density (FAVD) distribution of tree crowns were measured indirectly. The solar radiation. diffused skylight, hemispherical radiation at the given test points within and below the crowns were measured. The model, measurements and validation result are presented in this paper, which show that the transmitted and reflected radiation is closely related to the architecture of crown and the geometry of the test site.