Modeling bidirectional reflectance of multicomponent vegetation canopies

Abstract

By combining geometrical optics and transport theory, a bidirectional reflectance model for multicomponent vegetation canopies is explicitly developed in this article. A precise expression of the overlap function is derived for vertically cylindric phytoelements randomly distributed in the canopy, and a mathematical model for describing the vertical distribution of leaf area density is presented and fitted with measured data. In addition, the size, shape, orientation, and nonrandom distribution of leaves and vertical inhomogeneity of the canopy are properly taken into account in this model. The model also reproduces the measured distributions of reflectance from a wheat canopy quite well. The significant effect of stems on the canopy reflectance is identified by comparison with the result of the corresponding leaf canopy model. The results of sensitivity analysis prove that, even for crop canopies like wheat in this study, the influence of stems cannot be neglected if the ratio of stem silhouette area to the total foliage area is greater than 10%. Since this multicomponent model is a hybrid one, it could serve as a primary model to develop more accurate reflectance models for inhomogeneous vegetation canopies, such as forest, in further studies.