A HYBRID GEOMETRIC OPTICAL-RADIATIVE TRANSFER APPROACH FOR MODELING ALBEDO AND DIRECTIONAL REFLECTANCE OF DISCONTINUOUS CANOPIES

A new model for the bidirectional reflectance of a vegetation cover co mbines principles of geometric optics and radiative transfer. It relie s on gap probabilities and path length distributions to model the pene tration of irradiance from a parallel source and the single and multip le scattering of that irradiance in the direction of an observer. The model applies to vegetation covers of discrete plant crowns that are r andomly centered both on the plane and within a layer of variable thic kness above it. Crowns assume a spheroidal shape with. arbitrary heigh t to width ratio. Geometric optics easily models the irradiance that p enetrates the vegetation cover directly, is scattered by the soil, and exits without further scattering by the vegetation. Within a plant cr own, the probability of scattering is a negative exponential function of path length. Within-crown scattering provides the source for singly -scattered radiation, which exits with probabilities proportional to f urther path-length distributions in the direction of exitance (includi ng the hotspot effect). Single scattering provides the source for doub le scattering, and then higher order pairs of scattering are solved su ccessively by a convolution function. Early validations using data fro m a conifer stand near Howland, Maine, show reasonable agreement betwe en modeled and observed reflectance.