A HYBRID GEOMETRIC OPTICAL-RADIATIVE TRANSFER APPROACH FOR MODELING LIGHT-ABSORPTION AND ALBEDO OF DISCONTINUOUS CANOPIES

A new model for light-absorption and albedo of a vegetation cover comb ines principles of geometric optics and radiative transfer. It relies on the gap probabilities and path length distributions to model the pe netration of the irradiance from a parallel source and the single and multiple scattering of that irradiance. The model is applied to the ve getation covers of the discrete plant crowns that are randomly centred both on the plane and within a layer of variable thickness (from h(1) to h(2)) above it. Crowns assume a spheroidal shape with an arbitrary height to width ratio (B/R). Geometric optics easily models the irrad iance that penetrates the vegetation cover directly, and is scattered by the soil, and exist without further scattering by the vegetation. W ithin a plant crown, the probability of scattering is a negative expon ential function of path length. Within crown scattering provides the s ource for the singly scattered radiation, which provides the source fo r double scattering, and then higher order pairs of scattering are sol ved successively by a convolution-like function. Early validations usi ng data from a conifer stand near Howland, Maine of USA show reasonabl e agreement between the modeled and observed albedo.