Effects of leaf-transmittance versus leaf-reflectance on bidirectional scattering from canopy/soil surface: An analytical study

Abstract

A simple single-scattering model for a surface with sparse ereetophile plants is developed as a plane-parallel canopy consisting of small leaves (relative to leaf-to-leaf spacing) with a spherical-shell distribution of the leaf area. The contributions to the overall surface reflectance in the visible spectral bands by the soil-reflectance, leaf-reflection, and leaf-transmission (which are assumed isotropic) are analyzed under different view/illumination geometries. High values of leaf reflectance r, relative to leaf transmittance t, produce significantly different patterns of bidirectional reflectance, because r and t control the backscattering and the forward scattering, respectively. These two effects, especially strong at large solar zenith angles, produce high canopy reflectance at large view zenith angles around the principal plane. Model inversion with the PARABOLA bidirectional reflectance measurements over the Konza Prairie yield values of r and t for grassblades of this grassland canopy. The inversion results point to a possibility of assessing canopy condition from its bidirectional reflectances, as both r and t are sensitive to plant vigor and phenology. In an inversion with satellite measurements over a desert-scrub surface in the northern Sinai, the optical thickness of these dark plants (inferred in the visible band) and the near-infrared reflectance of the plant elements were inferred. The value of the optical thickness of this sparse canopy essentially did not depend on the assumed plant-element transmittance, but the inferred infrared reflectances of the plant elements were appreciably dependent. The canopy structure representation (the spherical-shell distribution of the planar leaf area) constitutes a rotation-invariant reflectance model. It allows formulation of the longwave exchanges identical to the conventional radiative transfer calculation through a layer of molecules or particles with a specified optical thickness.