Calculated sensitivities of several optical radiometric indices for vegetation canopies

A radiative transfer model for a vegetation canopy is used to compute several quantities used in remote sensing applications and to determine the sensitivity of these radiometric quantities to several of the important problem parameters. In particular, the following quantities are considered: 1) the ratio of near IR (700–1100 nm) to visible (600–690 nm) reflected intensities in the nadir direction, 2) the ratio of the fraction of incident energy reflected in the near IR to that in the visible (ratio of canopy hemispheric reflectances), 3)the normalized difference between the near IR and visible nadir reflected intensities, 4) the visible intensity transmitted in a given downward direction, and 5) the visible energy incident on the soil surface. A realistic radiative transfer model is proposed for calculating these radiometric quantities. Also simple approximate models are derived for the radiation transmitted through the canopy. With these models, the sensitivity of the reflected and transmitted quantities to various canopy and illumination conditions are then determined. Specifically, the effects of 1) sun angle, 2) the ratio of the diffuse-to-direct incident radiation, 3) the leaf-normal distribution for the canopy, 4) the scattering properties of the leaves, and 5) the soil albedo are considered. The sensitivity of the calculated quantities are presented as a function of the leaf area index of the canopy, and, for the reflected quantities, also as a function of the fraction of visible light absorbed in the canopy.