| The solar radiation regime within plant canopies influences many environmental processes ranging from photosynthesis to transpiration to surface energy balance. This study extended and applied a hybrid Geometric-Optical and Radiative-Transfer (GORT) model of the interaction of solar radiation with discontinuous plant canopies. First, an analytical approximation of the radiative transfer equation for a horizontally homogeneous layer of finite thickness was derived. Comparisons of the approximations with numerical solutions indicate that the method is robust for any finite horizontally homogeneous layer. Second, combined with gap probabilities as a measure of the discontinuity of plant canopies, this approximation was used to estimate the spectral reflectance of surface materials. Combining these spectral reflectances with an existing geometric-optical model results in a simplified GORT model of the bidirectional reflectance of discontinuous plant canopies. Comparison of model results with directional measurements over boreal forests in Saskatchewan, Canada, indicate the ability of this simplified model to capture the salient properties of canopy bidirectional reflectance. Third, this simplified hybrid model was extended to the spatial domain to estimate directional effects on the spatial variance of images as a function of scene properties. This approach captures the basic features of observed directional variances from images over discontinuous plant canopies. Fourth, a modified GORT model was applied to the question of solar radiation transmission in conifer forests for the purpose of estimating the downwelling solar radiation to snowpacks. Use of model estimates of transmission leads to improved estimates of the rate and timing of snowmelt in forested areas.;At the most general level, this research contributes to an improved understanding of the interaction of solar radiation with plant canopies, and in particular discontinuous plant canopies. More practical benefits are: (1) the basis for improved retrieval of surface parameters from satellite data, and (2) improved models for estimating solar fluxes within and below plant canopies which can be applied to a wide range of environmental processes. |
