MODELING RADIATIVE-TRANSFER IN HETEROGENEOUS 3-D VEGETATION CANOPIES

The DART (discrete anisotropic radiative transfer) model simulates rad iative transfer in heterogeneous 3-D scenes that may comprise differen t landscape features; i.e., leaves, grass, trunks, water, soil. The sc ene is divided into a rectangular cell matrix, i.e., building block fo r simulating lar er scenes. Cells are parallelipipedic. Their optical properties are represented by individual scattering phase functions th at are directly input into the model or are computed with optical and structural characteristics of elements within the cell. Radiation scat tering and propagation are simulated with the exact kernel and discret e ordinate approaches; any set of discrete direction can be selected. In addition to topography and hot spot, leaf specular and first-order polarization mechanisms are modeled. Two major iterative steps are dis tinguished: 1) Cell illumination with direct sun radiation: Within cel l multiple scattering is accurately simulated. 2) Interception and sca ttering of previously scattered radiation: Atmospheric radiation, poss ibly anisotropic, is input at this stage. Multiple scattering is store d as spherical harmonics expansions, for reducing computer memory cons traints. The model iterates on step 2, for all cells, and stops with t he energetic equilibrium. Two simple accelerating techniques can be us ed: 1) Gauss Seidel method, i.e., simulation of scattering with radiat ion already scattered at the iteration stage, and (2) decrease of the spherical harmonics expansion order with the iteration order. Moreover , convergence towards the energetic equilibrium is accelerated with an exponential fitting technique. This model predicts the bidirectional reflectance distribution function. of 3-D canopies. Radiation componen ts associated with leaf volume and surface mechanisms are distinguishe d. It gives also the radiation regime within canopies, for further det ermination of 3-D photosynthesis rates and primary production. Accurat e modeling of multiple scattering within cells, combined with the fact that cells can have different x,y,z dimensions, is well adapted to re mote sensing based studies, i.e., scenes with large dimensions. The mo del was successfully tested with homogeneous covers. Preliminary compa risons of simulated reflectance images with remotely acquired spectral images of a 3-D heterogeneous forest cover stressed the usefulness of the DART model for conducting studies with remotely acquired informat ion. (C) Elsevier Science Inc., 1996.