Phase: RAMI 3
4SAIL2 is a surface reflectance model that was evolved from the GeoSAIL model through an extension with a non-lambertian soil BRDF model and the additional consideration of vegetation with ground or crown coverage below 1 (see table 1). These extensions allow the more realistic simulations of directional acquisitions and of forests. As shown in Figure 1, 4SAIL2 follows a 4-stream concept, which divides the modeled fluxes in their direct and diffuse, upward and downward contributions. The input parameters to 4SAIL2 are also listed in Figure 1. They describe structural and physiological information on the vegetation, soil properties and the observation geometry. 4SAIL2 incorporates a submodel for the soil reflectance and its variation with moisture. The canopy model is a two-layer version of the model SAILH (Figure 2).
The canopy is modeled in 2 layers to mimic the vertical leaf color gradient often seen in agricultural canopies. The structural properties in both layers (leaf angle distribution and leaf size) are assumed to be identical in both layers, but the LAIs for green and brown leaves may differ. The leaf angle distribution is described by two parameters, a and b, of which a determines the average leaf slope, and b expresses the so-called bimodality of the distribution. The division of the LAI (leaf area index) for both types of leaf over both layers is governed by the parameters fraction brown leaves fB and the so-called dissociation factor D. For the extreme values D = 1 (complete dissociation) and D = 0 (homogeneous mixture) - see Figure 3. The spectral information on the optical properties of the leaves (i.e. spectral reflectance and transmittance of green and brown leaves) is usually calculated using the PROSPECT model.
