Effect of the vegetation clumping on the BRDF of a semi-arid grassland: comparison of the SAIL model and ray tracing method applied to a 3D computerized vegetation canopy

A 3D model of vegetation, the AMAP Software (Atelier de Mode´lisation de l'Architecture des Plantes, CIRAD), is used with a ray-tracing method to compute radiative transfer of sparse canopies. In a first step, the ray-tracing model is validated against two well-known radiative transfer models: a turbid medium model, SAIL, in the case of an homogeneous canopy, and a geometrical model of cylinders for clumped vegetation. Then, botanical measurements of semi-arid grassland acquired during the SALSA project in 1997 (Sonora, Mexico) are used to create 3D realistic scenes of vegetation. The BRDF of these 3D scenes is compared to the BRDF of a turbid medium equivalent in terms of vegetation parameters (leaf area index, leaf angle inclinations…). It is then possible to evaluate the error committed when sparse clumped canopies are treated as turbid media