BRDF behaviour of a tropical forest surveyed from space

Survey of tropical forest evolution and functioning with remote sensing is hampered by the variability of their BRDF (bi-directional reflectance distribution function); e.g. automatic classifications may be totally erroneous if view and illumination conditions are not taken into account. The authors used a new radiative transfer model (DART, Discrete Anisotropic Radiative Transfer) to analyze BRDF behaviour of a tropical forest plot in Central Sumatra, Indonesia. Simulations stressed that BRDF anisotropy, especially for low Sun zenith angles (θ_s), may be large enough to make difficult the study of forest evolution with satellite data time series. Variations were up to 30% for VIS (visible), 20% for NIR (near infrared) and 25% for SWIR (short wave infrared), for viewing zenith angles (θ_v) smaller than 25°. Larger variations occurred in the hot spot configuration and for variable Sun zenith angles θ_s: up to 50% for VIS, 30% for NIR, 40% for SWIR, whereas local topography and diffuse atmospheric radiation (SKYL) had a much smaller influence. On the other hand, variations due to a 50% cover degradation were 14% for VIS, 23% for NIR and 18% for SWIR at nadir and θ_s=35