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 (/spl theta//sub 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 (/spl theta//sub v/) smaller than 25/spl deg/. Larger variations occurred in the hot spot configuration and for variable Sun zenith angles /spl theta//sub 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 /spl theta//sub s/=35.