EFFECTS OF SOIL COLOR AND BRIGHTNESS ON V EGETATION INDEXES

Variations in the spectral properties of soils related to their colour and brightness considerably influence the detection of sparse vegetat ion in heterogeneous environments using vegetation indices. During the last decade, a new generation of vegetation indices (NDVI, PVI, SAVI, MSAVI, TSAVI, TSARVI, ARVI, GEMI, and AVI) was developed in order to minimize these effects. To evaluate the sensitivity of these indices t o soil colour and brightness and to test their potential for a more pr ecise description of the vegetation cover for different cover rates, a number of simulations were carried out using a first order radiative transfer model. The model was adapted for studying directly the contri bution of the optical properties of bare soils on the vegetation index . The results show that the first order radiative transfer model const itutes a valuable tool for analysing and understanding the interaction s between the electromagnetic radiation, the vegetation cover and bare soil. It makes it possible to analyse the effect of colour and bright ness on the reflectance factor and, consequently, on the vegetation in dex. The GEMI, AVI, NDVI, ARVI and PVI indices show lower performance for the management of sparse or moderately dense vegetation environmen ts. They are marked by non-negligible errors related to the optical pr operties of bare soils. The AVI leads to results that do not reflect t he theoretical behaviour of vegetation indices. As to the TSAVI, TSARV I, SAVI and MSAVI indices, they are more resistant to changes in the o ptical properties of soils and permit better discrimination between th e vegetation from the bare soil background in an heterogeneous and rel atively complex environment.