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.