1114 
Figure 2 : Comparison between simulated aboveground biomass and measurements. 
COUPLING WITH A REFLECTANCE MODEL 
The modelling of the optical behaviour of a sahelian landscape must be done at a spatial scale compatible with 
the (coarse) spatial resolution of the sensors used for regional applications (a few km 2 ). The landscape is 
typically a sandy soil partially covered with mainly three vegetation layers: a herbaceous layer (20-80% cover) 
dominated by annual grasses, a shrub layer (0-20% cover) and a sparse tree layer (0-5% cover) (Le Houirou, 
1989). In the field of view of the remote sensor, however, the heterogeneous landscape is seen as an integration 
of contributions from components with contrasting spectral behaviours. At that scale, the effects of the shrub 
and tree layers have been neglected as for the ecosystem model, but the herbaceous layer being rarely totally 
covering, bare (bright sandy) soil must be considered. 
A Simple Area Additive reflectance model of the landscape 
The soil-vegetation composite reflectance problems have already been investigated by Richardson and Wiegand 
(1989) with the experimental data of Huete (1985) for a developing cotton canopy. Two composite scene 
reflectance models were compared: the additive-independent reflectance model in which the response of 
different components are considered independent and are added after being linearly weighted by their 
respective cover fractions, and the transmittance-reflectance interactive model where light reflecting both from 
the soil through the canopy and from the canopy itself are considered. Although their results confirm that the 
composite canopy reflectance is an interactive mixture of plant, soil and shadow reflectances, in the present 
study, the soil and the vegetation components will be considered independent This assumption, which has also 
been made in several studies on semi-arid environments (Graetz and Gentle, 1982; Pcch et al., 1986; Rambal et 
al., 1990; Hanan et al., 1991) is reasonable considering the fact that in the Sahel the grass canopy is rarely very 
tall such that shadowing and adjacency effects at our spatial scale have been neglected. Besides, the effect of 
soil beneath the canopy is implicitly accounted for in modelling the reflectance of the vegetation component 
The vegetation itself can be separated into green and dry components. However, they have been considered 
independent components and their complex interactions overlooked, mainly because after maturation they 
actually coexist in comparable amounts only for a short period of time. Litter biomass also influences the 
optical properties of the ground, but it is too intimately mixed with the soil to be considered a separate 
component. Instead of being neglected, it is included in the soil component So, the scene reflectance R in a 
given waveband and at any date during the growing season is written :