SENSIBLE HEAT-FLUX AND RADIOMETRIC SURFACE-TEMPERATURE OVER SPARSE SAHELIAN VEGETATION .2. MODEL FOR THE KB(-1) PARAMETER

To estimate sensible heat flux from radiometric surface temperature ov er sparse vegetation, it is necessary to add an excess resistance to t he aerodynamic resistance calculated between the canopy source height and the reference height. This excess resistance is classically expres sed as a function of the kB(-1) parameter. By using the Shuttleworth-W allace two-layer model, together with the linearity hypothesis on radi ometric temperature, an analytical expression of the kB(-1) parameter has been obtained. This expression and the numerical simulations show that kB(-1) is not a constant parameter: it varies as a function of th e structural characteristics of the vegetation, the level of water str ess and the climatic conditions. The model predictions have been confr onted with experimental data, obtained on a fallow savannah during HAP EX-Sahel. The model performs fairly well, with a slight under-estimati on which has been explained. From these results, it emerges that the k B(-1) parameter does not seem to be an appropriate and accurate tool t o estimate sensible heat flux over sparse vegetation. Since it does no t depend only on structural characteristics, it is not a constant para meter for a given vegetation, and thus, it can not be used in an opera tional way.