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 over sparse vegetation, it is necessary to add an excess resistance to the aerodynamic resistance calculated between the canopy source height and the reference height. This excess resistance is classically expressed as a function of the kB(-1) parameter. By using the Shuttleworth-Wallace two-layer model, together with the linearity hypothesis on radiometric 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 the structural characteristics of the vegetation, the level of water stress and the climatic conditions. The model predictions have been confronted with experimental data, obtained on a fallow savannah during HAPEX-Sahel. The model performs fairly well, with a slight under-estimation which has been explained. From these results, it emerges that the kB(-1) parameter does not seem to be an appropriate and accurate tool to estimate sensible heat flux over sparse vegetation. Since it does not depend only on structural characteristics, it is not a constant parameter for a given vegetation, and thus, it can not be used in an operational way.