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In models of global change, the course of the general circulation of the atmosphere is dependent on the planetary distribution of sources and sinks of energy; during periods of balance, energy is transported by atmospheric air and the water of the oceans. The atmosphere must thus transport the energy of the continents towards the oceans during the summer and that of the oceans towards the continents during the winter. The study of these phenomena on the continents uses data related to studies of actual evapotranspiration on a local level, which is the level presented here. Recent models of actual evapotranspiration dissociate evaporation of the soil from canopy transpiration and interception, and simulate the evolution of the structure of cover, stomatal resistances, and factors which govern vapour water flux resistance and the balance between evaporation and transpiration. In today's models which take a mechanistic approach, the difficulty of choosing income parameters has become significant, especially the problem of their initial state under a given set of environmental conditions. The objective of this paper is to point out the potential use of the Penman-Monteith formula, which has worked well for the actual evapotranspiration phenomenon of close covers and can be adapted to sparse vegetation (herbaceous savannah with areas of bare soil), while treating leaf transpiration and bare-soil evaporation separately. The actual evapotranspiration of the herbaceous savannah is limited by a boundary layer resistance of the same order of magnitude as surface resistance plotted to the stomata and leaf area index, whereas that of woody covers is limited only by surface resistance: the stomata of trees are more sensitive to the drying of the air and soil than those of herbaceous plants.