In Costa Rica, Erythrina poeppigiana is used as shade tree in most coffee plantations. The balance between benefits and disadvantages of this association for the coffee production probably depends on the local environmental conditions. The use of simulations from computer models could help to find out where the conditions are met to ensure the beneficial effects of the presence of Erythrina in a plantation. Calculating the radiative and energy balance of individual leaves is likely to substantially improve the transpiration estimation, but requires the calculation of net radiation on an individual leaf scale. This was made possible by the development of specific models, MIR and MUSC, simulating radiative transfers on computerized plant mock-ups. The model described in this paper consists of six main modules, for the calculation of leaf radiative balance, one dimensional turbulent transfers above and within the canopy, leaf energy balance, leaf stomatal conductance, sap flow, and gross photosynthesis of each leaf. The model effectively reders the main parameters of hydric functioning in a plant. However, a major modification concerning aerodynamic exchanges between the leaf and the atmosphere had to be made. 3-D modelling of turbulent transfers is a serious theoretical problem and efforts need to be made in this direction. The model, which was tested on a few coffee plants, functions correctly. However, the precise description of the architecture of coffee then becomes necessary