This is the third in a series of papers describing a measurement program and modeling approach to the estimation of surface heat and moisture fluxes over a tallgrass prairie. We describe the design and formulation of an experimental biosphere model (Ex-BATS) which follows the development of Dickson's biosphere-atmosphere transfer scheme (BATS). Ex-BATS has been designed to incorporate in situ measurements and satellite parameterizations of certain canopy variables which are slowly varying in the course of a growing season. All components of a multistage biosphere process model used to simulate the exchange of heat and moisture between the canopy and the atmosphere are presented here. The remote sensing aspects of the model are described in a companion paper. The procedures used to optimize the model and the validation of the model against First ISLSCP Field Experiment (FIFE) observations taken during 1987 are described. Validation was carried out for three of the four intensive field compaigns (IFCs): 1, 2 and 3. The validation intercomparison shows that the Ex-BATS model reproduces the diurnal behavior of the surface fluxes very closely. The rms differences between in situ measurements of sensible and latent heat fluxes and their model counterparts are of the order of 35 W m-2. Biases over the three IFCs, which ranged in duration from 10 to 17 days, are typically less than 20 W m-2. The overall bias for the 44 days within the first three IFCs is less than 10 W m-2. The biases and rms differences are of the same order as the accuracy and precision uncertainties of the measured fluxes, therefore the model provides a useful experimental tool to explain radiative, hydrological, and physiological controls on surface fluxes over vegetated canopies and the explicit sources of water flux to the atmosphere from transpiration, foliage evaporation, and soil evaporation. ¿ American Geophysical Union 1993