In this study we analyze observations of water vapor and carbon dioxide fluxes above a bog vegetation in the Netherlands. The observations are used to evaluate an assimilation photosynthesis (so-called A-gs) model on canopy scale. An A-gs model couples the CO2 assimilation, A, to the stomatal conductance, gs and offers a physiological technique to simulate plant transpiration. Most parameters of this model are closely linked up with micrometeorological variables, hence, data from common meteorological weather stations can be used to simulate H2O and CO2 fluxes. It is believed that the observed correlation between A and gs indicates a plant strategy, in which different plant species behave more or less similar. This means that the A-gs model is robust and appropriate to be used in meso-scale, or, hydrological models. However, the effects of nutrient shortage, pests, diseases and weed competition are not taken into account in most A-gs models. Nevertheless, the mechanism how it affects the assimilation process in plants is known and has been implemented in the present model. Transpiration simulations resulted in an excellent agreement with the measurements over the bog area. Next, a simple model to simulate the soil respiration has been added to the model of the present study. The total simulated fluxes of plant assimilation and soil respiration have been compared with the measured total CO2 flux. The agreement between simulations and experiments appeared to be reasonable well.