In this study we analyze observations of water vapor and carbon dioxide fluxes above a sparsely vegetated bog area in the north of the Netherlands during the early growing season. The observations are used to evaluate a combined assimilation photosynthesis (so-called A-gs) model on canopy scale and an open water model. An A-gs model couples the CO2 assimilation, A, to the stomatal conductance, gs and offers a physiological technique to simulate plant transpiration. For the patches open water of the bog area, a so-called open water evaporation model has been applied. Transpiration and open water evaporation has been mixed together by applying a weighing technique using the surface characteristics. 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 A-gs model. Evapotranspiration simulations resulted in an excellent agreement with the measurements over the bog area. To simulate the total net CO2 flux, a simple soil respiration model has been added to the A-gs 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.