The need to increase the predictive power of risk assessment for large tritium releases implies a process level aproach for model development. Tritium transfer for atmosphere to plant and the conversion in organically bound tritium depend strongly on plant characteristics, season, and meteorological conditions. In order to cope with this large variability and to avoid also, expensive calibration experiments, we developped a model using knowledge of plant physiology, agrometeorology, soil sciences, hydrology, and climatology. The transfer of tritiated water to plant is modelled with resistance approach including sparce canopy. The canopy resistance is modelled using Jarvis-Calvet approach modified in order to directly use the canopy photosynthesis rate. The crop growth model WOFOST is used for photosynthesis rate both for canopy resistence and formation of organically bound tritium, also. Using this formalism, the tritium transfer parameters are directely linked to known processes and parameters from agricultural sciences. The model predictions for tritium in wheat are closed to a factor two to experimental data without any calibration. The model also is tested for rice and soybean and can be applied for various plants and environmental conditions. For sparce canopy the model uses coupled equations between soil and plants.