The ETMOD (Environmental Tritium Model) computer code calculates the potential radiological impact of short duration releases of HT and HTO. The code is now able to predict key input parameters, such as the deposition velocity of HT to soil and the rate of HTO exchange between terrestrial surfaces and the atmosphere, on the basis of weather conditions, land use, season and time of day. These features are illustrated by presenting model simulations of an HT field release experiment.