On the basis of studies of the behaviour in time of natural H-3 content in hydrological systems mathematical models can be developed for the water system being studied which permit extrapolation to the future. The purpose of this is to identify the transfer function and the relation-ship between input and output concentrations. Since the behaviour of water containing tritium and ordinary water is almost the same, this task is equivalent to determining the age distribution of water at the output of the hydrological system. The commonly used method is to calculate the output functions from the known input variations assuming different flow models and impulse responses. The model providing the best fit of measured and calculated output data is then adopted to describe the flow system. A number of models such as exponential and Gaussian distribution have been used to describe aquifer behaviour. Some examples of the application of model calculations of groundwater systems are discussed. For this, the tritium content of precipitation was used as the input function. The anticipated trend in concentrations at the output of the system was calculated for different mean residence times and compared to measured data.