A watershed model is developed capable of predicting hydrologic behavior of a natural watershed for which data concerning historical relationships between rainfall and runoff events are unavailable. The computer program used to implement the model together with sample data and its output is given in an appendix. The model uses a distributed analysis approach and involves the subdivision of a catchment into a grid of small elemental areas, the mathematical characterization or modelling of the various physical processes occurring within each element and the numerical integration of the responses from each watershed element into a comprehensive description of not only the discharge at the outlet, but of the complete hydrologic response of every element of which the catchment is composed.