This paper presents an original integrated approach to rainfall-runoff modelling for partly urbanized watersheds. A digital terrain model (DTM), allowing use of GIS techniques, was built not only for representing the undeveloped part of the catchment but also the urbanized area. For each cell of the DTM grid, a water budget is computed, providing runoff and interflow amounts. The water volumes generated at each cell are moved along the steepest slopes with a celerity depending on this slope, until they reach the outlet and contribute to the resulting hydrograph.
The model was tested in a partly urbanized catchment, specially equipped with rain and flow measurement station. Comparisons with other procedures, mainly with SWMM and WALLRUS models, shows that the proposed model seems to be one of the most accurate. Moreover, the sensitivity analysis confirms the flexibility of the tool. Its concept, based on GIS routines, allows much of the computation to be done automatically, avoiding a lot of tedious tasks. For example, it becomes relatively easy to check the consequences of urban planning options.