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When applying hydrological models at basin scale, it is essential to take into account the contribution of groundwater and its behaviour, together with the hydrological state before and after storm events. The main objectives of the present study were the evaluation and discussion of factors contributing to the formation of river discharge, how and to what extent these factors affect the risk of flood and, finally, how they are related to the type, duration and intensity of rainfall events, especially associated with flash floods. This was achieved by applying HEC-HMS models to a small watershed in central Italy, characterised by high hydrogeological risk. The watershed is equipped for total rainfall, river discharge and groundwater level measurement. In addition, double ring infiltrometric tests were performed during different hydrological periods. The results highlighted a direct correlation between flood risk and the hydrological period; the insertion of groundwater data in the model allowed for a sensitivity analysis of the relationship between the level of risk and the hydrogeological settings and properties of the area. Moreover, the analysis of some “extreme” events occurring in the summer period highlighted river behaviour in very different hydrological states. [ABSTRACT FROM AUTHOR]