An analysis of runoff generation through ephemeral streams in karst semi-arid areas is here presented. The analysis is based on a full 2D approach, integrating the hydrological and hydraulic models, in order to account first for the dynamic of catchment response to rainfall and activation of the streams, and then for the hydraulic behavior of the streams. This analysis entails the simulation of extreme events corresponding to low, medium and high return periods, in order to identify the event, which presumably activate the ephemeral streams. The analysis focuses on a case study located in southeast Italy.