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Since 2002, within the framework of the Cetemps Centre of Excellence at the University of L'Aquila, a distributed grid-based hydrological model (CHYM) has been developed to provide a general purpose model for operational flood warning activity. This paper presents two new cellular automata (CA) algorithms used respectively for drainage network extraction and rainfall data assimilation. The first is a cellular automaton-based algorithm for the extraction of a drainage network from an arbitrary digital elevation model. It has been implemented and tested on a large number of different domains. This algorithm is able to define the flow direction at every point on the digital elevation model where singular points are present (pits or flat areas). The second is a CA-based numerical technique for assimilating different data sources of rainfall to rebuild the rainfall field on a grid. This technique has been shown to produce a reasonable rainfield shape without any geometrical artefacts that often produce unrealistic rain gradients in the rainfield.

Abstract. The increased number of extreme rainfall events seems to be one of the common feature of climate change signal all over the world (Easterlin et al., 2000;Meehl et al., 2000). In the last few years a large number of floods caused by extreme meteorological events has been observed over the river basins of Mediterranean area and they mainly affected small basins (few hundreds until few thousands of square kilometres of drainage area) . A strategic goal of applied meteorology is now to try to predict with high spatial resolution the segments of drainage network where floods may occur. A possible way to reach this aim is the coupling of meteorological mesoscale model with high resolution hydrological model. In this work few case studies of observed floods in the Italian Mediterranean area will be presented. It is shown how a distributed hydrological model, using the precipitation fields predicted by MM5 meteorological model, is able to highlight the area where the major floods may occur.