Regional frequency analysis and geoadditive modeling for design storm estimates in the Arno river basin (Italy).

Investigations on extreme hydrological events are often at the basis of environmental studies related to hydrologic cycle changes and more in general with climate change. Design storm represents an important variable for its implications on flood risk assessment and territory protection measures definition. A regional frequency analysis for studying and understanding the annual maxima of daily rainfall, based on the index variable method, is implemented here on the Tuscany Region (Central Italy). According to the hierarchical approach on three levels, the studied area is divided into homogeneous regions and sub-regions; the statistical homogeneity within the regions is verified through several homogeneity tests. Furthermore, the Two-Component Extreme Value (TCEV) probability distribution of the extreme rainfall is considered identical within each homogeneous region unless a scale factor, called index rainfall, given by a multivariate model based on climatic and geomorphological characteristics. A GeoAdditive Model (GAM) for extremes assuming that the observations follow Generalized Extreme Value GEV distribution whose locations are spatially dependent is also carried out on the catchment area of Arno River, the main river of Tuscany Region that extends for a large part of the region area. The application of the two methods is discussed considering the comparison of the maps of the design storm for daily duration and 50 years return period.