Areal reduction factor. The effect of the return period

For the study and modeling of hydrological phenomena both in urban and rural areas, a proper estimation of the areal reduction factor (ARF) is crucial. The ARF is defined as the ratio between the average rainfall occurring on a specific area and the point rainfall. In literature there exist several methodologies to estimate this ratio, as a consequence the corresponding ARFs have different properties. Spite the importance of the topic for the definition of design rainfall events, the ARF estimation has still some open issues. Throughout the time, many authors have recognized several factors influencing the ratio between the spatial average rainfall over a specific area and the rainfall measured in a point. These factors can be related to the characteristics of the rainfall itself, to the characteristics of the catchment under study and to the data and methods used. In this paper, we investigate the influence of the return period (Tr) on ARF estimation, for heavy rainfall events observed in the Lazio Region (Italy).