Areal reduction factor estimate for extreme rainfall events

The areal reduction factor (ARF) is of paramount importance in the hydraulic structures design against hydrological hazards. In fact, the knowledge of areal-average rainfall over a basin is essential in rainfall-runoff models commonly adopted in the hydrological practice. Nevertheless, rainfall data are generally available at a point scale; ARF allows to transform such kind of data to an areal-average information. In this chapter, after an introduction providing a general overview over the topic, the main factors influencing ARFs are described. Then, the main empirical and analytical approaches to estimate ARFs available in the scientific literature are presented and critically discussed. The crucial issue of the transposition and applicability of ARFs values developed for a certain area to other regions is also deepened by presenting the results of several studies. Finally, a new empirical formulation for the ARFs estimate recently developed in the Umbria region (Central Italy) is proposed. On the basis of the specific case study and available data quality, indications to the reader in order to select the most suitable method to be applied for ARF estimation are provided.