Fixed-area vs storm-centered areal reduction factors: a Mediterranean case study

The areal reduction factor (ARF) is a concept widely used in many hydrological applications. In this paper, assuming a “storm-centered” approach, new empirical laws for ARF estimation have been derived for the Calabria region (southern Italy), which are distinguished according to rainfall generation mechanisms typical of the Mediterranean area. Specifically, using data from the regional rain gauge network for the period 2002–2015, extreme events were separated into Convective Cells and Frontal Systems through a heuristic criterion to identify type-dependent ARFs. Significant differences were observed: more intense convective precipitation events evidenced a more pronounced decrease in empirical ARFs than storms related to frontal systems, with reductions up to approximately 50% for the median values at about 2000 km2. Results were also compared to some of the most widely used “fixed-area” laws for ARF evaluation; overall, these lead to larger values compared to storm-centered ARFs derived for the investigated region. The proposed storm-centered methodology can find important applications in providing reference limits of ARF values to be properly used in the engineering design of hydraulic and hydrological infrastructures, and appears particularly suited for effective and reliable flood defense in early warning systems.