Influence of climate on the flood frequency distribution within a large region of southern Italy

In the theoretical derivation of flood frequency distribution as well as in the statistical regional analysis, climate plays a decisive role, being closely related to values and to patterns of variability of physically consistent parameters. We focused on the average annual number of flood peaks, Aq, and on the ratio of Aq over the average annual rate of rainfall events, Ap, as crucial parameters of extreme value distributions based on a Poisson process of occurrences. The link between climate and these statistical parameters is first empirically analysed and then explained as to how they relate to a characteristic water loss parameter dependent on climate, namely, a dimensionless loss factor f representing the storm rainfall threshold that determines if runoff is generated or not. Data from 20 basins in southern Italy, with hyper-humid to semiarid climates, show meaningful relationships between the degree of aridity and estimates of Aq/Ap and f.