Regional rainfall and flood frequency analysis for Sicily using the two component extreme value distribution

This paper first presents a duration-dependent hypothesis regarding the parameters of the two component extreme value (TCEV) distribution and proposes a simple model to obtain the rainfall depth-duration relationship both at ungauged sites and at short record gauged sites in the Sicilian region. Then, by using the annual maximum peak flood and the annual maximum mean daily discharge recorded in Sicily, a TCEV hierarchical regional procedure is developed. An empirical estimation criterion which links the mean annual flood with the watershed area, and a modified rational formula in which a mean runoff coefficient is introduced, are proposed. Finally, two relationships are established for estimating the mean runoff coefficient, making use of the permeable watershed area and the wooded area.