Analysis of Extreme Rainfall Trends in Sicily for the Evaluation of Depth-Duration-Frequency Curves in Climate Change Scenarios

Abstract

The design of urban drainage systems and the development of flood mitigation strategies commonly require knowledge of extreme rainfall for a given return period. In the context of climate variability that is no longer imputable to only natural forces, the estimation of this variable represents a critical issue. Climate change effects, particularly on precipitation, should be considered in the design and planning of hydraulics infrastructures. The aim of this study is to provide an assessment of the effects of statistically significant trends in extreme rainfall on the rainfall depth-duration-frequency (DDF) curves for the return periods typically used in design in order to obtain useful information for the definition of DDF curves in climate change scenarios. The study was applied in Sicily, in the center of the Mediterranean Area, because the study may provide interesting insights in a highly populated coastal region. The first step of the study consisted of the detection and quantification of trends in the annual maximum rainfall series of different durations (1, 3, 6, 12, and 24 h) observed in Sicily. The annual maximum rainfall series recorded using 65 rain gauges over the period 1950–2008 has been selected and analyzed. For each duration, the moving averages have been computed and then the Mann-Kendall test has been applied, considering different levels of confidence. The results showed that, for all the durations, increasing and decreasing trends occurred over the examined period. Once trends have been detected and quantified, the generalized extreme value distribution (GEV) has been employed to compute extreme rainfall with return periods equal to 5, 10, and 20 years. Finally, the parameters of the DDF curves have been updated for the Sicilian rain gauges, incorporating the information obtained from the trend analysis carried out on the local scale. The updating of the DDF parameters has been carried out by defining two climate scenarios on the basis of the trend analysis results. For each duration, results showed that, in the area of study, extreme rainfall series have been affected by both negative and positive statistically significant trends.