Discussion of "Analysis of extreme rainfall trends in sicily for the evaluation of depth-duration-frequency curves in climate change scenarios" by Lorena Liuzzo and Gabriele Freni

In the original paper, the Authors investigated the presence of trends in extreme rainfall depths of duration d = 1, 3, 6, 12 and 24 hours, in Sicily. Trend analysis was performed by using the Mann-Kendal non-parametric test, which was applied at-site, for a number of 65 raingauges well distributed in the region, for the period 1950-2008. In order to overcome the gap of missing data, six years moving average series were considered in the Author’s analysis. In this discussion, the discussers highlighted some important observations about the paper presented by the Authors: i) The Authors applied the non-parametric Mann Kendall test to moving average series of rainfall data. The use of moving average series introduce serial correlation affecting the ability of statistical test to assess the significance of trend; in particular, positive serial correlation inflates the variance of Mann Kendall test statistic, S, by increasing the possibility of rejecting the null hypothesis of no trend (von Storch, 1995), whereas negative serial correlations shrinks the S-variance, underestimating significant trends (Yue and Wang, 2002). ii) The conflicting results obtained by the Authors at a very small spatial scale (Sicily area 26000 km2), comparing to the scale at which climate changing should be studied, can reflect the need of a regional scale analysis, which could be more relevant and rigorous than that at a-site studies (Douglas et al., 2000; Yue and Wang, 2002).