For assessing risk of highly unusual events extreme value statistics needs to be applied, which plays an important role in engineering practices for water resources design and management. In hydrology, the typical application of extreme value theory concerns floods in river basins or landslides. The present paper is, instead, focused on the analysis of extreme wet and dry periods in a sample area (Sicily). First, we have studied monthly precipitation extremes both using the annual maximum and partial duration methods, and return times have been estimated by standard statistical techniques. Next, we studied the extremes of the Standardized Precipitation Index (SPI), which has been proposed as an indicator for monitoring wet and dry conditions. We found considerable differences both in the return periods and in the time location of the extremes. From our study it appears that the SPI better describes wet and dry periods than the precipitation does. Maps of return times for extreme conditions in Sicily are also presented, which cluster the territory into areas of different extreme return periods. Finally, the occurrence of extremes in Sicily has been related to large-scale atmospheric circulation. © Springer-Verlag 2006.

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