SPI-Based Probabilistic Analysis of Drought Areal Extent in Sicily

Drought is a natural phenomenon that presents spatial and temporal features whose knowledge is fundamental for an appropriate water resources management. In particular, the assessment of probabilities and return periods of areal extent of droughts of different severities over a region can provide useful information for planning drought management. In this study, an analytical methodology to characterize probabilistically the relationship between meteorological drought severity (computed in terms of Standardized Precipitation Index, SPI) and areal extent, expressed as Drought severity-Area-Frequency (SAF) curves, is proposed. In particular, analytical expressions of SAF curves describing the proportion of the total area of the region of interest where the SPI values are below a fixed threshold are derived. The developed curves enable to characterize a given drought event in a region, by computing the probability of occurrence of SAF curves exceeding the one observed. The proposed methodology is validated through the investigation of the spatio-temporal features of drought occurrences over Sicily, Italy, for the period 1921-2005.