Detecting Structural Changes in Regional Drought Characteristics via Non-parametric Control Charts

Monitoring drought requires methodologies capable of capturing changes not only in severity, but also in duration and frequency. In this study, we propose a non-parametric monitoring framework based on time-between-event and amplitude control charts to detect changes in regional drought characteristics. The analysis focuses on the Emilia-Romagna region (Italy) and is based on a weighted regional standardized precipitation evapotranspiration index. Two complementary approaches are proposed: an exponentially weighted moving average control chart and a change-point control chart based on the Kolmogorov-Smirnov statistic. The results reveal coherent signals of structural changes in drought behavior, including an increase in the frequency and severity of drought episodes since the early 2000s.