Future drought episodes over the North of Italy

Droughts are a normal and recurrent climate feature, but they have the greatest consequences, compared to all other natural hazards and can produce severe effects on natural and socioeconomic systems. The northern part of Italy (Po Valley) is historically rich in water resources, and one of the most fertile and productive agricultural areas. Recently drought events increased affecting the hydrological behaviour of the Po river. In fact, since the turn of the century, an increase of the prolongation of the Po river lean period was recorded. In this work near future (2021-2050) and far future (2071-2100) weekly drought events for the Po Plain region, based on 12 daily EURO-CORDEX and MED-CORDEX Regional Climate Models (RCP 4.8 and RCP 8.5) at 12 km² resolution were identified and mapped. First, the model validation was performed, 10 daily precipitation and temperature series (equally distributed in the plain/hill and mountain sector) were extracted and compared with the corresponding quality-controlled and homogenised gridded data obtained from ground stations. The statistical comparison between reference and candidate series for the control period (1971-2000) was performed using the Co.Temp software for temperature and Co.Rain software for precipitation. Then, on the more reliable models, the bias in the annual mean maximum and minimum temperatures between reference and Euro- and Med-CORDEX simulations was estimated. For precipitation, the ratio between reference and simulations was used. Subsequently, daily values were converted to weekly means and the reference evapotranspiration (ETo) was estimated by means of the Hargreaves ETo equation. Severe and extreme drought episodes were detected by mean of two drought indices (SPI and SPEI) calculated at 12-, 24- and 36- month time scales. Trends were analysed, and the main future drought events were characterised, identifying duration, magnitude and extent. For both RCP scenarios, the results have indicated an intensification of droughts in northern Italy for the period 2021-2050, with the Alpine chain being heavily affected by an increase of drought severity and duration. A North-to-South spatial gradient of drought duration was observed for the far future. This study indicates that at least two different triggering factors influence the characteristics of drought events, and it highlights the importance of using multiple drought indicators.