A weekly spatio-temporal distribution of drought events over the Po Plain (North Italy) in the last five decades

The aim of this research is to identify and characterize, in terms of length, intensity, and spatial propagation, the main drought events which took place in the Po Valley (Italy) from 1965 to 2017. Two drought indices were applied, the Standardized Precipitation Evapotranspiration Index (SPEI) and Standardized Precipitation Index (SPI). Daily precipitation and temperature series belonging to the National System for the Collection, Processing, and Dissemination of Climatic Data of Environmental Interest (SCIA) database were collected. Subsequent to an accurate quality control, the converted weekly climatic values were spatialized on a 20 × 20 km cell grid, and for each index, weekly severe and extreme drought episodes at a 12-, 24-, and 36-month time scale were calculated. Results showed that the application of two indices is fundamental in the study of drought episodes, and that different triggering factors act over time. Especially, since the 2001 drought, episodes have become stronger in terms of frequency and length, and they seem to be mostly related to changes in the intra-annual precipitation distribution. An analysis of the spatial propagation also indicates that two spatial gradients follow each other during the analysed period.