Spatial and temporal changes in dry spells in a Mediterranean area: Tuscany (central Italy), 1955-2017

Global change affects precipitation and small modifications in rainfall regimes could compromise water availability, especially in areas characterized by a strong seasonality and a climate ranging from dry sub-humid to semi-arid. Drought, in particular, is a creeping phenomenon defined as an important temporary decrease in water availability below the expected value in a certain period and a specific area. Among the different methodologies for analysing drought periods, one of the most used is the evaluation of dry spells. The aim of this study is to investigate spatial and temporal changes in dry spells in Tuscany during the period 1955-2017, by using daily precipitation data. Changes in the number of dry days, maximum duration and 75th percentile of the duration of the dry spells were investigated. Three precipitation thresholds were selected: 1, 5 and 10 mm. The analyses were carried out at annual, September-May, September-November and December-May level. Theil-Sen nonparametric method was used to estimate the slope; Mann-Kendall test and a modification of it, which applies a trend-free pre-whitening (TFPW) method to the series having a serial correlation, were applied to look for statistically significant trends. The drought indicators showed increasing trends (some significant) especially at December-May and to a lesser extent at the annual and September-May level; the most pronounced increases were found in the southernmost areas of Tuscany. During September-November period, on the other hand, a prevalence of decreasing trends (rarely significant), with less pronounced decreases in the south of the region, were observed. Mediterranean area is considered a climate change "hot-spot" from the viewpoint of vulnerability. An increase in long-lasting dry spells, especially in southern Tuscany, can exacerbate the problems linked to water management and demand.