Comparative Analysis of SPEI and WEI+ Indices: Drought and Water Scarcity in the Umbria Region, Central Italy

The purpose of this study is to assess the possibility of relating two phenomena: first, meteorological drought, which is exclusively dependent on climate; second, water scarcity and its uses, which are predominantly anthropogenic in nature. Sometimes these phenomena may overlap, with the former amplifying the latter, but direct correlation is not always highlightable due to the anthropogenic character of water shortage and the variability of water supply sources. In the literature, many papers evaluate these two phenomena separately: in particular, the SPEI (Standardized Precipitation-Evapotranspiration Index) is widely used for detecting meteorological drought, while the link between water shortage and its uses is assessed through an index of water resource exploitation, WEI+ (Water Exploitation Index Plus), which is based on the calculation of an anthropogenic factor, withdrawals net of restitutions. Specifically, this study examines the SPEI and WEI+, respectively, calculated for the July–August–September quarter (SPEI3 sept) and during the low-flow period (WEI+EF low flow), according to the environmental flow constraint. These periods are considered seasonally overlapping in the study area of the Umbria region. The results analyzed by spatial method show the more critical areas, where SPEI3 sept and WEI+EF overlap their critical values, respectively, <−1.0 and >100%. The proposed methodological approach provides stakeholders in the water sector with essential information to adopt a proactive approach to drought phenomena.