Assessment of the drought hazard in the Tiber River Basin in Central Italy and a comparison of new and commonly used meteorological indicators

Drought is one of the most common natural hazards with adverse impacts on agriculture and the water resources. This study aims to spatially analyse the drought hazard in the Upper Tiber River basin and find a representative indicator on the basis of meteorological data which are widely available. To this end, the significance of using solely precipitation versus including evapotranspiration (ET) in drought characterisation is thoroughly investigated. Three relevant indicators are considered: 1) a new index SP*ETI (Standardized effective Precipitation EvapoTranspiration Index) incorporating, besides ET, the losses due to runoff; 2) the commonly used SPI (Standardized Precipitation Index) and 3) the RDI (Drought Reconnaissance Index). A comparison is undertaken at various time scales (9, 12, 24-months) using precipitation and temperature data from 2 stations for the period 1953-2011, for which complete rainfall and temperature timeseries are available. This analysis demonstrates: i) the very similar evolution and behaviour of the 3 indexes; ii) the reliability of the SPI for drought monitoring and characterisation in the case of the Upper Tiber River basin, also using observed hydrological impacts as well. Based on these findings, a longer data set of available precipitation data (45 stations, 96 years long timeseries from 1916-2011) is used to calculate the SPI12, and to derive four new sub-indicators 2 reflecting the intensity, magnitude, duration and frequency of drought events. These sub-indicators, once classified, are blended into a Drought Hazard Index (DHI), thus providing a more holistic characterization of the drought hazard on a scale 1-4. A spatial analysis is finally performed across the resulting DHI values in order to investigate the spatial variability of drought hazard and identify drought prone areas. It is found that the most vulnerable areas are located in the southern and eastern part of the Upper Tiber River basin, while the north-central part is less affected by drought conditions.