Case Study Assessment of the Drought Hazard in the Tiber River Basin in Central Italy and a Comparison of New and Commonly Used Meteorological Indicators Pamela Maccioni 1 ; Maggie Kossida 2 ; Luca Brocca 3 ; and Tommaso Moramarco, M.ASCE 4 Abstract: Drought is one of the most common natural hazards with adverse effects on agriculture and the water resources. This study aims to spatially analyze the drought hazard in the Upper Tiber River basin and find a representative indicator on the basis of meteorological data that are widely available. To this end, the significance of using solely precipitation versus including evapotranspiration (ET) in drought characterization is thoroughly investigated. Three relevant indicators are considered: (1) a new index Standardized Effective Precipitation EvapoTranspiration Index (SPETI) incorporating, besides ET, the losses due to runoff; (2) the commonly used Standardized Precipitation Index (SPI); and (3) the Reconnaissance Drought Index (RDI). A comparison is undertaken at various timescales (9, 12, and 24 months) using precipitation and temperature data from two stations for the period 1953–2011, for which complete rainfall and temperature time series are available. This analysis demonstrates (1) the very similar evolution and behavior of the three indexes and (2) the reliability of the SPI for drought monitoring and characterization in the case of the Upper Tiber River basin, also using observed hydrological effects. Based on these findings, a longer data set of available precipitation data (45 stations, 96-year-long time series from 1916 to 2011) is used to calculate the SPI 12 and to derive four new subindicators reflecting the intensity, magnitude, duration, and frequency of drought events. These subindicators, once classified, are blended into a Drought Hazard Index (DHI), thus providing a more holistic characterization of the drought hazard on a scale of 1 to 4. A spatial analysis is finally performed across the resulting DHI values in order to investigate the spatial variability of a 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, and the north-central part is less affected by drought conditions. DOI: 10.1061/(ASCE)HE.1943-5584.0001094. © 2014 American Society of Civil Engineers. Author keywords: Drought; Standardized precipitation index; Trend analysis; Water resource management; Evapotranspiration. Introduction Drought occurs when precipitation is significantly lower than aver- age and differs from other natural hazards because its onset, extent, and offset are difficult to identify; it develops slowly, and its effects may persist for years after the event (Ali 1999). Different defini- tions of drought are available in the literature depending on the du- ration of the phenomenon, its spatial extent, and its effect on human activities. In particular, there are four conditions that are generally referred to as drought (Hughes and Saunders 2002): (1) “agricul- tural, ” in which soil moisture isn’t enough to support average crop production, (2) “meteorological, ” with a prolonged deficit of precipitation, (3) “hydrologic, ” for below-normal streamflows and lake and groundwater levels, and (4) “socio-economic, ” when the water demand for an economic good exceeds the water supply as a result of a weather-related shortage. By now, droughts are relatively frequent, and the entire Mediterranean area has suffered greatly in recent years (Kossida et al. 2012; Cacciamani et al. 2007). The Intergovernmental Panel on Climate Change (Solomon et al. 2007) predicts that the Mediterranean region will suffer from a reduction in water resource availability in the coming years. Thus, appropriate tools to develop an in-depth understanding of drought triggering and effects and appropriate mitigation strategies are necessary. Among the developed techniques for drought analysis and monitoring, unbiased indexes should be widely used, but the subjectivity in the definition of drought has made it very difficult to establish a unique and universal drought index (Heim 2002; Vincente-Serrano et al. 2010a). Different indexes were developed during the last few decades for drought quantification, monitoring, and analysis (Pisani et al. 1998; Heim 2002; Keyantash and Dracup 2002). The Palmer Drought Severity Index (PDSI) (Palmer 1965), based on a soil-water balance equation, is one of the most widely used. Nevertheless, the PDSI has several deficiencies (Vincente-Serrano et al. 2010a), mainly because of the parameters involved in the water-balance equation that significantly depend on the calibration period. Thus, Wells et al. (2004) proposed the self-calibrated PDSI (sc-PDSI), which still presents some issues in adaptation to the intrinsicly multiscalar nature of drought. McKee et al. (1993) illustrated that the timescale over which water deficits accumulate is very important and represents the differentiating factor between hydrological, meteorological, agricultural, and other drought types. 1 Engineering Fellow, National Research Council, Research Institute for Geo-Hydrological Protection (IRPI), Via Madonna Alta 126, 06128 Perugia, Italy (corresponding author). E-mail: pamela.maccioni@irpi.cnr.it 2 Researcher, National Technical Univ. of Athens, Athens, Greece. E-mail: mkossida@chi.civil.ntua.gr 3 Researcher, National Research Council, Research Institute for Geo- Hydrological Protection (IRPI), Via Madonna Alta 126, 06128 Perugia, Italy. E-mail: luca.brocca@irpi.cnr.it 4 Researcher, National Research Council, Research Institute for Geo- Hydrological Protection (IRPI), Via Madonna Alta 126, 06128 Perugia, Italy. E-mail: tommaso.moramarco@irpi.cnr.it Note. This manuscript was submitted on March 20, 2014; approved on August 27, 2014; published online on October 23, 2014. Discussion period open until March 23, 2015; separate discussions must be submitted for in- dividual papers. This paper is part of the Journal of Hydrologic Engineer- ing, © ASCE, ISSN 1084-0699/05014029(11)/$25.00. © ASCE 05014029-1 J. Hydrol. Eng. J. Hydrol. Eng.