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Due to the lack of a general drought definition, water users and managers have developed and used different indices. Many studies using drought indices have been made so as to detect drought events or just to compare their results and assess their advantages and disadvantages. In Portugal, these studies have been done for common drought indices; however, an integrated evaluation and comparison using recent data is needed. Therefore, this study is intended to give an updated overview of the behaviour of the proposed indices. This study proposes the usage of PDSI, scPDSI, SPI and SPEI. With the exception of the PDSI, all indices have been calculated through R packages. The results for the studied regions in mainland Portugal suggest that the drought situations are, in general, most significant and frequent than the wet periods. From our results, we can conclude that the SPI model is more sensitive to extreme drought events and can detect them earlier. The PDSI, scPDSI and SPEI are more reliable for drought monitorization at medium and long spells, which might represent the environmental interactions more closely to the reality. Also, the scPDSI tends to reduce the importance of short period recovering. It is then advisable that impact and scientifical studies consider all of these indices or at least some of them to have a broader and complete understanding of the drought situations to be studied. [ABSTRACT FROM AUTHOR]

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Climate describes how the atmosphere behaves over a long period of time informing us on the "average of weather", as well as the description of other aspects of weather patterns and meteorological parameters' distribution, including anomalous, rare and extreme events. Some examples of extreme weather or climate events include heat waves, cold waves, floods, extreme precipitation, drought, tornadoes and tropical cyclones (WMO-CCL, 2018).Drought is a prolonged dry period in natural climate cycle. It is a slow-onset phenomenon caused by precipitation deficit combined with other predisposing factors. Drought is a natural hazard that occurs in all climates and differs from other extreme events in that it has a slow onset, evolves over months or even years and can affects a large spatial region. Unlike other extreme events, such as floods, which are weather events that are immediately detectable, droughts develop slowly, making it difficult to determine the severity of the event and also the onset and end.Drought is a recurrent climatic event occurring worldwide and intensively perceived by the Mediterranean countries (Pereira et al., 2009). Portugal is one of the countries strongly affected by drought, which creates the strong need for impact reduction strategies as an integral part of drought preparedness plans.To better understand the risk of droughts in mainland Portugal, was used indices that rely on precipitation deficits and allows us to characterize and monitor meteorological drought in mainland Portugal, determining drought onset, duration and intensity. In this study was used the PDSI (Palmer Drought Severity Index), SPI (Standardized Precipitation Index) and SMI (Soil Moisture Index).These indices were computed using daily and monthly data and based on the recommendations of CCL task team on the definition of extreme weather and climate events: Threshold (is determined based on historical values of the index); Temporal (Station-level information on starting date, ending date, and duration of the event); Spatial (calculates the area affected, by providing the distribution of stations where the threshold was surpassed and uses a geographical information system (GIS) to determine the area affected by the event, the magnitude, and severity).In order to characterize an extreme weather and climate event was used the following properties: 1) Magnitude: measures the departure from the threshold and reflects unusualness or extremity of the event; 2) Duration: defined by the time at which the event begin and end; 3) Extent: defined as the geographical area affected by the extreme event; 4) Severity: severity should indicate the potential damages and impacts that can be associated to the event. It is a combination of magnitude and persistence of a drought.It is also aimed the framing of this extreme event with climate changes based on trend analyses and simulation of scenarios with variables such as precipitation, temperature and evapotranspiration. [ABSTRACT FROM AUTHOR]