Assessment and Monitoring of Droughts in Southeastern Europe: A Review - page 01

Definition and Classification of Drought

Due to the complexity of the phenomenon of drought, the differences in hydrologic, meteorological and socioeconomic variables around the world, there is no universally accepted definition of drought (Mishra and Singh, 2010). Yevjevich (1967) stated that widely diverse views of drought definitions are one of the principal obstacles to investigations of droughts. Nevertheless, some of the more commonly used definitions are noted here.

Some definitions of drought include: 1) "drought means a sustained, extended deficiency in precipitation" (WMO, 1986), 2) "drought means the naturally occurring phenomenon that exists when precipitation has been significantly below normal recorded levels, causing serious hydrological imbalances that adversely affect land resource production systems" (UN Secretariat General, 1994), 3) "drought is defined as the percentage of years when crops fail from the lack of moisture" (FAO, 1983), 4) "drought is an extended period – a season, a year, or several years – of deficient rainfall relative to the statistical multi-year mean for a region" (Schneider,1996). Drought definitions vary based on the variable that is used to describe the drought, therefore it can be useful to classify definitions of drought.

Droughts are generally classified into four categories (Wilhite and Glantz, 1985; American Meteorological Society, 2004). Meteorological drought is defined as a lack of precipitation over a region for a period of time. Hydrological drought is related to a period with inadequate surface and subsurface water resources for established water uses of a given water resources management system. Agricultural drought refers to a period with declining soil moisture and consequent crop failure without any reference to surface water resources. The socioeconomic definition of drought associates the supply and demand of some economic good with elements of meteorological, hydrological, and agricultural drought. Another, fifth, category of drought has been introduced as groundwater droughts (van Lanen and Peters, 2000), and deals with how groundwater recharge, levels and discharge are affected by droughts.

Drought Impacts in See

Drought in Southeastern Europe is a hazard with a wide range of transboundary, environmental and socio-economic impacts on various sectors including agriculture, energy production, public water supply and water quality. The vulnerability to drought impacts in South East Europe (SEE) is higher in comparison to neighboring regions. Ineffective water management is seen as the fundamental problem and can induce additional problems when drought occurs.

It is expected that drought will become one of the dominant factors limiting the agriculture production in southern and eastern Europe (Olesen et al, 2009).Annual river flow is projected to decrease in southern and south-eastern Europe (Milly et al., 2005). In summer and autumn, river flows are projected to decrease in most of Europe (Dankers and Feyen, 2008), especially in southern and southeastern Europe, in line with the predicted increase in the frequency and severity of drought in this region.

Within the Danube River Basin, drought and low flow events, as well as water scarcity situations, are likely to become more intense, longer and more frequent (ICPDR, 2013). The frequency could increase especially for moderate and severe events. Due to less precipitation in summer (Figure 1), these extreme events will occur more frequently in summer than in winter.

In some parts of the Danube River Basin, the drought risk is expected to increase drastically in the future, leading to possible economic loss, potential for water conflicts and water use restrictions. The Carpathian Area, particularly the southern parts of Hungary and Romania, as well as the Republic of Serbia, Bulgaria and the region of the Danube Delta, are likely to face severe droughts and water stress resulting in water shortages. In alpine areas (e.g. some parts of Austria), no clear trend or even a slight improvement of the mean annual low flow and drought situations were identified (Mauser and Prasch, 2012). Therefore, alpine head watersheds remain important for downstream areas during drought periods. The future low flow situation also depends on changes in water use, which could worsen or improve the general trend.


Figure 1: Increase of mean annual and summer temperature in the Danube River basin for 2021-2050 and 2071-2100 for A1B scenarios according to different model results (ICPDR, 2013).


Drought Indices

In order to compare measures of drought from region to region, and to compare past drought events, a numerical standard is needed. A drought index value is typically a single number, far more useful than raw data for decision making. Because of the complexity of drought, no single index has been able to adequately capture the intensity and severity of drought and its potential impacts on the wide range of economic sectors on which it has an impact. The World Meteorological Organization defines a drought index as "an index which is related to some of the cumulative effects of a prolonged and abnormal moisture deficiency" (WMO, 1992).

The percent of normal precipitation is a meteorological drought index that describes the drought as the precipitation deviation from the normal (average) (Zargar et al, 2011).

The normal usually corresponds to the mean of the past 30 years. Percent of normal is calculated by dividing a given precipitation by the normal. The time scale of the analysis can vary from a single month to a year. The main advantage of this index is its simplicity and transparency, which makes it favorable for communicating drought levels to the public (Keyantash and Dracup 2002), The Standardized Precipitation Index, or SPI(McKee et al. 1993), is an index based on the probability of precipitation for any time scale. The SPI can be computed for different time scales, can provide early warning of drought and help assess drought severity, and is less complex than the Palmer.

The Palmer drought severity index (PDSI), sometimes called the Palmer drought index, is a measurement of dryness based on recent precipitation and temperature (Palmer, 1965). The Palmer Drought Index is based on a supply-and-demand model of soil moisture.


Figure 2: Spatial distributions of PaDI in SEE region for several years (Gregorič, 2012).


PAI (Palfai's Aridity Index) (Palfai, 1990) is a relative indicator, which characterizes the drought with one numerical value in connection with one agricultural year. It expresses the evaporation (temperature) and precipitation relations (the last one with time-varying water demand of plants), and is in consideration of groundwater level state. Over the course of the DMCSEE project, a new drought index was developed based on the PAI to allow usage in the Southeastern Europe region and meet data availability demands. The modified index was named Palfai's Drought Index (PaDI), and expresses the strength of a drought for an agricultural year (Palfai and Herceg, 2012). Figure 2 shows the resulting map of the spatial distribution of PaDI in SEE for several years.