Droughts are common naturalphenomena that arise from a substantial deficit of precipitation and can takeplace around any corner of the world, in different climatic regions (Zargar, et al.,2011). The deficits adversely affect both surface and groundwater resources andlead to decrease in water supply, poor water quality, declined agriculturalproduction, reduced hydro-power generation, distressed riparian and wetlandhabitations, and so on (Mpelasoka,et al., 2008).In 1997, drought has beenclassified into four types by The American Meteorological Society;meteorological or climatological, agricultural, hydrological, and socioeconomic(Mpelasoka,et al., 2008; R.
& Junior, 2002). The impacts of droughts are one ofthe most significant among all the natural hazards because it is not onlycapable of affecting various economic sectors of a nation, but also affectingthe lives and livelihood of people. The agriculture, food security, industry,human health, animal health and livelihood security of a region can be severelydamaged by the occurrence of a drought event (Svoboda& Fuchs, 2017). Considering theimpacts and frequency of drought, it is essential to evaluate drought severity,but the accurate assessment of drought is not easy (Keyantash& Dracup, 2002). The extent to which a drought canaffect a particular region depend on a various factors, such as thesocioeconomic condition, geographic location and so on (Svoboda & Fuchs, 2017; R.
& Junior, 2002). Hence,the type of impact relevant to a specific region bears significantconsideration to decide what indices to select.Drought indices can be defined asquantifiable measures that evaluates drought by integrating data from one orseveral variables (Zargar, et al., 2011). More than 100 drought indices have so farbeen proposed which quantifies severity levels andduration of drought (Zargar, et al.
, 2011, Keyantash &Dracup, 2002). Several indices that can be used to evaluate droughts are, theStandardized Precipitation Index (SPI), Standardized Runoff Index (SRI), StandardizedPrecipitation Evapotranspiration Index (SPEI), Soil Moisture Index (SMI),Reconnaissance Drought Index (RDI) and Palmer Drought Severity Index (PDSI).Among these, SPI and PDSI are the most commonly practiced indices (Tsakiris & Vangelis, 2005). The droughtindices are generally continuous functions of some hydro-meteorological variables,such as; rainfall, potential evaporation and temperature (Mpelasoka,et al., 2008).These indices correspond to different forms of drought, includingmeteorological, agricultural and hydrological drought and aid in a variety of operations, including drought earlywarning and monitoring and contingency planning (Zargar, et al., 2011). Thusdrought indices can work as a toolto track droughts.
But the choice of an index on a combination of differentindices depends on the characteristics of the specific region and the impacts of drought to the stakeholders (Svoboda & Fuchs, 2017).Determination of the best fit is quite a lengthyprocess which arises the dilemma while selecting which index to use (Svoboda & Fuchs, 2017). Hence, theindices that are reviewed in this paper are considered to provide options ofdifferent indices that can be best suited for river basin areas with differentclimatic and geographical characteristics. Thepurpose of this paper is to look into some of the most widely applied drought indicesthat are being used across drought-prone river basins regions around the globeand to come up with applicable indices. Basin Specific Climate, Geography andDrought-ImpactsThe papers that have beenselected to review in this article are based on 10 different river basinregions of 6 different countries.
In this section the climatic and geographicalfeatures of these basin areas are briefly discussed along with the impacts ofdrought in the respective regions. This section will help to provide the ideaof climate, geography and impact wise suitability of indices. ChinaTheriver basins that are studied from China are located in the subtropical climateregion of the country (Fluixá et al., 2017; Li, Q. et al., 2015).
The Jinsha Riverbasin is a watershed with the area of about 1, 32, 000 sq. km, ranging inelevation from 263m to 5910 m from sea level (Yang et al., 2013).
The average annual rainfall of this basin rangesfrom 239mm in dry years to1154.9mm in wet years (Meng et al.,2012). The Huaihe river basin expands in the east of the country with the basinarea of 270,000 sq.
km and has an average annual precipitation of883mm (Mingkai & Kai, 2017). In China, droughts areconsidered to be the most severe natural threat for socioeconomic growth andecosystems because, a wide range of areas are affected by droughts that resultsin great agricultural losses (Fluixá et al., 2017). In the southernpart of China a number of significant drought event took place during the lastdecade and resulted in crop failure, shortage of pure drinking water,degradation of ecosystem, health issues, and even deaths (Fluixá et al.
,2017; Li, Q. et al., 2015).IndiaThe Ken andGhataprabha river basins of India consist of a geographical area of 28,692 sq.km and 7231 sq. km respectively (Pathak et al., 2016;Jain et al.
, 2012). The ken river flows within the highestaltitude of 550m and the lowest to be 87m above the sea level (Jain et al., 2012).
The subtropical monsoonclimate is the climatic feature of the Ghataprabha river basin, which has 4monsoon months followed by 8 dry months. The average annual rainfall is around1248mm (Pathak et al., 2016).
On the other hand, the Ken river basin is located in asemi-arid to sub-humid climatic zone of India, where the mean annual rainfallranges from 800mm to 1250mm (Jain et al.,2012). The major effects of drought around these regions are:threat to the food security and increase rate of human mortality (Nath et al.,2017). ThailandThe Sakae Krang and the Chi river basinsof Thailand are located in a tropical savanna climatic region. The area of SakaeKrang river basin and Chi river basin is 5191 sq. km and 49,476 sq. kmrespectively (Homdee et al.
, 2016; Wichitarapongsakun etal., 2016). The averageprecipitation over the considerable small area of Sakae Krang river basin isaround 1000 mm and Chi River basin is approximately 1150 mm per year(Homdee et al.
, 2016; Wichitarapongsakun etal., 2016). Drought hits the basin areas annually resulting insevere impact on crop production, human health, environment and socioeconomicconditions (Wichitarapongsakun et al., 2016)EthiopiaThe Awash river basin of Ethiopia have atemperate climate and consist of an area about 701,000 sq. km. (Anon., 1965). The highest pointof this basin area is 3000m above the sea level whereas the lowest is at 250m (Anon.
, 1965). This river basinhas a mean rainfall of 710mm per year (Edossa et al., 2009). In 1888 a historicdeadly famine took place as an impact of drought when 30% of the populationdied and 90% of the animal perished (Edossa et al.
, 2009). Moreover, loss of assetsin the form of crops, livestock, and other productive capitals takes place asimpacts of drought (Edossa et al.,2009).
ZambiaThe Kafue river catchment is located underthe humid-subtropical region of Zambia where the average annual precipitationranges from 800mm to 1300mm (Anon., 2007).The area of this river basin is 155,000 sq. km (Anon., 2007).This basin is the mostdeveloped region of Zambia’s where 50% of the total population and a variety ofanimal species live (Lweendo et al., 2017).Majority of the industrial, municipal water supply, agriculture and miningactivities takes place around this river basin (Lweendo et al.
, 2017). An event of a severe drought can affectthe whole system of these aspects of this basin.BangladeshOnly7% of the whole GBM basin lies inside Bangladesh and about two-third of thecountry is comprised by this basin (Banglapedia, 2014). The climatic zone ofthis river basin area falls under is subtropical climate. The total area ofthis basin is 1,761,300 sq. km among which 123,291 sq.
km falls under theterritory of Bangladesh (Kattelus et al.,2015). The northern region of the country faces frequent incidents of droughtas a result of which crop production gets affected, malnutrition takes place(Rakib et al., 2015).Indices Used for River Basin AreasThissection covers the different indices that have been applied to the consideredriver basins. The purpose is to try finding out which index or indices havebeen the best fit under what conditions and why.About the IndicesThe use of StandardizedPrecipitation Index (SPI) has been reviewed to be predominant as a droughtindex for river basin areas (Fluixáet al.
, 2017; Lweendo et al.,2017; Nury& Hasan, 2016; Homdee et al.,2016; Wichitarapongsakun et al., 2016; Jainet al.
, 2015; Li, Q. et al., 2015; Rafiuddin et al., 2011; Edossa et al., 2009).
The SPI is anindex for drought severity level analysis. Its calculation is based on rainfalldata at various periods of 1 month to 24 months or longer, over a period of 30or more years of time (Svoboda & Fuchs,2017). In a couple of study based on two different river basins ofThailand; the Krang river basin and Chi river basin; the drought severity levelwas determined by SPI (Homdee et al.,2016; Wichitarapongsakun et al.
, 2016). The drought conditions of Awash River Basin of Ethiopia and Upper KafueRiver Basin of Zambia have also been analyzed, primarily by the use of SPI (Lweendo et al.,2017; Edossa et al., 2009).
Acouple of studies have been conducted for drought detection and assessment onthe Jinsha river basin and Huaihe river basin of China (Fluixá et al., 2017; Li, Q. et al.,2015). Both the studies used SPI drought index onthe purpose of formulating new indices; MDI, and indicators; ODE and ODI (Fluixá et al., 2017; Li, Q.
et al.,2015).Ganges-Brahmaputra river basin of Bangladesh has also been brought underconsideration to diagnose the drought scenario across the country (Nury , 2016; Rafiuddin, M. et al., 2011). According to the outcomes, SPI turned out tobe a consistent drought indicator in the context of Bangladesh (Nury , 2016; Rafiuddin, M. et al.
, 2011). No other indices were found to be popularlyused to study drought conditions in Bangladesh. A comparison study of differentdrought indices was conducted based on Ken river basin of central India whereSPI was one of the considered indices (Jainet al., 2015).Inthe study of the Upper Kafue River Basin of Zambia a number of standardizeddrought indices were selected to evaluate different types of droughts alongwith SPI (Lweendo et al.
, 2017). Thetechnique that is used to compute SPI, was extended to calculate SRI and SMIwhere both of the indices use the process of fitting a probability distributionto data and converting it to normal distribution (Lweendo et al., 2017). Similar to SPI, they are also quantified on multipletime scales.
SRI uses runoff time series as input data and both the input datafor SRI and SMI computation was derived from the hydrological SWAT model (Shukla & Wood, 2008). The agriculturaldrought index, SMI was compared to historical droughts to validate itsefficiency and its performance turned out to be fair in terms of capturing themajor agricultural droughts but it was less sensitive than the other indices (Lweendoet al., 2017). It is evident that all drought indices demonstratedcomparable and consistent results, even though they were calculated based ondifferent input parameters. The paper also drew the conclusionthat, all of the indices could successfully detect temporal variability ofmajor drought events in this humid subtropical basin of Upper Kafue River (Lweendoet al., 2017).InIndia, a couple of studies were conducted based on the application of severaldrought indices in Ken river basin and Ghataprabha river basin to measure theirapplicability (Pathak et al.
, 2016; Jain et al.,2015). Both the studies were focused on drawing comparison among anumber of drought indices including SPI, SDI, SRI, EDI, CZI, RD, and RDDI (Pathak et al.,2016; Jain et al.
, 2015). Inthe study of Ghataprabha basin the suitability of SDI and SRI to assesshydrological droughts was compared and both indices were proven to beappropriate for the area (Pathak et al., 2016). According to thestudy on Ken basin, the comparatively new index called Effective Drought Index (EDI)was found to be more responsive and showed a better performance (Jain et al.,2015). SPEIwas applied to study the scenario of hydrological drought in the studies ofUpper Kafue river basin of Zambia and Chi river basin of Thailand (Lweendo et al., 2017; Homdee et al.
, 2016). The SPEI considers thedifference between precipitation and potential evapotranspiration (PET) (Homdeeet al., 2016). Formulationof new indicators and a drought index took place in the studies based on theJinsha river basin and Huaihe river basin of China (Fluixá et al., 2017; Li, Q.
et al.,2015).A new Multivariate Drought Index (MDI) was developed in the study of Huaiheriver basin and was being compared to SPI and PDSI; as a result of which, MDIturned out to have certain benefits over SPI and PDSI in drought evolutionmonitoring (Li, Q. et al.
, 2015).On the other hand, two new indicators; Overall Drought Extension (ODE) andOverall Drought Intensity; were developed during the study of Jinsha riverbasin using the outputs from several meteorological drought indices, such as,SPI, RAI, PN and DEC (Fluixáet al., 2017).
The newly developed indicators and index showed a decent agreement with therecords of historical drought events in both the river basins of China (Fluixá et al., 2017; Li, Q. et al.,2015).Whys and Wherefores SPI has been most commonly used by the aforementionedstudies on the purpose of understanding the drought condition of river basins. The simplicity and versatility of this index has madeit more popular among the researchers. One of the benefits of SPI is that it requiresonly one input variable that is precipitation data (Svoboda & Fuchs, 2017).
On the purpose of analyzing meteorological droughts, SPI has been recommendedby the World Meteorological Organization to be used all across the world(Lweendo et al., 2017). SPI is consideredto be a versatile index because its calculation can be done on any timescaleand it is appropriate for both agricultural and hydrological applications.
Thestudies chose to work with this index because of its application to monitor thedynamics of a drought, including its development and decline (Lweendo et al.,2017; Nury & Hasan, 2016; Wichitarapongsakunet al., 2016; Rafiuddin et al., 2011; Edossa et al., 2009). Moreover,SPI is not negatively affected by topography and can work as an input to watermanagement planning, risk management and decision analysis. However, theproblem with using SPI is that, it requires continuous data of at least 30years and cannot accommodate missing data (Jainet al.
, 2015).Acouple of the abovementioned studies have applied other indices apart from SPI.SPEI, SRI, and SMI has been successfully used in the study based on Upper KafueRiver Basin of Zambia. Newly developed index MDI turned out to be a great fitfor the study of drought condition in river basins as well. That means, alongwith SPI; SPEI, SMI, SRI and MDI can be considered to be well suited toevaluate drought condition around river basin areas. In one of the studies, thePDSI index failed to monitor a number of typical drought events (Li, Q.
et al., 2015). As a result, this index can beconsidered to be less applicable for river basins. Even though PDSI is a widelyused index, its nine-month time scale results into a lag in determining droughtconditions (Svoboda & Fuchs, 2017).
Because of this lag, rapidly emerging droughts can be left unidentified whichmakes it less suitable for particular regions. SPEIis suitable for drought assessment around river basins because it offers versatility as SPI and uses monthlyprecipitation and temperature data as input which allows this index toacknowledge the impact of temperature on a drought event (Svoboda & Fuchs, 2017). The issue that makes it less popular that SPI is itsrequirement of a serially complete record of data without any missing months. Moreover,this index failed to representextreme drought events in the Chi river basin of Thailand during 1993–1994 andcould only detect a slightly dry period (Homdee et al., 2016).
TheMDI performed the best because it evaluates droughts by using a simple tacticwhich considers multiple easily obtainable hydrological parameters, such as;precipitation, evapotranspiration, soil moisture and runoff (Li, Q. et al.,2015). The feasibility of MDI to be applied on river basin droughtcondition is up to the mark because, the calculation is done based on localclimate and the each parameter is weighed by the hydrological conditions of thestudy area (Li, Q. et al., 2015).
Among the considered variables, thecalculation of evapotranspiration is considered to be a difficult one. Thisvariable along with soil moisture and runoff, were simulated using theXin’anjiang model (Li, Q. et al., 2015)The study of Ken river basin successfully used ChinaZ-Index (CZI) and Rainfall Departure (RD) and suggested these as simpleapplicable indicators of drought conditions for a given time over specificregions (Jain et al.
, 2015). A straightforward measure of rainfall deviation is used by this index from its long-termmean or median based on the regional weather conditions. In general, thistechnique is appropriate for defining local weather condition for rainfall deficienciesof smaller duration (Jain et al.,2015). It has been reported that CZI and SPI provide similar results, hence,CZI can be preferred over SPI for its ease of use (Jain et al.
, 2015). EDIcan sometimes perform better than other indices such as, SPI, RD etc. becauseit is free from the time scale problem, effective for both long and shortdrought and could identify drought earlier than the other considered indices, (Jain et al.,2015). On the other hand, RDDI showed the poorest performance comparedto others (Jain et al., 2015).