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Using the SPI to Identify DroughtThis article was written in response to the recentanalysis of drought in Turkey by Komuscu (1999).The study showed the relationship between droughtduration, drought frequency, and drought time scaleusing the Standardized Precipitation Index (SPI):whereSPI = (X ik- X i)ó ió i= standardized deviation for the ith stationX ik= precipitation for the ith station and kth observationX i= mean precipitation for the ith stationThe index has the advantages of being easilycalculated, having modest data requirements, andbeing independent of the magnitude of mean rainfalland hence comparable over a range of climatic zones.It does, however, assume the data are normally distributed,and this can introduce complications forshorter time periods. Komuscu claims that the SPI hasnot been widely applied or tested and employs thedrought classes suggested by McKee et al. (1995),reproduced here in Table 1.The SPI is of course the same as the StandardizedRainfall Anomaly, defined by Jones and Hulme(1996) and widely used in the analysis of desiccationin drylands. Figure 1 shows a typical example of suchuse, depicting the widely reported downward trend inSahelian rainfalls (only continental Sahelian stationsare employed after the suggestions of Ba et al. [1995],Janicot et al. [1998], and Nicholson and Palao [1993]that other parts of West Africa belong to a differentclimate regime). Komuscu’s assertion that the SPI isunderused for drought assessment appears to becorrect, in that it is the persistence of the negativeanomalies that receives most attention rather than anexamination of their intensity or impact (for example,see Hulme, 1992). That is, rainfall anomalies are usedto investigate desiccation rather than drought (seeAgnew, 1995, for further discussion). The purpose ofthis paper is then to question the values assigned to theSPI for drought classes and to suggest alternative,more rational thresholds. The effect of using differentdrought classes is investigated using annual rainfallsfrom the Sahelian region of West Africa, and theSPI Probability Komuscu (1999) and Proposed newof occurrence McKee et al. (1995) drought classesdrought classesLess than -2.00 0.023 Extreme droughtLess than -1.65 0.050 Extreme droughtLess than -1.50 0.067 Severe droughtLess than -1.28 0.100 Severe droughtLess than -1.00 0.159 Moderate droughtLess than -0.84 0.201 Moderate droughtLess than -0.50 0.309 No droughtLess than 0.00 0.500 Mild drought No droughtTable 1. Probabilities for different standardized rainfall anomalies.6 Vol. 12, No. 1, Winter 1999–Spring 2000
21.5Standardized Rainfall Anomaly10.50-0.5-1-1.5-21930 1940 1950 1960 1970 1980 1990Figure 1. Standardized annual precipitation anomalies for the continental Sahel (Burkina Faso, Mali, and Niger), using the1961–90 base period.problem of changing the base averaging periods ispresented.What is Meteorological Drought?This question has been addressed time and again(Agnew and Anderson, 1992; Wilhite, 1993), and ithas often been stated that no universal definition ofdrought exists. There is little to be gained by reproducinga long list of conflicting definitions that merelyillustrate the diverse interests of those who investigatedrought. Most definitions anyway can be resolvedinto a generic statement that drought is caused by animbalance between water supply and demand. Hencedrought can be defined in terms of both supplyreduction and demand increase, and there are numerousdefinitions of hydrological, agricultural, ecological,and economic drought that demonstrate this.Many, however, agree with Palmer (1965) and Beranand Rodier (1985) that drought is essentially a meteorologicalphenomenon. The analysis below adoptsthis perspective—that examining the occurrence ofmeteorological drought is the most fundamental requirementof any investigation.The second premise of this account is that droughtis an abnormal occurrence. This is an equally importantpoint and it is the reason why it is suggested thatTable 1 should not be used without modification fordrought analysis. In McKee’s classification (McKeeet al., 1995), all negative indexes (SPI) are taken toindicate the occurrence of drought; this means for50% of the time, drought is occurring. This is clearlynonsense! It also raises the notion of “persistentDrought Network News7
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