Some Aspects Of The Spatial And Temporal Characteristics Of Drought In Nigeria A Statistical Approach

Mean rainy season (April-October) precipitation seriesfor fourty one stations in Nigeria for a long period ofyears, are used to derive a Bhalme and Mooley-type droughtindex for the region. Various statistical analysis indicatethat precipitation series are more variable Over theSahelian, eastern Midland and western sub-sectors of theGuinea-Savanna and Coastal zones. The rainfall series forthe Sahelian and northern Midland zones indicate a significanttrend towards increasing aridity, especially in thelast 30 years.Map-pattern correlation method stratifies 46 per centof the eighty two years, and produces comparatively goodresults as the more sophisticated eigenvector method, sixmoisture amomaly patterns, are identified by map-patterncorrelation method indicating distinct differences betweenareas north of 10°N and the rest of the country. Maps ofthe mean seasonal areal coverage of drought reveal remarkablechanges in size and position from year to year.Drought does not cover the entire country and no particularareas were persistently affected until the 1970s and 1980s.Since the beginning of the 1970s areas affected by droughthave increased in extent with a consequent decrease in nearnormal and wet conditions particularly in the Sahelian andthe Midland zones. The 1970s and 1980s were characterizedby exceptionally high frequency of drought conditions with apresence of consecutive years of large-scale droughts since1982, confirming persistence of drought in Nigeria.vAll stations are subjected to Non-Integer method ofspectral analysis and low-pass filter method in order toidentify any trends and regular periodicites in the droughtindex. The low-pass filter results confirm negative trendfor the Sahelian and Midland zones (from the mid 1950s andlate 1960s respectively), indicating increasing aridity.The drought spectrum as indicated by the Non-integer methodis dominated by peaks in the 2.1-7.9 frequency range(Coastal and western Guinea-Savanna zones) and longerfrequencies with peaks between 11.0-11.9 and 13.0-13.9 yearsin the Sahelian and the Coastal zones. Based on the numberof stations with significant quasi-11-year oscillations andtheir spatial distribution a linear relationship betweendrought index fluctuation and 11-year sunspot cycle may notbe realistic, as it is often portrayed in the literature.In general, drought in Nigeria is a complex randomphenomenon, generated by non-linear atmospheric processessporadic in spatial occurrence, making its predictiondifficult.

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