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Ahmadu Bello University (1993)

SOME ASPECTS OF THE SPATIAL AND TEMPORAL CHARACTERISTICS OF DROUGHT IN NIGERIA : A STATISTICAL APPROACH

IWEGBU, IRINA ANDREEVNA

Titre : SOME ASPECTS OF THE SPATIAL AND TEMPORAL CHARACTERISTICS OF DROUGHT IN NIGERIA : A STATISTICAL APPROACH

Auteur : IWEGBU, IRINA ANDREEVNA

Université de soutenance : Ahmadu Bello University

Grade : Doctor of Philosophy (PhD) 1993

Résumé
Mean rainy season (April-October) precipitation series for fourty one stations in Nigeria for a long period of years, are used to derive a Bhalme and Mooley-type drought index for the region. Various statistical analysis indicate that precipitation series are more variable Over the Sahelian, eastern Midland and western sub-sectors of the Guinea-Savanna and Coastal zones. The rainfall series for the Sahelian and northern Midland zones indicate a significant trend towards increasing aridity, especially in the last 30 years. Map-pattern correlation method stratifies 46 per cent of the eighty two years, and produces comparatively good results as the more sophisticated eigenvector method, six moisture amomaly patterns, are identified by map-pattern correlation method indicating distinct differences between areas north of 10°N and the rest of the country. Maps of the mean seasonal areal coverage of drought reveal remarkable changes in size and position from year to year. Drought does not cover the entire country and no particular areas were persistently affected until the 1970s and 1980s. Since the beginning of the 1970s areas affected by drought have increased in extent with a consequent decrease in near normal and wet conditions particularly in the Sahelian and the Midland zones. The 1970s and 1980s were characterized by exceptionally high frequency of drought conditions with a presence of consecutive years of large-scale droughts since 1982, confirming persistence of drought in Nigeria. v All stations are subjected to Non-Integer method of spectral analysis and low-pass filter method in order to identify any trends and regular periodicites in the drought index. The low-pass filter results confirm negative trend for the Sahelian and Midland zones (from the mid 1950s and late 1960s respectively), indicating increasing aridity. The drought spectrum as indicated by the Non-integer method is dominated by peaks in the 2.1-7.9 frequency range (Coastal and western Guinea-Savanna zones) and longer frequencies with peaks between 11.0-11.9 and 13.0-13.9 years in the Sahelian and the Coastal zones. Based on the number of stations with significant quasi-11-year oscillations and their spatial distribution a linear relationship between drought index fluctuation and 11-year sunspot cycle may not be realistic, as it is often portrayed in the literature. In general, drought in Nigeria is a complex random phenomenon, generated by non-linear atmospheric processes sporadic in spatial occurrence, making its prediction difficult.

Mots clés : ASPECTS SPATIAL TEMPORAL CHARACTERISTICS DROUGHT NIGERIA

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