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34. TREND ANALYSIS, CYCLES AND PERIODICITIES IN ANNUAL MAXIMUM DAILY RAINFALL DISTRIBUTIONS OVER SOUTHERN NIGERIA by Emeribe C.N. Butu A.W. and Laka S.I. Volume 52 (July & Sept., 2019 Issue)
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TREND ANALYSIS, CYCLES AND PERIODICITIES IN ANNUAL MAXIMUM DAILY RAINFALL DISTRIBUTIONS OVER SOUTHERN NIGERIA

Emeribe C.N. Butu A.W. and Laka S.I.

National Centre for Energy and Environment, Energy Commission of Nigeria, University of Benin, Benin, City, Edo State

Department of Geography, Faculty of Arts and Social Sciences, Nigerian Defence Academy, Kaduna, Kaduna State

Department of Geography and Planning, University of Jos, Plateau State

Abstract

The study is aimed at investigating patterns and frequency of annual extreme daily rainfall events over southern Nigeria using the Correlogram method.  Maximum daily rainfall data were collected for a period of 1969-2017 for Warri, Benin City, Calabar, Port Harcourt, Ikeja, Ondo, Oshogbo, Enugu and Owerri, while 1986-2017 for Uyo. Maximum daily values were extracted using descriptive statistics of mean, median maximum and minimum. Cycle and periodicity were determined using autocorrelation function white noise and based on asymptotic chi-square approximation. Trend was examined using the Mann-Kendall Test, Spearman’s Rho Test and Linear regression test statistics at α =0.10, α <0.1, α <0.05 and α <0.01 levels of significance. Results of correlogram for all the synoptic stations showed evidence of white noise weakly stationary time series, rapid decaying trend and periodicity. The value of Box-Ljung Q statistics for all the stations indicate that the residuals are independent and random, thus it can be concluded that the model  provided an adequate fit for the forecast of future maximum daily rainfall in the study area. The autocorrelation plots show, that most residual autocorrelations except those at Lag 3 (Calabar), Lag 2 (Uyo), Lag 8 (Ondo), Lag 13 (Enugu) and Lag 3 (Owerri) fall inside the 95 % confidence bounds indicating the residuals appear to be random. The general trend in annual maximum rainfall distribution over southern Nigeria ranges from absence of trend to strong evidence of statistical trend.  In locations like Benin City, Calabar, Enugu, Ikeja, Ondo and Warri,  annual maximum values did not show any evidence of statistical significant change at α = 0.1. However, for locations like Port-Harcourt, there is possible to strong evidence of increasing trend, while in Oshogbo, Owerri and Uyo, there is a general strong evidence of increasing trends at α < 0.05. For locations such as Calabar, Port-Harcourt, Uyo, Oshogbo, Enugu, Owerri and Ikeja which show evidence of either double cycles and/or the probability of every year extreme event, and this suggest that these areas are highly vulnerable to extreme meteorological-induced events, in particular urban, river flooding and soil erosion. These patterns are further buttressed by results of annual daily fluctuations around means. 

Keywords: Correlogram, Autocorrelation Function, Maximum Daily Rainfall, Cycles and Periodicity 

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