Comparative Analysis of the Distribution of Daily Rainfall Amounts at three Stations in Northern Nigeria

Abstract

The distributions of daily rainfall amounts at three stations in the savanna vegetative zone of northern Nigeria were studied. The lengths of the rainfall data series ranged from 66 to 81years and covered the period from 1916 to 1999. The data series of each station was divided into two segments with those predating 1961 (i.e. up to year 1960) as at first. The data series for the period from 1961 onwards, which generally showed a declining trend of annual number of rainy days, constituted the second segment. Comparison using cumulative probability ploys and a non-parametric statistical test showed that corresponding distributions in both segments were similar when the data partitioned to two antecedent rainfall-states (i.e. rainy day preceding rainy day and dry day preceding rainy day) and also when partitioned to the early, middle and late parts of the wet season. The result suggested that the data from both segments belonged to the same population and were therefore combined to derive the parameters of a gamma distribution model used stochastically generate daily rainfall data for the stationary part of the series. The Statistical properties of the simulated and observed data were comparable suggesting that the derived model parameters are representative of the series.

JARD Vol. 2 2003: pp. 70-86