Abstract:
Analysis of rainfall distribution is important in studying the impact of climate change on water
resources planning and management. This study examined the performance of three different
probability distribution models, namely: Generalized Extreme Value Distribution (GEV),
Lognormal Distribution (LNG) and Gumbel (EVI) Distribution to describe the rainfall distribution
patterns in some selected stations (Gusau, Yelwa and Sokoto) in North-Western Nigeria. Thirty
years of daily rainfall data for the period of (1985-2014) for the selected stations were obtained
from the archives of the Nigerian Meteorological Agency (NIMET), Abuja. The aim of the study
is to determine the distribution model that best describe the distribution of daily precipitation in
North-Western Nigeria and also to identify the effect of plotting position on existing models. Root
mean square error (RMSE) was used to determine the efficiency of the different plotting formulae
on the existing model. The model performance was evaluated based on the statistical goodness of
fit test, namely Kolmogorov-Smirnov (KS) at 95% (α=0.05) significant level. The method of
moment (MOM), Probability weight-moment and maximum of likelihood were used for
estimating the models parameters. The results show that the probability distribution model that
best fit the data based on statistical of goodness of fit test is the EV1 followed by LNG then GEV.
The EV1 model gave the smallest value of KS test for all stations except Gusau station Where
LNG model was the most suitable. The best plotting position formula with the entire distribution
model was also observed to be the Weibull plotting position formula followed by Chegodayev,
Gringoten and lastly Hazen plotting position respectively. Hazen plotting position gave minimal
errors with the EV1 and GEV probability distributions, while Weibull gave minimal error with the
LNG probability distribution for all the stations. The EV1 model was found to be the most suitable
distribution for modelling the daily rainfall distribution in two out the three stations investigated,
while the LNG was observed to be only suitable for Gusau. The result of this work has provided
information on rainfall probabilities as a vital tool for the design of water supply and supplemental
irrigation schemes and the evaluation of alternative cropping and of soil water management plans.
This study has examined the pattern of rainfall in the North-western Nigeria and observed some
irregularities in the pattern. For government’s efforts towards improving agriculture to be fruitful,
a more advanced technology of ensuring constant rain source is very vital.
