Rainfall intensity-duration-frequency models for Lokoja Metropolis, Nigeria
Intensity-Duration-Frequency (IDF) relationship is one of the most commonly used tools in water resources engineering. The purpose of this study was to develop rainfall intensity-duration-frequency models/curves for Lokoja Metropolis, Kogi State, Nigeria. Rainfall data was obtained from Nigeria Meteorological Agency (NIMET) and sorted for frequency analysis. Five different frequency analysis techniques namely; Normal, Log-Normal, Gumbel, Pearson Type III and Log-Pearson Type III distributions were used to develop the IDF relationships for Lokoja. Storm durations of 5, 10, 20, 30, 45, 60, 90 and 120 minutes, and return periods of 2, 5, 10, 25, 50 and 100 years were adopted for the derivation of the models. Kolmogorov-Smirnov and Anderson Darling goodness of fit tests were conducted using Easy Fit software to ascertain the best distribution that fits the data. A power-law model was adopted in developing the desired IDF models/ curves for the study area. The results of the goodness of fit showed that all the five distributions were not rejected both at 5% and 1% significance levels except Pearson Type III which recorded Anderson-Darling value of 3.0814 at 30 minutes’ duration which is above the critical value of 2.5018 at 5% level of significance. Although, all the distributions gave good results, Log-Pearson Type III distribution was adjudged the best for the study area because of its best ranking. It is recommended that the IDF models/curves derived in this study should be used as tools for prediction of rainfall events for design of hydraulic structures in the study area. Also, more meteorological stations should be created in the country and properly equipped to generate requisite data for planning and design of water resources systems.
Keywords: Rainfall-Intensity-Duration-frequency models, Log-Pearson Type III distribution, Power model, rainfall events, hydraulic structures