Frequency modeling of rainfall intensities using a probability and a non-probability distribution function for Abuja metropolis, Nigeria

Year 2020, Volume: 3 Issue: 1, 13 - 33, 09.07.2020

Gabriel Apken Ejeh Omale Oloche Ekwule

Abstract

The aim of this study is to analyze rainfall data so as to develop rainfall intensity – duration – frequency (IDF) models and curves for Abuja metropolis, Nigeria. A thirty-one years data set was obtained, checked for consistency and then used for the modeling. The method of Annual Series was used to select the data sets for the rainfall analysis. The data were fitted to Gumbel Extreme Value Type – 1 (GEVT – 1), Log Pearson Type III (LPT – 3), Pearson Type III (PLT – 3), Normal and Log Normal (LN) Distributions. Probability distribution function (PDF) and Non-Probability distribution function (nPDF) were calibrated to obtain the IDF models. Chi-squared goodness-of- fit test was used to confirm the appropriateness of the fitted distributions for the location. The Gumbel Extreme Value distribution has the best fit and was adopted in developing models using the PDF (power model). The location parameters resulting from GEVT – 1 were determined thus: c = 23.16, m = 0.21 and e = 0.55. Five IDF models were developed using the nPDF (quotient model) for return periods of 2, 5, 10, 16 and 32 years. The coefficient of correlation (R2) for different return periods for all models developed showed very high values ranging from 0.7538 to 0.9927, an indication of good curve fitting. The IDF models developed for Abuja showed no significant difference when compared with results from locations within the same zone using t-test of significance though durations of 12 and 24 hours had exceptions. The application of IDF models (power law and quotient) with return period of 10 years and rainfall duration of 2 hours for design of a typical rectangular drainage channel was found to be okay. The IDF models and curves are recommended for the prediction of rainfall events for Abuja.

Keywords

Frequency Modelling, Probability and Non-probability Functions, Intensity Duration frequency Curve, Abuja

Supporting Institution

Nil

Thanks

thanks to NIMET for making available rainfall data

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