ESTIMATION OF PRECIPITATION INTENSITY-DURATION FREQUENCY MODEL RELATIONSHIP DESIGNED FOR SOME REGIONS IN SOUTHERN NIGERIA

Abstract

Abstract
Precipitation intensity-duration-frequency (IDF) Model was developed for designated regions in Southern Nigeria. Thirty-two years (1983-2014) daily rainfall data were collected for Lagos, Ibadan, Benin, Calabar, Port Harcourt and Warri from Nigeria Meteorological Agency (NIMET) Oshodi, Lagos State for the study. The procedure of annual maximum series was used to select data sets for precipitation investigation. The advanced storm, a pattern developed by United State Department of Agriculture (USDA) Soil Conservation service was used to in the development of the models, which was used to break down daily rainfall into shorter durations. Gumbel and Log Pearson Type III distributions were used to compute the observed rainfall intensity values at durations of 10, 15, 20, 30, 60,120,180,240, 300, and 360 minutes for return periods of 2, 5, 10, 20,50, and 100 years. To obtain parameters for the IDF models for each location, the computed rainfall intensities were subjected to non-linear regression analysis using Microsoft Excel Optimization Technique Solver wizard for the respective durations and return periods. The performance of the models were analyzed by determining the chi-square(χ2 ), coefficient of determination(R2 ), and Root Mean Square Error(RMSE) of the fitted distributions. Coefficient of determination values, R2 obtained from the fitted IDF Models adopting Log Pearson Type 111 and Gumbel distributions gave perfect value of 1 for both regions. Gumbel distribution RMSE values ranged from 0.20 -11.73 for Port Harcourt, 0.50 – 10.94, 0.21 – 9.25 for Calabar and Warri, also Benin, Lagos and Ibadan regions has 1.31 – 9.04, 1.28 – 15.92, 0.49 – 13.08 while, Log Pearson Type 111 RMSE values ranges from 0.23 – 9.68 for Port Harcourt, 0.41 – 11.47, 0.01 – 10.23 for Calabar and Warri while Benin, Lagos and Ibadan regions has 0.29 – 10.0, 0.51 – 15.69, 0.79 – 10.37. However, there was no significant difference amongst the predicted intensities of the various IDF models.