DEVELOPMENT OF RAINFALL INTENSITY-DURATION FREQUENCY MODELS FOR SELECTED LOCATIONS IN SOUTH EASTERN NIGERIA
The development of a rainfall intensity-duration-frequency (IDF) Models for selected locations in humid rainforest zones of Nigeria is the subject of concentration on this research. Thirty-two (32) years (1983-2014) daily rainfall data were collected for Umuahia and Owerri from Nigeria Meteorological Agency (NIMET) Oshodi, Lagos State for the study. The method of annual maximum series was used to select data sets for rainfall analysis. For the development of the models, the advanced storm, a pattern developed by United State Department of Agriculture (USDA) Soil Conservation service 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 analysed 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. Also for the Log Pearson Type 111 distribution, RMSE values ranged from 1.57 ─ 15.33 and 0.46 – 11.69 for Umuahia and Owerri regions. Gumbel distribution and RMSE values ranged from 0.05 – 14.47 and 0.21 – 11.08 for Umuahia and Owerri regions. However, there was no significant difference amongst the predicted intensities of the various IDF models.
Keywords: Gumbel distribution, Rainfall, intensity- duration –frequency (IDF), Log Pearson type III