Estimating Annual Maximum and Minimum Flows at Ona River Gauging Station in Ibadan, Nigeria Using a Modified Mixed Gamma Distribution

Abstract. Reliable estimates of extreme flood flows are required for general flood risk management and planning. To assess the risk and nature of flood discharge, we apply the mixed-gamma distribution to different plotting positions in taking care of shortfall in gamma distribution and observe how best it will estimate flood frequency at Moniya Gauging Station along Ona River. Goodness of fit between the observed and predicted flood flows was measured. Gamma distribution matched with Weibull plotting position gave the highest coefficient of determination (R²) of 0.9966; mixed gamma with California gave 0.9947 while modified mixed gamma with Weibull gave 0.9964 respectively. The minimum absolute differences of 49.653, 3.123 and 20.922 at return periods of 50-, 100-, and 200-year were obtained under the gamma when matched with Weibull, California and Weibull plotting positions respectively. California gave minimal error with the root mean square error (RMSE) of 92.85 for gamma with some information lost while Weibull gave minimal errors of 159.20 and 93.91 for mixed gamma and modified mixed gamma distributions respectively. The RMSE due to modified mixed gamma with the use of Hazen, Weibull and California are 104.34, 93.91 and 103.22 respectively. Hence, we conclude that modified mixed gamma distribution with Weibull plotting position is the most suitable for gauging station.