Regional Low Flow Analysis: A Case Study on Upper Awash River SubBasin, Ethiopia

Abstract

Prediction of low flow of a river in magnitude as well as in frequency is necessary for the planning and design of water resource projects since low flow affects significantly in water supply, water quality and river ecological status. However, there is a high level of gap in the country Ethiopia in understanding the regional low flow and frequency analysis compared to flood studies. Thus, the main objective of this study was to perform appropriate regional low flow analysis on Upper Awash river basin of Ethiopia. This paper discusses regionalization of the Upper Awash river basin using three approaches: flow duration curve, low flow frequency analysis and base flow index. L- Moment ratio diagrams were used for identifying and grouping of stations in to hydrological homogeneous regions and hence the underlying statistical distribution. Accordingly, the basin was delineated in to two homogeneous regions, Region one and Region two covering 34.66% and 65.34% of the sub basin respectively. Discordancy measure of regional data of the L-moment statistics was performed using MATLAB R2013a. Both regions have shown satisfactory results for discordance measures and homogeneity tests. For the candidate distribution from l-moment ratio diagram, best fit distribution is selected by using Easy Fit software. All the three goodness of fit test used in this study shows a first rank for Generalized Pareto and Generalized Extreme Value for region one and two respectively. For the selected distribution parameter estimation technique was selected by performing a Robustness Assessment. Accordingly, GPA with PWM selected for region one and GEV with MOM was selected for region two. Based on best-fit distributions for the two regions, regional low flow frequency curves were constructed for the return periods of 2, 5, 10, 20, 50 and 100 years resulting 0.125, 0.084, 0.070, 0.064, 0.059 and 0.038 m3 /s quantile for region one and 1.206, 0.964, 0.807, 0.659, 0.611 and 0.570 m3 /s for region two . Base flow separation has been made using BFI+ software using a local minimum method. Accordingly 40% of the stations has BFI ranging from 0.15-0.39, 50% of the station constituting most part of the Region two shows BFI of 0.4-0.65. Similarly, 10% of the stations has BFI >0.7. The results indicate that the region one in Upper Awash river basins shows a high diminished in low flow quantile relative to the region two.