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.
