REGIONAL LOW FLOW ANALYSIS OF ABAYA- CHAMO SUB BASIN

Abstract

The purpose of this study was analyzing and characterizing low flow in Abaya­ chamo river sub basin. Flow duration curves of 1, 7, 10, 30 day have been plotted for 17 stations. Base flow separation has been computed using one day annual minimum by USGS computer program called Institute of Hydrology Method, Low flow frequency has also been done for the Abaya- chamo river basin using annual daily minimum low flow series. L- Moment ratio diagram was used to group stations to same region and identifies the underlying statistical distribution. Accordingly four homogeneous regions were delineated using a simple test based on the variability of at-site values of Cv. In assessing the suitability fitted regional distribution, the goodness of fit measure statistic (z) and minimum standard error of estimate were used. The GLL distribution provides a good fit to low flows in regions one, while Gpa distribution fits well in region two, GEV fits region three and wakeby to region four. The method of probability weighted moments is considered as the best parameter estimation procedure compares with the method of moments. Moreover regression analysis has been applied to develop regression models for the delineated four regions to predict mean annual daily minimum flow from un-gauged catchments using catchments characteristics. The multiple coefficient of determination, R2, is used as a measure of the ability of the regression model to describe variations in the dependent variable. The closer R2 is to one, the better the regression model fits the data. The value of R2 is equal to 0.75 for region one and two, R2 is equal to 0.916 for region three and 0.792 for region four . Also a regional low flow frequency curves are established for the entire delineated regions using standardized flow data for the purpose of estimating low flow quintiles of un-gagged catchments