REGIONAL LOW FLOW ANALYSIS OF ABAYA CHAMO SUB BASIN

Abstract

In early days of application of analytical techniques in the filed of hydrology, low flow studies of a river stream were basically carried out to analyze its ability to supply a particular water demand. The main categories of water demand were i) domestic ii) agriculture and iii) industrial. If it was found that the flow of the river was insufficient to supply a demand, further analysis were carried out to ascertain how much water to be stored in order to meet the demand. Even in the present context, low flow studies remain important for the supplying of such direct demands, either directly or by a suitable storage. However the greater awareness of environmental impacts of the use or the abuse of water courses by mankind such as the disposal of industrial and domestic effluents has oriented low flows studies in another important direction. Extreme low flow events are more diligently analyzed these days in the context of ascertaining whether a certain course can take up a given load of effluents. The effects of every low flows on the ecological and recreational of rivers also adds to the newly emerging of low flow studies. Low flow characteristics can be determined by Regional frequency analysis and flow duration analysis. The estimation of flow regimes at un-gauged sites may be achieved by transfer of statistics derived from gauged catchments using regionalization procedures. The term regionalization in hydrology refers to grouping catchments in to homogenous regions. 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. Flow duration curves of 1, 7, 10, 30 days have been plotted for 17 stations. Base flow separation has been made by the standard method called the Institute of Hydrology Method, Moreover regression analysis has been applied to develop regression models for the delineated three 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