Hydrologic Regionalization and Geomorphoclimatic Instantaneous Unit Hydrograph Application (The case of Abbay River Basin, Ethiopia)

Abstract

To make accurate and reliable quantile estimates at ungauged catchments, two approaches were followed and studied in Abbay river basin: hydrologic regionalization and application of the event based Geomorphoclimatic Instantaneous Unit Hydrograph ( GcIUH) concept. Abbay river basin was divided into 129 watersheds and 45 Geomorphologic and climate parameters were extracted for each watershed using Arc Hydro/GIS, WinDisp and New_LocClim softwares. Out of these the 21 most important parameters were selected through Principal Component Analysis. Cluster analysis option of SPSS was used to cluster (group) these 129 watersheds into a convenient riumber of groups depending on the similarity of the 21 input parameters and with a reasonable classification threshold. This grouped Abbay river basin into hydrologically homogeneous regions. This hydrologic regionalization was perceived from a hydroclimatological and physiogeographical point of view; and verified with representative recorded flow data from within the basin. After all, the research resulted in the delineation of five stable regions. Regions one and two cover much of the area of the eastern half of the basin, while region three is the area covering most of the boundary of the lakes Tana and Finchaa. Region four mainly covers the area following the western border of the study area where as region five covers the southern portion of the western half of the basin. Out of these five, regions one, two, four and five have adequate number of recorded flow data. These four regions were tested for homogeneity and heterogeneity using discordance tests and H statistics respectively, and found homogeneous. On the other hand, the prediction power of the Geomorphoclimatic Instantaneous Unit hydrograph concept was assessed in one of the catchments in Abbay river basin, Andassa. The findings are very encouraging, suggesting that the GclUH concept can be indeed become a useful tool in many regions where very little or no data are available to the hydrologist Further study on the companson of data and information transfer from a region of completely different regional taxonomy and synthetic flow generation methods would make the study more complete and useful for decision support system. Key words: Abbay river basin; Regionalization; GcIUH; Ungauged catchments; Cluster analysis