Application of the Geomorphologic Instantaneous Unit Hydrograph Concept for Runoff Prediction in Ungauged Catchments

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Addis Ababa University


This study was aimed at development of unit hydrograph by relating the geomorphological characteristics of a catchment with the basic characteristics of the catchment IUH through the concept of Geomorphological Instantaneous Unit Hydrograph (GIUH) to address the problem of predictions in ungauged catchments. Two conceptual unit hydrograph models, ED-GIUH model and the GIUH based Nash model, have been used in the study. The geomorphological characteristics including the Horton‘s ratios of the catchments were extracted using GIS software called ILWIS. DEM-hydro processing, a new routine developed in ILWIS, was used for the extraction of the geomorphological characteristics. The geomorphological characteristics of a catchment were related with the shape and scale parameters of the Nash IUH to derive the complete shape of the GIUH based Nash model. ED-GIUH was developed from geomorphological characteristics of the catchments and probability density function of travel time of rain fall excess to the catchment outlet. These two models were developed in four catchments in Abay basin with range of areas (185-670km2). The velocity parameter of the two models was calibrated using unit hydrograph ordinates developed through rainfall-runoff analysis and the optimum velocity was determined. The performances of the calibrated models were evaluated using error functions, namely, Nash-Sutcliffe efficiency (NSE), percentage error in peak discharge (PEP), and percentage error in time to peak (PETP) and the models were compared together with the observed average unit hydrographs. From the results of the calibration as well as verification of the models it was found that both models were adequately simulate the shape of the unit hydrographs of the catchments. The ED-GIUH was successful in capturing the peak discharge of the hydrographs but the Nash based GIUH model was better in simulating the time to peak as well as the shape of the unit hydrographs. The optimized velocity parameter of ED-GIUH model was related to the geomorphologic characteristics of the catchments. The linear equation, relating the velocity with the slope and longest flow path of the catchments under the study was found to be best fit. A validation test was carried out to check the reliability of the derived equation in an adjacent catchment which was not used in the calibration of the models and a reasonable result was obtained in four rainfall-runoff events. Finally, it is clear that the predictability of the developed velocity equation in the region will be improved if the number catchments included in the calibration of the models were increased.