Predicting Discharge At Ungauged Catchments Using Rainfall-runoff Model (case Study Omo-gibe River Basin)

This paper deals with predicting discharge at ungauged catchments, the case of Omo-Gibe riverrnbasin, using the WATBAL conceptual lumped rainfall-runoff model. Parameters calibrated withrnthe model are extrapolated from gauged catchments to ungauged catchments of similar physicalrncharacteristics with Regionalization technique. In the Omo-Gibe river basin, most of therncatchments are ungauged. Hydro-meteorological data from twenty four metrological and twentyrntwo hydrological gauging stations of the basin are used to calibrate and validate the modelrnparameters. Sizes of gauged catchments whose data are used vary from about 40 to 3242 kmrn.rnKey model parameters considered consist of sub-surface runoff coefficient (α), surface runoffrncoefficient (ε), and maximum water holding capacity of catchments (Smax). These parameters arerncalibrated using an automatic optimizing routine of the WATBAL model and through routinerniteration; Smax is found to vary from 3403mm to 466mm, α from 64mm/day to 0.26mm/day and εrnfrom 9.6 to 2.6. Some more model parameters, namely, direct runoff coefficient (β), subsurfacernrunoff coefficient (γ), and base flow (Rb) are calibrated manually. The hydrograph characteristicsrnof observed and simulated events are compared using various evaluation criteria consisting of rnNash-Sutcliffe coefficient (NS) greater than 0.6, relative volume error (RVE) between -10 andrn10%, and coefficient of determination (rrn2rn) greater than 0.7. Among the catchments of the basinrnthat are used in the modeling work, 73% fulfilled the criteria. Based on availability of requiredrndata, six physical catchment characteristics (PCCs) are selected among those commonly used inrnmany regionalization studies. These are catchment area, mean annual precipitation, mean annualrnevapotranspiration, average catchment slope, longest flow path and mean catchment elevation. Inrnaddition to the PCCs, input data including precipitation, observed flow, temperature, relativernhumidity and sunshine are used in the WATBAL rainfall-runoff model. Four parameter rn2rnregionalization methods (multiple regression, spatial proximity, area ratio and sub-basin mean)rnwere applied to transfer model parameter values from the gauged to the ungauged catchments.

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