REGIONALIZATION OF PARAMETERS FOR UNGAUGED CATCHMENT USING HEC-HMS MODEL: THE CASE OF MIDDLE OMO-GIBE BASIN, ETHIOPIA.

Abstract

Transfer of hydrologic model parameters from gauged to ungauged catchments is perhaps the most common procedure in hydrology. In developing countries like Ethiopia most of the Rivers are ungauged. Therefore, applying regionalization techniques for ungauged or poorly gauged river basin is crucial. This paper deals with predicting discharge at ungauged catchments using conceptual rainfall-runoff model HEC-HMS in middle Omo-Gibe sub-basin. Four regionalization methods (linear regression, spatial proximity, area ratio and sub-basin mean) were applied to transfer model parameter values from the gauged to the ungauged catchments. In regional model, gauged catchments model parameters and physical catchment characteristics of ungauged catchments were used to develop the equations in order to predict discharge from ungauged catchments. To have better understanding of model parameter performance, the sensitivity analysis of ten model parameters were performed manually by trial and error. The evaluation shows that crop coefficient (CC), storage coefficient(R), constant rate (CR), and time of concentration (TC) are more sensitive than others. The model performance was evaluated using Nash Sutcliff efficiency greater than 0.75, percent difference lies between +10% and -10%.and coefficient of determination greater than 0.8 during calibration and validation period The model parameters in ungauged catchments were determined by using regional model, sub-basin mean, area ratio and spatial proximity methods and simulated the discharge by using HEC-HMS model. . In predicting model parameters from ungauged catchments in regional model method p-value ≤ 0.1 and determination coefficient (R2 ) = 0.91 for crop coefficient(CC) and 0.99 for maximum deficit(MD), constant rate (CR), maximum storage(MS),initial storage(IS),storage coefficient(R), and time of concentration (TC) were obtained by linear regression. Because of this river flow from ungauged catchments simulated by regional model method was the best. Therefore, regional model method is recommended in predicting discharge for ungauged catchments for this research and obtained 30m3 /s and 15m3 /s averaged maximum monthly averaged simulated flow for Denchiya and Mansa ungauged sub catchments respectively.