EVALUATING THE PERFORMANCE OF SWAT AND HEC- HMS MODELS ON RAINFALL-RUNOFF ESTIMATION AT THE GREAT AKAKI RIVERWATERSHED, ETHIOPIA.

Abstract

The present study was conducted to examine the accuracy and applicability of the hydrological models Soil and Water Assessment Tool (SWAT) and Hydrologic Engineering Center (HEC)- Hydrologic Modeling System (HMS) to simulate stream flow. Models combined with the ArcGIS interface have been used for hydrological study in the Great Akaki River watershed. The critical focus of the stream flow analysis was to determine the efficiency of the models when the models were calibrated and optimized using observed flows in the simulation of stream flow. Daily weather gauge stations data were used as inputs for the models from the 1995-2017 periods. Other data inputs required to run the models included land use/land cover (LU/LC) classes resulting from Map agency and related offices, soil map and digital elevation model (DEM). For evaluating the model performance and calibration, daily stream discharge from the catchment outlet data was used. For the SWAT model calibration, ALPHA_BF (Base flow alpha factor (days), curve number (CN) and GW_DELAY(Groundwater delay(day)) are identified as the sensitive parameters. SCS.lag (MI), Curve number(AMC_II) and Initial abstraction (Ia) are the significant parameters identified for the HEC-HMS model calibration. The models were subsequently adjusted by auto calibration for 1995-2010 to minimize the variations in simulated and observed stream flow values at the catchment outlet (Aba Samuel). The hydrological models were validated for the 2011-2017 period by using the calibrated models. For evaluating the simulating daily stream flow during calibration and validation phases, performances of the models were conducted by using the Nash-Sutcliffe model efficiency (NSE) and coefficient of determination (R2). The SWAT model yielded high R2 and NSE values of 0.85 and 0.82 for daily stream flow comparisons for the catchment outlet at the calibration and validation time, suggesting that the SWAT model showed relatively good results compared to the HEC-HMS model. Also, under modified LU/LC and ungaugedstream flow conditions, the calibrated models can be later used to simulate stream flow for future predictions. Overall, the SWAT model seems to have done well in stream flow analysis for hydrological studies.