Application of HEC–HMS for runoff simulation of Gojeb Watershed, Southwest Ethiopia

The objective of this study is to apply HEC–HMS for hydrological modeling of the Gojeb watershed. The daily precipitation and stream flow, Land Use Land Cover (LU/LC), soil, and Digital Elevation Model (DEM) were the data used to achieve the objective. The sensitivity analysis of the watershed parameters showed that Curve Number (CN), initial abstraction ratio (λ), and lag time (t) were most sensitive, whereas wave travel time (K) and channel storage coefficient (x) were moderately sensitive. The sensitivity of these parameters signifies that LU/LC, soil types, topography, and channel geometry, which are the dominant runoff causative factors, are lumped by these parameters. Model calibration and validation were conducted from 2002 to 2008 and 2009 to 2014, respectively. The selected objective functions during the calibration and validation periods were PEP (Percentage Error in simulated Peak), PEV (Percentage Error in simulated Volume), Nash–Sutcliffe efficiency (NSE), Root Mean Square Error (RMSE), Percent bias (PBIAS), and Coefficient of Determination (R). Their respective values during the calibration period were 16.44%, 10.3%, 0.81, 10.42m/s, 11.64% and 0.87; and during the validation period, 3.92%, 22.95%, 0.551, 12.85 m/s, 22.91% and 0.73. The result showed that HEC–HMS well simulated the daily runoff of the Gojeb watershed. The average monthly observed and simulated flow during the entire simulation period were 1,044.9m/s and 841.4m/s, respectively, whereas the annual average observed and simulated runoff values of the watershed were 12,538.8 m/s and 10,096.8m/s, respectively. Hydrological modeling of the Gojeb watershed helps to better understand the effect of watershed management practices on surface runoff. The HEC–HMS model can also be used for water scarce areas with limited data or for ungauged watersheds.