Application of HEC-HMS Model for Rainfall Runoff Simulation of Weyb River Watershed, Genale Dawa River Basin, Ethiopia

Abstract

Hydrologic studies on rainfall-runoff have been extensively conducted by water resource planners to simulate the hydrological response in many regions around the globe to fulfill various desirable needs with a purpose of effective and proper planning and managing of water resources for present and future uses. Whereas such study is not well drawn much attention to Weyb watershed (Genale Dawa River Basin, Ethiopia) in which may prevail to water insecurity. Therefore, this research thesis is intended to apply the Hydrologic Engineering Center – Hydrologic Modeling System (HEC–HMS) for the simulation of the rainfall-runoff of this watershed to evaluate whether the model performs sufficiently in this study area. Long term daily rainfall data from 6 rain gauging stations for 30 years from(1985-2014), daily River flow of 1 stream gauging station for 15 years from (1992-2006), land use and soil data of the watershed, and DEM were obtained from relevant sources. These data were then analyzed and interpreted and used to set up the HEC-HMS mode. In this study SCS-curve number (loss), SCS unit hydro-graph (transformation), monthly constant (base flow) and Muskingum (Routing) Methods were adopted. In order to clearly understand the horologic characteristics of each watershed, rainfall-runoff relation of the watershed was calibrated using 11 years stream flow data (1992-2002) and the remaining 4 years data (2003-2006) for model validation. The model simulation has been conducted using reasonable approximation and the initial results showed that there is a clear difference between the observed and simulated peak flows and the total volume. Therefore, a model calibration with an optimization method and sensitivity analysis was carried out and the model was run with the most identified sensitive parameters. After parameter optimization the difference between observed and simulated and error functions were so reduced and the results indicate values of Percent bias (%) =13.6, root mean square error (RMSE) =0.4, Nash-Sutcliffe Efficiency (NSE) =0.867 and coefficient of determination (R 2 ) =0.936. Moreover the calibrated model with optimized parameter was also used for model validation and found percent bias (%) =-3.306, RME=0.4, NSE=0.819 and R 2 =0.929. The results obtained showed that the model is appropriate for hydrological simulations and to analyze rainfall-runoff in the Weyb River watershed. In this study, the rainfall-runoff relationship was also analyzed for the data used for validation using the scattered plot and found to have very mush relations. Finally, in this study the storm flows for different return period were also predicted using HEC-HMS and compared with other statistical models namely gamble, normal distribution and log Pearson (3p) to minimize the risk caused by flooding and drought. Using Kolmogorov-Smirnov (KS) test, log Pearson (3p) was found to be the best next to HEC-HMS model. The peak discharges obtained by HEC-HMS for the 2, 5,10,25,50 and 100 year storms are 196.2, 300.7, 375.1, 515.2, 692.4 and 850.0m3∙s–1 respectively. In doing so, this thesis will help and become an input in flood risk mitigation process.