MODELING RAINFALL-RUNOFF PROCESS FOR HIRNA WATERSHED, WABI-SHEBELE RIVER BASIN, ETHIOPIA

Abstract

Modeling Rainfall runoff process is essential for the analysis of watershed hydrological response toward the received precipitation under the influence of watershed parameters. The aim of this study is to simulate the rainfall-runoff Process for Hirna watershed using Hydraulic Engineering Center Hydrological Modeling System (HEC-HMS) Model. The data used for this study were soil, Land Use Land Cover (LULC), Digital Elevation Model (DEM) with 30 m x 30 m resolution, 31 years (1989 -2019) daily rainfall and 16 years (1999-2014) daily stream flow data. The Soil Conservation Service Curve Number, SCS Unit Hydrograph, monthly constant base flow and Muskingum method were employed for loss computation, excess rainfall computation, base flow modeling and flood routing computation, respectively. The model was calibrated and validated with stream flow data of eight years (2001-2008) and six years (2009- 2014), respectively. Nash-Sutcliffe Efficiency (NSE), Root Mean Square Error (RMSE), and Coefficient of Determination (𝑅 2 ) were used to evaluate efficiency of the model. The model had shown good performance both during calibration and validation with (NSE = 0.82, RMSE = 0.40 and 𝑅 2 = 0.95) for calibration and (NSE = 0.87, RMSE = 0.40 and 𝑅 2 = 0.91) for validation respectively. These indicate very good performance rating of the model. It can be concluded that well calibrated and validated HEC HMS can be used to simulate the runoff at reasonable accuracy in Hirna watershed. The runoff potential of Hirna watershed was generated after validation of the model. The runoff potential of Hirna river watershed was generated from 01Jan2001 to31Dec2014. The generated average yearly runoff discharge of the watershed was about 69.83 𝑚3 /𝑠. This resulted the potential runoff volume of the watershed at the outlet 132M𝑚3 . Therefore, the study will be helpful for efficient water resources and watershed management for Hirna watershed. It can also be used as a reference for any future hydrological investigations in the nearby poorly gauged watershed.