Abstract:
Rainfall was the main source for generation of runoff, which flows through Rivers and streams.
The study was conducted in Gojeb River. In this area, there is lack of planning and inadequate
water resources management. Therefore, the main objective of this study was simulation of
rainfall runoff processes using Hydrologic Engineering Center Hydrological Modeling System
(HEC-HMS) model. The data used were meteorological data from (2002-2014), Stream flow
data from (2002-2014), Digital Elevation Model (DEM), Soil and Land use/land cover. The
data were collected from National Meteorological Agency of Ethiopia and Ministry of Water,
Irrigation and Electricity. After data collection, the missing data were filled by using
Arithmetic Mean method and the consistency of data was checked by using double mass curve.
The point rainfall data was changed to areal rainfall using Thiessen polygon. By using
Hydrological Engineering Center Geospatial Hydrologic Modeling System (HEC-GeoHMS),
basin model was generated which is imported to HEC-HMS. Using DEM, soil data and land
cover data curve number for each sub basin was also computed using HEC-GeoHMS and the
methods for determination of loss, transformation, channel routing and base flow SCS loss,
SCS unit hydrograph. Muskingum and monthly constant methods was selected respectively.
For this study, there are five watershed parameters available these are curve number, initial
abstraction, lag time, time of passing of a wave (k) and Muskingum coefficient(x). By using
these initial values of parameter, the first run was conducted and there is a variation between
simulated and observed flow. Therefore, calibration was done using optimization. The result
indicates that the wave travel time and channel storage coefficient are the most sensitive
parameters. After calibration, there was an agreement between the simulated and observed
flow. The model performance evaluation was conducted using percentage error in simulated
volume (PEV), percentage error in simulated peak (PEP), Nash-sutcliffe efficiency (NSE) and
coefficient of determination (R
2
). In calibration (PEV=16%, PEP=10.3%, NSE=0.810,
R2=0.866 and PBIAS=16.44%) and for validation (PEV=23%, PEP=3.9%, NSE=0.55,
R2=0.728 and PBIAS=22.9%). These values were in the range of very good, good and
satisfactory therefore, the model was adopted for runoff simulation on Gojeb watershed. In the
watershed, the peak flow during calibration and validation was116.5m3/sand 85.9m3/s
respectively. The total runoff volume of the watershed was 1700.8mm. Determination of stream
flow from the watershed is required for policy makers to make decisions on water planning
and management. This study can be used as reference or benchmark for any other future
studies that will be conducted.
