The result of the sensitivity analysis showed that the curve number is the sensitive parameter. Thereafter, a model calibration with an optimization method and sensitivity analysis was carried out. Initial results showed that there is a clear difference between the observed and simulated peak flows and the total volume. The rainfall-runoff simulation was conducted using six extreme daily time series events. To account for the loss, runoff estimation, and flow routing, Soil Conservation Service Curve Number (SCS-CN), Soil Conservation Service Unit Hydrograph (SCS-UH) and Muskingum methods were used respectively. The meteorological model was developed within HEC-HMS from rainfall data and the control specifications defined the period and time step of the simulation run. The catchment was delineated and its properties were extracted from a 30 m × 30 m Digital Elevation Model (DEM) of the Lake Tana Basin. The surface runoff was simulated using the Hydrologic Modelling System (HEC-HMS) for the Gilgel Abay Catchment (1609 km2), Upper Blue Nile Basin, Ethiopia. Understanding the complex relationships between rainfall and runoff processes is necessary for the proper estimation of the quantity of runoff generated in a watershed. From all the results it can be said that the overall performance of the HEC-HMS model is reasonable and trustworthy and can be applied in similar area of research interest. To evaluate the model performance, Relative Error (%), Root Mean Square Error (RMSE), Correlation Coefficient (r) and Nash Sutcliffe Efficiency (CE-NSE) are used along with simulated and observed hydrographs and scatter plots. The Soil Conservation Service Curve Number (SCS-CN), SCS Unit hydrograph, and time interval were selected for each component of the inbuilt processes of loss method, transform method, and channel routing respectively while developing the HEC-HMS Model. Consequently, present study aims in predicting the Koyna reservoir inflow using-a conceptual model, Hydrologic Engineering Centre's Hydrologic Modelling System (HEC-HMS version 4.8). Koyna dam reservoir is one of the major reservoirs in India which serves the multipurpose functions including Hydroelectric power generation and thus for efficient reservoir operation, accurate inflow prediction is the need of the day. As reservoir inflow pattern predominantly depends upon the rainfall-runoff process, land use, land cover and climate change within a catchment area, accurate prediction of it, is a difficult task.
Reservoir inflow plays a key role in hydrological predictions such as drought and flood predictions as well as in reservoir operations and thus precise prediction of reservoir inflow is of utmost importance.
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