The main goal of this study is to estimate the hydrologic parameters of Kocanaz watershed located in Western Black Sea Region, using a semi-distributed hydrologic model, Hydrologic Engineering Center – Hydrologic Modelling System (HEC-HMS). In this study, the hydrologic model was set up for two flood events occurred in 2002 and 2013, in which one was used for calibration while the other one was used for validation of the calibrated hydrologic parameters. The watershed was introduced into the model as a single basin. Hydrologic parameters used for transformation from rainfall to runoff and base flow estimations were searched using Nelder and Mead method and calibrated using the peak-weighted root mean square error. The model results were deliberated by statistical indicators such as Nash-Sutcliffe efficiency coefficient (E_NS), coefficient of determination (R²), mass balance error (MBE) and peak flow rate error (PE). The model results were found to be very good for calibration and satisfactory for validation while the peak flow rate was under estimated for both calibration and validation. It can be concluded that small watersheds such as Kocanaz might be modelled as a single basin without sacrificing the estimation capability of the model and increasing the model simulation time. Though, slightly improved peak discharge estimations were obtained in case of using sub basins.  

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