Hydrological Modeling and Calibration with Swat+ in the Arıt Stream Basin, Türkiye

This study develops and calibrates a SWAT+ hydrological model for the Arıt Stream Basin in Bartın Province, Türkiye. The basin (approximately 187 km² based on watershed delineation) was modeled using the QSWAT+ interface in QGIS, and an automatic calibration was performed with the SWAT+ Toolbox (v3.1.0). The model was driven by a 25 m DEM, 2018 CORINE land cover, FAO soil map, daily meteorological data (2010–2024), and monthly streamflow observations (2013–2024) at the Darıören gauge (basin outlet). After a 3-year warm-up (2010–2012), the model was run for 2013–2024 on a daily time step; calibration (2013–2018) and validation (2019–2024) were evaluated at monthly flows. Key performance metrics—Nash–Sutcliffe Efficiency (NSE), coefficient of determination (R²), percent bias (PBIAS), root mean square error (RMSE), and RMSE-observations standard deviation ratio (RSR)—indicate that the model reproduces the basin’s streamflow dynamics within acceptable ranges. Monthly calibration results yielded NSE ≈ 0.56 and PBIAS ≈ +6 %, and validation NSE ≈ 0.51 and PBIAS ≈ +9 %, with RSR ≈ 0.66–0.70 (satisfactory per literature criteria). The calibrated model successfully captures seasonal flow patterns and baseflow contributions, providing a reliable tool for scenario analyses and water resources management in the region. These results mark one of the first comprehensive SWAT+ applications in the Bartın region, demonstrating the model’s utility and laying a foundation for future climate change, land-use, water quality, and water–energy nexus studies. Based on 2010–2024 MGM monthly climatology, the basin’s climate is humid temperate with no dry season (Cfb): the warmest-month mean is 21.7 °C (August,