The SWAT model was applied to the watershed of Rusenski Lom River, North Bulgaria. The model is implemented using QSWAT, which is an interface between SWAT and QGIS. The data used are the measured values for river discharge at the NIMH hydrological station in the village of Bozhichen, Ruse region. The model was fed with data for the period 2007–2021. Calibration, validation and sensitivity analysis were performed using SWAT-CUP 2019, SUFI-2. Five factors were used for the estimation of model performance: R2 (coefficient of determination), Nash-Sutcliffe performance index (NSE), PBIAS (percentage bias), P-factor (model accuracy) and R-factor (model uncertainty). Eleven parameters were applied for the sensitivity analysis, with six (CN2, SOL_AWC, ESCO, REVAPMN, GWQMN, RCHRG_DP) found to be more sensitive (P < 0.05) to the river discharge. The SUFI-2 algorithm interfaced with SWAT-CUP was able to capture the behavior of the model with calibration results showing an R2 of 0.37 NSE index of 0.24, PBIAS of -1.0, while validation results revealed an R2 of 0.28, NSE of 0.26 and PBIAS of -2.0. The model produced a P-factor of 0.69 and an R-factor of 1.35 during calibration and during validation, 0.62 and 0.97 respectively.