Rainfall-runoff modeling is one of the most complex tasks in hydrological studies because it requires accounting for the diverse characteristics of watersheds. Such modeling is crucial for forecasting runoff at watershed outlets by replicating the hydrological dynamics of the basin. This study employed the Hydrologic Engineering Centre - Hydrologic Modeling System HEC-HMS model to simulate runoff in the Kalar urban watershed, situated in Sulaymaniyah Governorate, Kurdistan Region of Iraq KRI, over the period from 2017 to 2023. Input parameters for the Hydrologic Engineering Centre - Hydrologic Modeling System HEC-HMS model were extracted using tools like Geospatial Hydrologic Modeling Extension HEC-GeoHMS and Arc Geographic Information System ArcGIS. Various methods were applied throughout the modeling process, including the Soil Conservation Services – Curve Number SCS-CN method for precipitation loss estimation, the Sol Conservation Services - Unit Hydrograph SCS-UH method for excess rainfall transformation, and the Muskingum method for flood routing. After model calibration and validation, the model's performance was assessed using Nash-Sutcliffe Efficiency (NSE), the correlation coefficient (R2), and Root Mean Square Error (RMSE) which yielded high values during both calibration (NSE =0.926, R2 =0.96, 0.63) and validation (NSE =0.923, R2 =0.96,0.31). The parameters influencing output include lag time, curve number, initial abstraction, flood travel time (Muskingum-k), and discharge weighting factor (Muskingum-x). From findings the difference between simulated and observed peak flow after model calibration was 9%, with an R2 value of 0.9575. Similarly, during the evaluation period, the peak flow discrepancy was 11%, with an R2 value of 0.9678. Furthermore, rainfall-runoff efficiency, improved from an average of 70% in 2017–2018 to higher levels in 2022–2023, particularly in basins with reduced losses, such as W270 (87.25%), W260 (87.04%), and W240 (which increased from 83.68% to 89.77%). In addition, the study underscores the significant impact of impervious surface expansion on runoff and peak discharge. For example, the peak discharge at the outlet rose by 6.89%, with moderate increases observed in sub-basins W240 (4.44%) and W260 (5.88%). These findings suggest that the Hydrologic Engineering Centre - Hydrologic Modeling System HEC-HMS model is highly effective for simulating hydrological behavior in the Kalar urban watershed and can be applied to other watersheds with similar characteristics.
