Rainfall-runoff modelling is one of the most complex tasks in hydrological studies because it requires accounting for the diverse characteristics of watersheds. This modelling is essential for predicting runoff at watershed outlets by replicating the basin's hydrological dynamics. This study employed the Hydrologic Engineering Centre - Hydrologic Modelling 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 HEC-HMS model were extracted using tools like HEC-GeoHMS and ArcGIS. Various methods were applied throughout the modelling process, including the SCS-CN method for precipitation loss estimation, the SCS-UH method for excess rainfall transformation, and the Muskingum method for flood routing. 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. The peak discharge at the outlet rose by 6.89%, with moderate increases observed in sub-basins W240 (4.44%) and W260 (5.88%).
