Land-use and land-cover (LULC) changes substantially alter hydrological processes by modifying infiltration, surface runoff, and flow routing, particularly in rapidly urbanizing arid watersheds. This study evaluated the hydrological impacts of LULC changes in Wadi Bani Malik, Jeddah, Saudi Arabia using a scenario-based hydrologic modelling approach. Historical (1999) and recent (2024) LULC maps were integrated with rainfall, soil, and topographic data to simulate watershed responses using HEC-HMS under multiple design storm scenarios. Simulations quantified changes in peak discharge, runoff volume, and hydrograph characteristics under historical and recent LULC conditions. The results indicated that urban expansion between 1999 and 2024 increased the peak discharge and runoff volume across all return periods. For the 2-year return period, peak discharge increased by 21.4% and runoff volume increased by 12.8%; whereas for the 100-year return period, these increases were 5.0% and 10.5%, respectively. The results revealed a nonlinear hydrological response, with the relative impact of LULC change being more pronounced for frequent, lower-magnitude storms than for extreme events.