Considering global warming trends, climate change, and fluctuations in precipitation and temperature, surface water resources and sustainable water management in Iran particularly in arid and semi-arid regions are under serious threat. In this context, the present study investigates the impacts of climate change on surface water resources in the Meymeh watershed, Ilam Province. The SWAT hydrological model was employed to simulate the watershed’s hydrological processes, coupled with the CanESM5 climate model and the LARS-WG statistical downscaling model under SSP1-2.6 and SSP2-4.5 scenarios for future climate projection. The SWAT model demonstrated good performance in hydrological simulation, with NSE and R² values of 0.72 and 0.74 for the calibration period (2010–2016), and 0.84 and 0.90 for the validation period (2017–2020), respectively. Future climate projections indicated an increase in temperature ranging from 2.5 to 2.9 °C and a decrease in annual precipitation of approximately 13% under the intermediate scenario. Hydrological analysis revealed that the mean annual river discharge is expected to decline from 5.63 m³/s to 1.61 m³/s in future periods, while summer runoff is projected to approach zero. These findings underscore the urgent need for adaptation strategies, including runoff storage, artificial groundwater recharge, modification of cropping patterns, and the development of flood early warning systems. By integrating LARS-WG and SWAT models with the latest IPCC scenarios, this study provides a scientific basis for smart water resource management and strategic regional decision-making.