In this study, the surface water of Salmas Plain were simulated using the WEAP model. For this purpose, the period 2015–2020 was used for model calibration, while 2021–2024 was considered for validation. The validation results, with (R^2 = 0.84), (NSE = 0.64), and (Bias = 0.11), indicated that the model achieved acceptable performance in simulating. Subsequently, using data from the Sixth Assessment Report on climate change, seven different scenarios were defined for the future period 2027–2036 under SSP126 and SSP585. The results showed that no considerable water shortage would occur in the urban domestic, rural, and industrial sectors, and the water demands of these sectors would be fully met. The highest level of unmet water demand was observed in the agricultural sector, indicating its high vulnerability to reduced water availability and the impacts of climate change. The assessment of environmental water requirements showed that flow shortages mainly occurred in the Zola River during September and October, whereas the Derik River did not experience environmental flow shortages. The sustainability of surface water resources in the Salmas Plain requires agricultural water demand management, improvement of irrigation efficiency, and particular attention to maintaining the environmental flow of the Zola River.