The stream flows lead to erosion and degradation of soil fertility, sedimentation in reservoirs and reduction in water quality of rivers. Therefore, deeper insights into these changes will help water resource managers and planners to address such challenges. This research aims to explore the effects of climate change on stream flow variation in the Urmia Lake basin as the wettest basin in Iran, which struggles with drought and diminished water level these days, under RCP2.6, RCP4.5 and RCP8.5 scenarios using HadGEM2-ES coupled earth system model and rainfall–runoff IHACRES model during 2041–2060 period based on the LARS-WG6 statistical downscaling model. The results of evaluating observational and simulated data by LARS-WG6 downscaling model with diverse statistical indexes revealed that there was no significant difference between the simulated and observational values with a critical error of 0.5. The results of the model’s projection also suggested that the average minimum and maximum temperature of the basin would increase by 2.6 and 3.1 °C, and the rainfall would drop by 4.5% compared to the base period (1981–2010). The performance analysis of the IHACRES runoff–rainfall model demonstrated the accuracy of this model in simulating stream flow changes in the studied basin. The results of studying surface flow changes also revealed that, based on all scenarios, the stream flow would diminish in the future, and there would be an average decrease of about 5.5% at the basin level. According to the results of different scenarios, it is predicted that the Urmia Lake basin face with decreasing stream flows, declining precipitation and rising temperatures in the future.

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