In recent decades, increasing the industrial activities and greenhouse gases emissions changed the Climate Balance on Earth. This phenomenon influence on the different systems involved in human life, so investigating this phenomenon became one of the most important scientific subjects in recent years. Water resource is one of the most important resources that affected by Climate Change, so the variation of temperature and precipitation directly affects these resources. The proper maintenance and operation of water resources, is one of the important duty for water professionals. Global warming has a significant impact on the hydrological cycle and related processes. However, the report shows there isn't enough water in many parts of the world to meet basic human needs. Iran is one of the Arid and semi-arid parts of the world, so it is essential to evaluating the effects of Climate Change on Water resources and improving the necessary actions. The aim of this study is investigate future periods of peak discharges in Behesht-abad the sub-Catchment basins of Northern Karun. For this purpose, the output of 7 general circulation models of the Fourth Assessment Report of Intergovernmental Panel on Climate Change of the basin in the periods 2015-2044 and 2045-2074 and 1971-2000 base period of two scenarios A2 and B1 were taken. The models output change into three models includes critical, ideal and medium micro-scale models that introduced exponential LARS-WG. The results showed the increasing of 16% and 9% precipitation on first and second periods and increasing the 1.4 and 1.2 °C temperature under scenarios A2 and B1. Finally, the best model was selected by comparing the output of each model on the basic period and observed data. The results of GFDL-CM2.1 model showed that this is best model in the region, so downscale data from this model under the A2 scenario - as the critical scenario- introduced to the HEC-HMS model to simulate runoff in future periods. Simulated runoff discharge compared with observations in the first and second periods, 21 and 19 percent increase, respectively.