Considering the need to supply water and food in future periods, this study assessed the effects of climate change on the water resource system of the Ballarood reservoir that located in the northwest of Khuzestan province, Iran. The outputs of ten general circulation models (GCMs) under two scenarios RCP8.5 and RCP4.5 have been used to simulate the effects of climate change. The results indicated an increase in average annual temperature by 2.34 (1.64)°C under RCP8.5 (RCP4.5). An increase in the amount of agricultural water requirement for downstream and decrease in the amount of runoff into the reservoir (31% under the RCP8.5 scenario and 11% under the RCP4.5) are other results of this study. Based on this results, the increase of vulnerability [46% (21%) under RCP8.5 (RCP4.5)] and decrease reliability [41% (18%) under RCP8.5 (RCP4.5)] of reservoir water supply system is simulated in the future period (2030-2055) relative to the basis period (1980-2005).