Prediction of behavioral changes in water resources systems under applying different scenarios and policies can help to optimum operation of these resources. In order to integrated operation of Choghakhor dam reservoir for meeting the downstream demands a system dynamic model was designed and implemented in VENSIM software. Also the amount of the runoff input to the reservoir and the evaporation from the reservoir surface was predicted based on the ARIMA model for the next five years. After defining these parameters and their interactions in the model, it was calibrated. Then different scenarios, including changes in the calculation method for environmental demand, increasing irrigation efficiency and changes in the irrigated agricultural area, was implimented. The results of running the model for next five years showed that the Choghakhor Dam reservoir can supply the water demands for 1600 Ha of farmlands in the most optimal condition. In this case, the agricultural and environmental supply were obtained about 83 and 95 percent, respectively. In addition, by changing of the irrigation system from surface irrigation to pressurized system and increasing irrigation efficiency by 30% the cultivated area, can be developed up to 1690 hectares.