Meeting the healthy nutrition needs of the increasing population in the arid and semi-arid climates of the different regions of the world such as Iran has become very important for the agriculture ministry and water resources managers. In this study, the system dynamics approach was used in the Vensim software environment to allocate the water of the Zonouz dam reservoir for irrigation purposes in the northwest of Iran. For this purpose, the existing surface water resources in the basin and the amounts of agricultural water and environmental water demands were determined and a water allocation plan was developed. In the first stage of the study, it was found that if the existing water resources and demands will not change, the amount of water stored in the reservoir will provide approximately 91% of irrigation water demands and approximately 99% of environmental water needs. The model created in the study was found to be sensitive to reservoir inputs and irrigation water demands. Within the scope of this study, the impact of two different scenarios that may occur as a result of climate change and irrigation management in the operation of the reservoir was evaluated. The decrease in the amount of water entering the reservoir in the first scenario and the increase in irrigation water needs in the second scenario are assumed within the next 10 years. According to the simulation results of the first scenario, irrigation water demands will not be met sufficiently with the decrease in the amount of water to be stored in the reservoir due to the decrease in the amount of water entering the reservoir in the next 10 years. According to the results of the second scenario, in the next 10 years due to possible climate change or if the cultivated area increases due to some new agricultural policies; The amount of water stored in the reservoir will not meet the irrigation demands and there will be water shortage in the system. In this case, it is necessary to make changes in irrigation water management and use new irrigation systems to save water. Based on the findings of the study, it has been observed that the impact of all types of irrigation water policies can be successfully evaluated within the scope of the system dynamics approach.