The high level of reliability of water resources is always an advantage for consumers, but in arid and semi-arid regions where the inflow to the reservoir is faced with severe fluctuations, it makes sense to decrease the percentage of reliability of the system and allocate less water to consumuption zones to prevent critical conditions such as emptying of the reservoir. In this research, the employed operaton modelis based on the simulation-optimization combination, which by considering the objectives of minimizing the violation of the allowed capacity of the new reservoir and maximizing the percentage of supplying the demands, the optimal hedging variables are specified through the reservoir simulator link to the MOPSO multi-objective optimization algorithm. According to the available data, the duration of the simulation and optimization period in the model is 360 months.After 1000 iterations, the optimal reservoir volume values are obtained at the hedging level and hedging coefficient in different months. Finally, the model results are compared with the results obtained from the standard operation policy (SOP). The results show that the proposed model is able to manage the allocation to needs in the dry months and prevent the tank from emptying. Also, by storing a part of the flow in the reservoir in watery months and consuming it in low water months, it increases the supply of needs by 20 to 35% and reduce the failure rate in dry months.