Due to long life and sedimentation, the capacity of Voshmgir reservoir has decreased. Therefore, it is important to think about how to save capital and reduce the shortage volume. The main purpose of this investigation is to estimate the amount of shortage volume of voshmgir reservoir by considering the two-reservoir system in Golestan province. In this research, for simplifying, the node method was used. The optimization of problem was done using linear programming method. By considering the importance of evaporation in reservoirs, the results showed that in optimum operation of Voshmgir and Golestan reservoirs, the evaporation decreased and in present situation and future condition, the Voshmgir reservoir capacity has to be increased by 9 million cubic meters to supply all water demands. The results showed that the linear programming was very useful method in water resources problems.