Water storage using dams is a perfect solution for agricultural, industrial, drinking water supply, flood control, hydroelectric power generation, and other purposes. Integrated management of water resources involves the development, management, protection, regulation and beneficial use of surface- and ground- water resources. The reliability of water supply reservoirs depends on several factors, e.g. the physical characteristics of the reservoir, the time series of river discharge, climatic conditions, the amount of demand, and the method of operation. If a portion of a dam's volume is kept empty for flood control, the confidence values of taking the bottom water demand will be reduced. In this paper, a yield-storage model developed in a MATLAB software environment is used to determine the optimal capacity of Darband dam in northeast Iran (the study phase). The reservoir's performance with respect to demand downstream, e.g. from industry and agriculture, and for potable use, was studied, and the results compared for scenarios in flood control volume change conditions. The results show that, for a capacity of 80 Mm3, the reliability values for meeting agricultural, environmental, and potable water demand are estimated at 0.922, 0.927, and 0.942, respectively. If the reservoir's capacity is changed from 80 to 350 Mm3, the reliability values increase by only about 7%.