Some of the most important challenges facing water managers are to increase water supply and reduce its demand. A single systematic method is needed to address both issues, such as the System Dynamics (SD) modeling approach. In this approach all the factors, parameters, and their influences on the problem are considered by causal loops and stock-flow diagrams. The multipurpose Dez Reservoir in southwestern Iran is a good case study for this approach, and we simulated 10 years under differing operation strategies to develop the most appropriate operation policy. A hydrologic time series analysis was conducted to generate simulated inflow to the reservoir, and differing policies, including hedging rules and a “goal-seeking hedge,” were applied. By using performance criteria and a new measure entitled “corrected reliability,”, the most appropriate scenarios were identified. We found that using the goal-seeking hedge in combination with water demand management offers the best chance for effectively meeting demands and minimizing supply shortages.