The output from most analytical procedures used to set firm output levels for water projects is not associated with any given probability of failure to meet those output objectives. As a result, such output cannot always be used with confidence in the design and operation of water resource systems. A simple procedure is proposed that uses Monte Carlo techniques in deriving the optimum contract levels for a multipurpose system subject to constraints imposed by stated maximum permissible probabilities of failure to meet firm contract deliveries. The procedure is tested by analyzing a particular reservoir system. Similar procedures can be designed to handle multireservoir, multipurpose systems with a large number of such constraints and can be used to choose between alternative designs and fix optimum contract levels and release rules. Probability tables derived from such analyses can be used to improve the operational efficiency of water resource systems.