An integrated Rough Set approach is proposed and implemented to discover the historical operating rules of a Sicilian irrigation purpose reservoir. Operating rules are derived by expressing monthly releases from the reservoir as functions of stored volume, inflow and release during a 35-years period. This is accomplished through the Rough Set approach as implemented in the Rose package and the use of some indices able to recognize and further screen out the effective rules used in water supply reservoir management. This approach represents a new mathematical tool quite different to classical fuzzy rule-based systems in the decision rules induction. Results show that the integrated Rough Set approach allows to individuate with acceptable reliability the real criteria used for the system management.