In this article, a fuzzy rule based model is developed for the operation of a single purpose reservoir. The model operates on an ‘if – then’ principle, where the ‘if’ is a vector of fuzzy premises and the ‘then’ is a vector of fuzzy consequences. The steps involved in the development of the model include, construction of membership functions for the inﬂow, storage, demand and the release, formulation of fuzzy rules, implication and defuzziﬁcation. The methodology is illustrated through the case study of the Malaprabha irrigation reservoir in Karnataka, India. Reservoir storage, inﬂow, and demands are used as premises and the release as the consequence. Simulated reservoir operation with a steady state policy provides the knowledge base necessary for the formulation of the Fuzzy rules.