This paper describes our experience with construction and use of a simulation model for a large Irrigation and Power Project in a state in India. The project has an estimated outlay of Rs.5000 million (U.S. $ 5 billion) and a project construction time to thirty years. The model was used by decision makers to fix key project parameters such as the main panel capacity and the capacity of the power plant. It integrates a variety of uncertain factors, such as upstream withdrawals, effect of rain-fall and use of ground water. The model has also been instrumental in a qualitative re-design of the project, by indicating substantial benefits from creation of additional storage in the command area. This project is now being actively investigate by the project authorities.