In this paper the feasibility of using risk an alysis for the planning and operation of a water supply system is evaluated through a case study where limited water resources have to be shared by Fukuoka city and the small neighbouring comm unities when drought occurs. Actual data of water demand and water supply together with actual reservoir inflow data are used. An application of the method which analyses two existing water supply subsystems is presented. The possible situations, in which drought occurs or the water demand target increases, are simulated and the risk is calculated and analysed .