Planning water supply systems is a complex task. Planners and managers often face with decisions to be made under uncertainties. The uncertainties could include changes in community growth (temporal and spatial), per-capita water use, public perceptions, regulations, and climate conditions. One of the most powerful, convenient and intuitive ways to represent these uncertainties in the planning stage is to use scenarios. Scenario planning analysis provides organizational flexibility by planning for multiple possible future scenarios; each of which may be, although is not necessarily, associated with a probability of occurrence. Here we present a methodology utilizing scenario analysis for optimal design of regional-scale water and wastewater infrastructure. The proposed methodology is demonstrated with an application to the planning of decentralized water supply and reuse system for a new development community located in the Tucson metropolitan area.