Community can grow faster or slower than expected, regulations will change, and public sentiment can shift. Under this uncertain and time varying planning environment, water system planners often face challenges for decisions: whether and when it is desirable to invest infrastructure. One of the most powerful and intuitive ways to incorporate uncertainties in planning stage is to use scenarios. Scenario-based planning is gaining wide popularity in water planning community. Preparing for a range of possible futures allows flexibility and adds robustness to the system so it can adapt to uncertain events at reasonable cost without community trauma. Here scenario based planning and optimization approaches are presented for optimal planning of regional-scale water supply infrastructure in a multistage process. Various water demand projections are uncertain parameters and multistage construction projects are decisions to be made for an objective of minimum economic costs including capital and O&M costs. The proposed models are applied for a decentralized water reclamation planning in Southeast green-field development area situated in Tucson, AZ where water reclamation is viewed as a critical component for a sustainable water supply.