Sustainability refers to systems that can endure over long periods of time while remaining productive and diverse. For communities, sustainability requires balancing environmental, social, and economic needs. A sustainable water supply ensures water continues to be available at the same quantity, quality, and cost over 15-20 years as when the system was installed. Assessing the sustainability of a community's water faces challenges as water resources are often shared between multiple communities, so problems in one area can impact others.