Decentralization within metropolitan areas has been a major aspect of population movement in the United States over the past two decades. The trend has great significance for all urban service activities. In particular, it affects water supply planning in urban areas. Both numbers of people and their spatial location affect forecasting of water supply demands which, in turn, affect fixed plant investments (source works, treatment works, transmission lines, distribution systems, and so forth) which are frequently built many years in advance of the resulting services. The paper describes, via a case study, the use of population and per capita demands as a basis for forecasting water requirements.