A methodology is presented for planning and managing water supply for metropolitan areas in arid ciiiates where water resources are severely limited. The methodology proposed here is intended to lead to a thorough analysis (involving economic, social and environmental considerations) of known alternative means of increasing water supplies or of reducing water demand, and to help select a least-cost mix of actions that will achieve a supply/demand balance for water throughout the selected planning period. The technique of cost-effectiveness analysis is recommended to help determine the optimum mix of strategies that will achieve the desired goal of matching water supply and demand at the least cost. "Cost" is used in a broad sense that involves a weighing, not only of economic costs, but also of social and environmental considerations.
Because social, cultural and environmental considerations differ from one area to another, the best solution for one metropolitan area will not necessarily be best for another. Therefore, this methodology must be considered within the framework of overall community goals, i.e., the desires of the citizens with respect to growth, environment, and quality of life as well as adequate water supply.
SCORE AND LUITATIONS OF DECISION-MAKING METHDDOLOGY
The water resources decision-making process described in this Paper is conceptually simple, although considerably more complex in execution. For any metropolitan region, the first step in planning for water resources is the definition of a study area-the land and people to be served. Ideally, the study area should encompass a complete system, i.e., the urban core and a realistic projection of all of the surrounding land in the designated metropolitan area which will have a demand for water and will be served by existing water suppliers throughout the planning period. The time period of the analysis should reach as far into the future as metropolitan population growth and development can be forecast with reasonable confidence. This will extend as far as statistical projections are available, particularly demographic data on population, land use, employment, industrial development, housing, etc., and data on municipal, industrial, and agricultural water use within the study area. Typically, this may extend 50 years into the future. It is recognized that forecasts lose accuracy the farther they reach ahead, and for this reason the term "planning time-horizon" is used. It signifies the haziness of distant vision and the existence of a limit beyond which the future is totally obscure. Nevertheless, an extended time-horizon is essential for water resources planning because the actions that may be necessary to enlarge water supplies may require decades to accomplish.