Methodology for long-term water supply planning : Mexico City case

Abstract

A complete methodology for long-term water supply planning is presented. Based upon the characteristics of the water resources development planning problem (nonlinearity of cost functions, and hydrologic variables), the author rejects the seeking of "optimal" solutions and supports the seeking of "good enough" solutions. To answer the questions that are involved in long-term water supply planning, it is proposed to break down the problem into two simpler ones to be solved in a sequential fashion. Although mathematical guarantee of optimality cannot be assured, the introduction of physical and engineering constraints greatly increases the confidence in the final results. The proposed methodology allows deep analysis of the hydrologic aspects involved in water resources planning. The depth of hydrologic analysis is only restricted by available data and technology. In this respect, a method for synthetic generation of monthly runoff records in ungaged streams is proposed. An application of the methodology to the development of a Mexico City water supply plan is presented in full detail to appreciate its usefulness. Mexico City population forecast for the year 2000 is 28 million people. The estimated water demand in that year is 105 m³/sec, more than twice the present water supply of 50 m³/sec. To satisfy this demand, water has to be brought from four basins more than 150 km distant, and located at elevations more than 1,000 m below Mexico City's elevation (2,300 m above mean sea level). The water supply plan which resulted from this study indicates the most recommendable sequence for the development of the four basins, and the amount of water to be obtained from each one.