Building new water resources projects or managing existing systems?

The planning of regional water resources systems and their subsequent management has to rest firmly on three foundations: availability of water of adequate quality, demand for water expressed quantitatively for specific periods of time, and direct, indirect, and externality costs. Mathematical models of hydrosystems represent only a segment of the real world The basic science of hydrology is an empirical discipline, and its basic law is the continuity equation. Optimization of operating rules based on this law of conservation of matter may face difficulties when the objective function is expressed in economic terms. Overcoming this obstacle requires the use of targets (for water releases and stored volumes) expressible in physical units. The optimization objective can then be expressed as the sum of the squares of the differences between the results of the analysis and the targets. An additional complexity is created by parties who are interested in regional hydrosystems for purposes other than the traditional water supply and flood control. They may require certain elevations of water levels in streams and reservoirs for wildlife or recreational purposes or minimal flows needed for fishing and fisheries. All these complexities lean to an expansion of regional water resources models so as to enable the manager to investigate factors and decision alternatives not included in the model. A regional water resources system is relevant only in so far as it advances the solution of socio-economic and political issues and promotes development rather than growth. Management of existing systems should take precedence over building new water resources projects.