The great human project of the next century will be learning how 10 or 12 billion people can live fruitfully and sustainably on a finite Earth. Questions of water use and allocation will be among the many contentious dimensions of the sustainability discussion. During the last century, global water consumption grew more than twice as fast as population. Roughly one-third of the world population now lives in areas subject to moderate to high water stress, and the U.N. projects that the proportion of people affected could double during the next 25 years. Allocation of water among countries bordering rivers like the Jordan, the Nile, and the Colorado is a source of international tension. Allocation of water between agriculture, manufacturing, household use, and ecosystem maintenance are politically sensitive issues that will impact food availability, human health, and culture.

To be effective, policy discussions on water use must consider issues of availability, efficiency, human equity, needs of ecological systems, and the well-being of future generations. To be seen as fair, they must involve the principal stakeholders. Negotiating this complex of competing needs and interests requires both a scientific understanding of how water resources are sustained and used and a moral understanding of how different participants value water and understand the notion of equity.

A full discussion of water use must therefore incorporate both scientific and moral reflection on water availability and allocation. Finding ways of connecting a scientific understanding of how the world works with a moral understanding of how humans ought to live has proven difficult, but is a vital part of the sustainability problem. We hope that exploring these aspects of water use will be helpful in understanding how best to approach the larger issue of sustainability.