Optimum management of global water resources presents one of the most crucial challenges of the 21st century. Global population will increase by three billion or more over the next 50–75 years, and the number of people living in urban areas will more than double. Most of the world's population growth will occur in developing countries where water is already critically short and many of the residents are impoverished. Even today, >1 billion people do not have access to safe and affordable drinking water and perhaps twice that many lack adequate sanitation services. In fact, inadequate drinking water quality is a leading cause of infant mortality worldwide. Food production may soon be limited by water availability. Agricultural water use is not sustainable in many locales around the world for reasons that include soil salinization, ground water overdraft, and the overallocation of available surface water supplies. This situation raises questions about whether there are sufficient water resources to support the existing population on a long-term basis, to say nothing of the significantly larger population that will have to be fed in the remaining decades of this century. Intensifying competition for water resources by agricultural, industrial, and domestic users has led to a sharp increase in stress on aquatic and wetland ecosystems. Moreover, the inadequacy of environmental water supplies in much of the world has been significantly exacerbated by declining trends in water quality. Many developed countries have addressed this problem by adopting laws to guarantee supplies for the environment, but such guarantees are contingent on having adequate water for urban needs and on the availability of sufficient quantities of food. No such guarantees can be provided in developing countries, which tend to neglect environmental needs and are unable to mount efforts to maintain and enhance water quality for financial reasons. There seems …