Water resources represent one of the most serious environmental issues of the twentieth century. Water is abundant globally, yet millions of people have inadequate fresh water, and droughts plague rich and poor countries alike. Water pollution compounds the scarcity of usable fresh water and threatens the health of millions of people each year. At the same time, the ravages of flooding and catastrophes associated with excessive rainfall or snowmelt cause death and destruction of property. To cope with these problems, water resource management has become a high priority worldwide.

Concerns about water scarcity seem unwarranted, considering that there are 326 million mi³ (1.36 billion km³) of water on earth. Although 99% of this water is either unusable (too salty) or unavailable (ice caps and deep groundwater storage), the remaining 1% can meet all future water needs. The total volume of fresh water on earth is not the problem. Scarcity of...