A shortage of water exists, not only in the arid regions of the world, but even in some moderately humid climates. This situation is a consequence of water require- ments for agriculture and industry in amounts greater than the natural surplus. Even in Europe there is increased anxiety over the state of water reserves, and shortages are forecast for the near future if industry continues to expand. During the past 50 years in the United States, water use has increased about twice as fast as the rate of population growth, and shortages have already appeared in some places. The need to conserve...