Introduction

The thin spherical shell of thickness about 10 km at the surface of Planet Earth is unique in the universe because it has abundant amounts of water substance in all three phases: solid, liquid and vapor. Water substance has played a critical role in the evolution of the earth’s atmosphere and surface environment, including the development of biological organisms.

The importance of water, particularly fresh water (as opposed to saline ocean water), in human activities cannot be overstated. Freshwater scarcity already is taking its toll on the African continent and likely will be a limiting factor in future sustainable economic development. Present full commitments of river and lake water for human consumption coupled with water shortages in many regions and expanding future water needs for industrialization, agriculture, and a growing population will put new strains on water supplies. The Director General of the United Nations Educational, Scientific, and Cultural Organization (UNESCO), in a recent visit to Iowa State University warned that water use by humans could well be a source of significant social unrest — even wars — in the 21^st century.

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