Seventy-one percent of the earth's surface is covered by water—an area called the hydrosphere, which makes up all of the oceans and seas of the world. Only 3% of the earth's entire water is freshwater. This includes Arctic and Antarctic ice, groundwater, and all the rivers and freshwater lakes. The amount of usable freshwater is only about 0.003% of the total. To put this small percentage in perspective, if the total water supply is equal to one gallon, the volume of the usable freshwater supply would be less than one drop. This relatively small amount of freshwater is recycled and purified by the hydrologic cycle, which includes evaporation, condensation, precipitation, runoff, and percolation. Since most of life on earth depends on the availability of freshwater, one can say "water is life."

Worldwide, agricultural irrigation uses about 80% of all freshwater. Cooling water for electrical power plants, domestic consumption...