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Basic Sources of Water

Water covers approximately two-thirds of the earth's surface, only 3 percent of this amount consists of fresh water. Of this already small percentage, only a third is in liquid form; the majority of the rest remains frozen in polar icecaps and glaciers. With most of the unfrozen fresh water is groundwater --- serving today as a primary source of water for humans --- an extremely small portion remains as aboveground freshwater, upon which most animals have thrived for millions of years.

Surface Water
- The water cycle involves the evaporation of water exposed to the sun; condensation of evaporated water vapor in the atmosphere and precipitation of condensed water, which allows it to fall back to Earth as rain, snow or sleet. "Surface water" doesn't become absorbed back into the earth, at least not completely. Augmented by rain and overflows from groundwater, basic sources of surface water include rivers, lakes, springs and streams.
  
  Although a small fraction is used for public drinking water supply --- such as in county and municipal water systems --- most of the withdrawn surface water serves industrial purposes, irrigation and in thermoelectric power plants, often for cooling purposes.
Groundwater
- Cleaner than surface water and drawn from the earth by pumps, groundwater constitutes the greatest percentage of public water supply distributed for commercial and domestic use. Absorbed through the earth's porous surfaces and collected in aquifers and water-tables, groundwater undergoes a natural process of purification. As with surface water, groundwater requires filtration and chlorination to become safe for human consumption. Groundwater is also widely used for irrigation and industrial purposes.
Rain
- For many animals, rain remains a primary source of water. Rain occurs in different amounts depending on the region. In tropical or forest areas, rain tends to be more plentiful, while in desert area, rain can be extremely rare. Over the past few thousand years, rain has played a critical role in agriculture; any lack of it can have a devastating effect on crop development.
Sea Water
- The oceans and seas serve as by far the most abundant source of water. The high costs of desalination and treatment make sea water impractical for domestic, agricultural and commercial use, however. The 1995 U.S. Geological Survey data on water use showed saline water limited to only mining, industrial and thermoelectric purposes.
  
  The oceans harbor tons of animal life, providing many types of habitats. Some animals --- such as whales, shrimps and salmon --- have been central to the fishing industry for centuries.

Nature