Sources of Water

Of all the water on the Earth, about 97% exists in the ocean as saltwater. The remaining 3% represents the amount of freshwater on the planet. Unfortunately, however, 90% of this freshwater is trapped in glaciers and ice caps, and in general humans cannot extract it for use. Ultimately, only 0.014% of the earth's total volume of water is easily available to us for agricultural, industrial, and domestic purposes. This water exists in a variety of forms, including soil moisture, groundwater, water vapor, and lakes and streams.

Surface Water

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Surface water results from precipitation that is not absorbed by the ground or evaporated back into the atmosphere. Surface water becomes surface runoff as it flows into streams, lakes, wetlands, and reservoirs. A watershed is the region from which surface runoff drains into the various bodies of water mentioned above. Approximately two-thirds of the world's runoff is carried into the ocean by seasonal flooding and is not available for human use. The remaining one-third represents the amount of reliable runoff that provides a stable source of freshwater for humans.

Groundwater

Groundwater results from precipitation that infiltrates the ground and seeps downward through fractures, pores, and other spaces in soil and rock. Because layer of bedrock that lies underneath the earth's soil cannot be penetrated by water, groundwater usually accumulates above this barrier. The layer of soil directly above bedrock that is completely filled with water is known as the zone of saturation. On top of the zone of saturation lies the water table. During wet weather, the water table becomes soaked with water and rises closer to the surface of the soil, and during dry weather, it falls closer to the zone of saturation. Above the water table lies the zone of aeration, a layer of earth in which rock and soil may be moist but are not saturated with water.

An aquifer. (Click to enlarge)

An aquifer is a porous, water-saturated layer of sand, gravel, or bedrock through which groundwater flows. It can be compared to a large, horizontal sponge that absorbs and transports water along its length. A confined aquifer is one that is bounded above and below by beds of rock, while an unconfined aquifer lies underneath a water table (see picture). Aquifers continuously receive more water at recharge areas, areas of land through which groundwater passes downward into the aquifer. This groundwater moves from the recharge area through the aquifer and out into a discharge area. Examples of discharge areas include lakes, geysers, streams, and oceans. Groundwater usually moves from regions of high altitude and air pressure to regions of low altitude and air pressure. It travels at an extremely slow rate, averaging about only a meter per year.

Water Around the World

Nature's distribution of water sources does not always correspond with the distribution of the world's population. Canada, for example, has 20% of the world's freshwater but represents only 0.5% of the world's population. China, on the other hand, contains 21% of the world's people and only 7% of its water supply. Sometimes, the most reliable sources of water exist far from the centers of population where such water is needed. For example, 60% of South America's Amazon River flows through remote areas where few people live. As the world's total population continues to increase, this uneven distribution of water sources will generate and intensify conflicts between countries and regions over scarce supplies of water. See also case study: water shortages in the Middle East.