Water Supply and Availability

It is interesting to note that, despite what appears to be enormous amounts of water in major rainfall and flooding events, 91.5% of Earth s water remains in the oceans and only 2.5% is freshwater. Furthermore, the greater portion of freshwater, 1.7% of Earth s total water, is held immobilized in ice caps in polar regions and in Greenland. This leaves only 0.77% of Earth s water, commonly designated as freshwater, potentially accessible for human use. 

The water that humans use is primarily fresh surface water and groundwater, the sources of which may differ from each other significantly. In arid regions, a small fraction of the water supply comes from the ocean, making use of huge water desalination plants, which is a source that is likely to become more important as the world s supply of freshwater dwindles relative to demand. Saline or brackish groundwater may also be utilized in some areas. 

In the continental United States, an average of approximately 1.48 x 10 L of water fall as precipitation each day, which translates to 76 cm per year. Of that amount, approximately 1.02 x 10 L per day, or 53 cm per year, are lost by evaporation and transpiration. Thus, the water theoretically available for use is approximately 4.6 x 10 L per day, or only 23 cm per year. At present, the United States uses 1.6 x 10 L per day, or 8 cm of the average annual precipitation, an almost tenfold inaease from the usage of 1.66 x 10 L per day in 1900. Even more striking is the per capita increase from about 40 L per day in 1900 to around 600 L per day by the end of the last century. Much of this increase is due to high agricultural and industrial use, each of which accounts for approximately 46% of the total consumption. Municipal use consumes the remaining 8%. 

FIGURE 3.5 Water distribution in the continental United States showing water deficiencies in the rapidly growing southwestern states precipitation amounts are in centimeters per year. 

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