Most people in the United States take for
granted the fact that when they turn on the water tap, safe,
clean, pleasant-tasting drinking water will emerge. But, did you
ever stop to wonder where the water comes from or whether there
will always be enough for everyone to enjoy? These are questions
that deal with water supply and demand and will be examined in
this section. Another important issue, the quality of water, is
examined in a separate section of the
program.
To learn more about these issues, explore the topics in the following menu.
Water is one of the Earth's natural resources. It is a finite resource, which means that the total amount of water is limited. Most of the world's water supply is saltwater stored in the oceans. Converting saltwater to freshwater is generally too expensive to be used for industrial, agricultural or household purposes.
The type of water we generally use in human activities is freshwater. Only 3% of the world's water supply is freshwater and two-thirds of that is frozen, forming the polar ice caps, glaciers, and icebergs. The remaining 1% of the total world water supply is freshwater available as either surface water or ground water; ground water accounts for two-thirds of this amount. Surface water is water that is visible above the ground surface, such as creeks, rivers, ponds and lakes. Ground water is water that either fills the spaces between soil particles or penetrates the cracks and spaces within rocks.
Most people get their water from ground water sources. Roughly 9 out of every 10 public water systems (PWSs) operate wells to tap ground water and just over half of the total population served by PWSs drinks water from a ground water source. Millions more get their water from private wells which also tap ground water sources. Ground water systems tend to serve smaller numbers of people, while surface water systems generally serve large populations. The quantity and quality of the world's water supply depends on how we choose to use water. Our use can be consumptive, which means that the water is not returned to nature (such as drinking water), or noncomsumptive. Nonconsumptive use returns water, polluted or not, to the system. Creating hydroelectric power is an example of nonconsumptive use.
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It is hard to understand
why there are water shortages when the United States' average
daily precipitation equals 4.2 trillion gallons. The earth's
total water volume is constant. Water goes through the hydrologic
cycle of evaporation, condensation, precipitation, infiltration,
percolation and runoff. However, water is not always located
where it is needed because of unequal distribution of
precipitation at any given time or location. Also, the demand for
water is always increasing.
The U.S. receives enough annual precipitation to cover the entire country to a depth of 30 inches. This 30 inches is known as the U.S. water budget. The eastern half of the country receives more rainfall than the western half. Most of this precipitation returns to the water cycle through evapotranspiration. Of the 30 inches of rainfall, 21 inches returns to the atmosphere in this manner. Water loss by plants, the transpiration portion of evapotranspiration, is most significant. One tree transpires approximately 50 gallons of water a day. Approximately 8.9 inches of annual precipitation flows over the land in rivers and returns to the ocean. Only 0.1 of an inch of precipitation infiltrates into the ground water zone by gravity percolation. The actual amount of water that enters the ground water zone for any specific area depends upon the annual rainfall in that area.
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In the United States, 450 billion gallons of water are withdrawn per day from ground and surface waters for a variety of uses. Of that 450 billion gallons, only 100 billion gallons are actually consumed. The remaining 350 billion gallons are withdrawn for nonconsumptive industrial and agricultural uses.
Industry is the largest withdrawer of water but not the largest consumer. Much of the water withdrawn for industry is returned to its source after being used for cooling or other purposes.
Agriculture is the largest user of water due to irrigation. It takes over 1 million gallons of water a year to irrigate one acre of farmland in arid conditions. Most of the water used by irrigation re-enters the hydrologic cycle through evapotranspiration. Although it has only 28% of the U.S. population, the arid West accounts for 80% of the average water consumed daily due to extensive irrigation.
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In the past 30 years, the U.S. population has grown 52% while the total water use has tripled. On average, Americans use approximately 75 to 80 gallons of water per person per day. Much of this water eventually returns to the water supply after having passed through a waste water treatment facility or domestic septic systems.
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