 |  Environmental Science: A Global Concern, 7/e William P. Cunningham,
University of Minnesota
Mary Ann Cunningham,
Vassar College
Barbara Woodworth Saigo,
St. Cloud State University
Water Use and Management
Be Alert BoxesBE ALERT FOR: Atmosphere Dynamics
Understanding why deserts developed where they did is easier if you understand the concepts of saturation point and convection currents, and the fact that warm air rises and cooler air sinks.
It takes energy to turn liquid water into water vapor. (Humidity refers to the water vapor present in air.) However, air at a given temperature can only hold so much water. When a body of air contains the maximum amount of water vapor it can hold, it is said to have reached its saturation point.
The greatest amount of energy available to evaporate water is present near the equator. As massive amounts of solar energy strike the equator, the air is heated and much water is evaporated into it. As this less dense, warm, moist air rises above the equatorial zone, it cools. But, since cool air cannot retain as much water vapor as can warm air, water condenses out of the rising air, forming clouds and dumping much of the water back to the equatorial surfaces below as rain.
The remaining cool, dry air eventually begins to sink back to earth as well. The sinking happens a little ways north and south of the equator, however, at approximately 23° N and S latitude. As this dry air nears the earth’s surface, it warms, and because it is so dry, causes huge evaporative water loss from the lands below—hence, deserts.
BE ALERT FOR: The Ogallala Aquifer
Take a careful look at the story of the Ogallala Aquifer in the text. This is an excellent illustration of some of the most important concepts of this chapter. Why is precipitation in the Great Plains as low as it is? Where did all the water of this gigantic aquifer come from to begin with? What eventually happens when we extract and use water at a rate faster than it is replaced by the hydrologic cycle? How can water use policy in one area of the country affect people living in other places? How do we resolve potential economic, social, and cultural dislocation when a crucial resource like water runs out? What are the elements of a national, sustainable policy for water use?
Our modern way of life is as dependent on water as on any other resource, including energy. Cultures do not survive without adequate supplies of this most precious resource.
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2003 McGraw-Hill Higher Education