Lee B. Hotchkiss and Beverly Stern
The earth’s water is used over and over in an endless cycle (figure 1). The ultimate source of energy for this cycle is the sun. The source of all water is rain—or snow, sleet, or hail. Most of the earth’s surface is water, for the oceans cover about seventy-one percent of the globe. Most of our fresh water is frozen in glaciers and polar ice caps. Water is found in all living things and is integral to the making of man-made things. Luckily for us, the recycling of water insures us of a continuing supply. It is interesting to think that perhaps the water we just drank may have been the same molecules that a thirsty Brontosaurus might have tasted. This liquid is so ordinary that we often take it for granted. In New Haven each resident uses about 115 gallons a day, or about 42,000 gallons a year. For a whole year this figures adds up to about five billion gallons for all 126,000 New Haven residents.
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This is the way the hydrologic cycle works: Fresh water falls from the condensation of cooled water vapor in clouds. When the water falls, most of it soaks deep into the ground and flows below the surface down hills and mountains into rivers and lakes. Only during heavy storms is there surface runoff. This gathering area forms a watershed. Eventually both runoff and groundwater flow into the ocean. When the sun hits the ocean surface, the ocean water is heated and great amounts of water are evaporated (vapor), leaving the salt behind like a huge distillation apparatus. Despite the evaporation, however, the salinty of the ocean continues constant because of the balance built into the water cycle. As the vapor rises, it cools, condenses, and physically changes back to a liquid to form clouds. This is a simplified version of the cycle because not all precipitation (rain) lands in rivers or lakes.
About half of the water that reaches the ground evaporates before it can reach a lake or a river. Some of the rain may be absorbed by plant roots. Some of the rain will end up in city sewers. Not all water vapor comes from the oceans and other bodies of water. We supply some in the forms of perspiration, urine and exhaled water vapor. And of course all organisms give up tissue water when they die. It all adds up to one life-giving cycle unique to our planet.
The water which does reach the land surface either directly or indirectly (by being intercepted) may move in different ways. If a soil is porous, the water moves down into it. A well-drained soil suitable for growing plants will retain water in its small and intermediate-size pores. The water in large pores will eventually move laterally beneath the soil surface until it enters a stream that will transport it to a lake or ocean. If the soil pores are filled with water at the time of precipitation, the water moves laterally across the soil surface as surface water runoff. But most water moves below the surface. As it does, it can leach out minerals (which is how “hard water” becomes hard). When the water reaches a stream or a river, it joins the flow capable of eroding and of changing the courses of waterways and the shapes of land.
Even in nature, water is never absolutely pure as it flows through the series of physical changes in the water cycle. Water can absorb gasses, chemicals, minerals, acids, and salts.
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Water facts of possible interest to teachers and students are as follows:
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1. Approximately 70% of our bodies is water.
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2. About 30,000 cubic miles of water fall as rain and snow on the earth’s land areas each year.
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3. The average annual precipitation on the United States is about 30 inches. The average runoff carried by its rivers is only about 9 inches.
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4. 1 cubic foot of water weighs 62.4 pounds.
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1 cubic foot of water equals 7.48 U.S. gallons.
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1 U.S. gallon of water weighs 8.33 pounds.
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1 imperial gallon of water weighs 10 pounds.
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5. How deep is an ocean? Up to 7 miles (Pacific), 5 miles (Atlantic).
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6. Long Wharf extended 3,480 feet into the New Haven Harbor because dredging would be very expensive.
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