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Main Curriculum Tie:
Background For Teachers:
Water does not disappear with our use of it in irrigation, manufacturing, or consumption. The water we have now is the water we had at the beginning of time. Water forms, dissipates, and forms again in a cycle called the hydrologic or water cycle.
The water cycle is a gigantic circulation system operating in the atmosphere and on the earth’s lands and oceans. Being a cycle, there is no beginning or ending, but for illustration, let’s begin with the waters of the ocean, which cover about threefourths of the earth.
Water from the surface of the ocean EVAPORATES, while water given off by plants TRANSPIRATES. This combined water is referred to as EVAPOTRANSPIRATION. Here the water enters into the atmosphere and in turn cools and CONDENSES into clouds, and falls back to the earth’s surface as PRECIPITATION.
Precipitation that falls as rain, hail, dew, snow, or sleet is important to all living things. After wetting the foliage and the ground, some of the precipitation RUNS OFF into streams and other waterways. This is the water that often causes erosion and is the main contributor to floods. Not all of the precipitation runs off. Some of it pools and becomes available for evaporation. Some of it slowly PERCOLATES or INFILTRATES (soaks in) through the ground. Some of it resurfaces at springs, while some seeps to maintain and replenish streams during dry periods. Streams eventually lead back to the oceans, where the water is again evaporated into the atmosphere.
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