The Water Cycle

Water is crucial to life. Cells contain 70 to 90 percent water, and water provides the environment in which most of life's chemical reactions occur. The availability of water is one of the key factors that regulate the productivity of terrestrial ecosystems. However, very little of the available water on Earth is trapped within living things at any given time. Bodies of water, such as lakes, rivers, streams, and oceans, contain a substantial percentage of Earth's water. The atmosphere also contains water—in the form of water vapor. In addition, some water is found below ground. Water in the soil or in underground formations of porous rock is known as groundwater.

The movement of water between these various reservoirs, known as the water cycle, is illustrated in Figure 18-12. Three important processes in the water cycle are evaporation, transpiration, and precipitation.

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