Why do people call water a "renewable" resource?
Water may be considered an inexhaustible resource because the total supply of water in the biosphere is not affected by human activities. Water is not destroyed by human uses, although it may be held for a time in combination with other chemicals. To be useful, however, water must be in a particular place and of a certain quality, and so it must be regarded as a renewable, and often scarce, resource, with cycling times that depend on its location and use.
Water that falls from the atmosphere as various types of precipitation and then runs off the land surface to form streams and rivers that eventually reach the ocean, generally operates on a one-year-renewable cycle known as the hydrologic cycle. From the ocean the water is evaporated by solar energy and returned to the atmosphere, from which it again falls as rain or some other form of precipitation.
In certain locations, however, water has a much longer cycling time; after entering the ground from rainfall, it may percolate slowly through underground channels until it reaches underground reservoirs. In certain arid regions, the total water supply may be underground water that accumulated during past ages, when the climate of the region was more humid. Since that time there may have been little or no addition to this supply because of the existing climatic conditions. Because its cycling time may be extremely long and dependent upon the frequency with which wet and dry climates alternate in a particular region, such a water resource can be virtually nonrenewable.
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