The Hydrologic Cycle

The total amount of water on the earth and in the atmosphere is finite, unless little icy comets melt in our atmosphere and contribute a small additional amount. The

Figure 6-2 Residential cold water use.
Figure 6-3 Residential hot and cold water use combined.

water we use today is the same water that was in Noah's proverbial flood. Ninety-nine percent of the earth's water is either saltwater or glacial ice. A quarter of the solar energy reaching the earth is employed in constantly circulating water through evaporation and precipitation, in a process known as the hydrologic cycle (Fig. 6-4).

The most accessible sources of water for our use are precipitation and runoff. Rain, snow, and other precipitation provide a very large but thinly spread supply of relatively pure water. Precipitation can be captured on a local basis in cisterns (containers for rainwater), a strategy that is rarely used in the United States but widely found in other parts of the world where rains are rare and water is precious. Water that runs off the earth's surface results in a more concentrated flow that is more easily captured in cisterns or ponds. Any daily precipitation that doesn't evaporate or run off is retained as soil moisture. After plants use it to grow, it evaporates back into the atmosphere.

Groundwater sinks into the soil and fills the open spaces with water. The upper surface of the ground-water is called the water table. Groundwater makes up the majority of our water supply. It can also be used to store excess building heat in the summer for use in the building in winter. Groundwater can harm building foundations when it leaks into spaces below ground.

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