Climatological concerns involve more than the effects of pressure and air masses on the weather. The conservation of soil and water is based on an understanding of the hydrologic cycle. Figure 16.4 illustrates the movement of water in, on, and above the earth.
The earth is a closed system in which all of the water circulates from one form to another. A study of the cycle can begin at any point, but we will start with precipitation.
Precipitation is caused by the condensation of water vapor in the atmosphere. Water vapor collects in the atmosphere as the sun evaporates water from the oceans, rivers, lakes, and plants. It falls to earth in the form of rain, hail, or snow, or forms on the surfaces of objects as dew or frost. Not all precipitation reaches the earth's surface; some evaporates as it falls, some reaches the surface but does not move through the cycle because it is held in the form of snow and ice and some is intercepted by plants. The moisture stored as snow and ice may be stored for a long period of time in glaciers and the polar ice caps. Precipitation follows different paths before it eventually returns to the atmosphere in the form of vapor.
A portion of the precipitation will infiltrate the soil. It is not unusual for the infiltration rate (inches/hour) of the soil to be less than the rainfall intensity (inches/hour). When this occurs, the excessive precipitation becomes runoff, which is one of two causes of erosion. Not all runoff reaches the ocean; some evaporates, some is collected in different size impoundments and then infiltrates into the soil,
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and some is used by vegetation. The water used by vegetation may be evaporated quickly or may become part of the plant processes. Water can be tied up in plants for extended periods, but it eventually returns to the cycle.
The precipitation that infiltrates into the soil also takes different paths. Some may fall on areas of deep soil and percolate deeply into the earth. Other infiltration may reach an impervious layer close to the surface and start to move horizontally quickly. The underground horizontal movement may end at some type of surface water, or may flow out of the ground as a spring or an artesian well. The water that infiltrates deeper into the soil may collect in large underground basins such the Ogallala aquifer, and may be pumped out of the ground for domestic, industrial, or agricultural use, or it eventually may reach the ocean. Horizontal movement through the soil may be as little as a few inches per year. Once the water reaches the ocean, the sun causes it to evaporate, and the cycle begins again.
The hydrologic cycle can be used to explain the importance of water conservation. Activities such as pumping, dam building, and so on, change the amount and direction of the flow of water. For example, any water captured by a dam, not only reduces the amount of water available to down stream users, it also changes the amount of water that infiltrates into the soil and evaporates into the atmosphere. Contamination, in the form of chemicals, silt, and so forth, added to the water at one point in the cycle may remain in the water and cause problems for the next user.
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