The hydrologic cycle

The unending circulation of the earth’s moisture and water is called the water or hydrologic cycle. It is a gigantic system operating in and on the land and oceans of the earth and in the atmosphere that surrounds the earth. The cycle has no beginning or ending.

Water from the surface of the oceans is evaporated into the atmosphere. That moisture in turn is lifted and then condenced and falls back to the earth’s surface as precipitation.

The part of the precipitation that falls as rain, hail, dew, snow or sleet on the land is of particular concern to man and agriculture.

About 80,000 cubic miles of water are evaporated each year from the ocean. About 15,000 cubic miles are evaporated from the lakes and land surfaces of the continent. Total evaporation is equaled by total precipitation, of which about 24,000 cubic miles fall on the land surfaces.

Circulation of the earth’s atmosphere and moisture can be thought of as starting in the belt around the Equator. Because more of the sun’s energy is received near the Equator than farther north or south, greater heating occurs there and the result is greater evaporation and a tendency for the air to rise. The warm, moist air flows outward from the Equator at high altitudes and because of the earth’s rotation moves in a generally northeasterly direction.

Water that infiltrates into the soil is known as subsurface water. It may be evaporated from the soil, it may be absorbed by the plant roots, or it may be percolate downward to ground water reservoir that feeds springs, streams, and wells.

In the hydrologic cycle the principal functions of the ground water zones are as follows:

1. Zone of aeration – receives and holds water for plants use in the belt of soil water and allows the downward movement of excess water.

2. Zone of saturation – receives and stores, and provides a nature regulated discharge of water to wells, springs, streams.

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