The hydrologic cycle.

Water.

Water is an important part of nature, which surrounds us. Water is more essential to a man’s life than food. There is no life without water. The great importance of water was realized by our remote ancestors. That’s why their settlements were always situated on the banks of rivers and lakes. The location of human settlements and the well - being of the people depended on the distance from the source of water.

Man can live as long as ninety days or more without food, but we cannot live long without water.

Where can water be found?

Everywhere. The seas and oceans, rivers and lakes cover about seventeenths of the Earth’s, surface.

Where else can you find water?

Water is also contained in the soil, in the atmosphere and in all living things more than half of the human body consists of water. Water also forms a large part of the food we eat, especially vegetables and fruit.

Water exists as a substance in three states: ice, liquid water, steam. Ice melts at 0˚ degree Centigrade. Steam is formed at 100 ˚C.

Water is one of the most important of all chemical substances. It is a major constituent of living matter and of all environment in which we live.

The formula of water is H₂O. The constituent parts of water are hydrogen and oxygen.

Water was thought by the ancients to be an element. H. Cavendish in 1701 showed that water is formed then hydrogen is burnt. And A. Lavoisier first recognized that water in a compound of the two elements- hydrogen and oxygen.

Water differs from all other liquids i.e. its physical properties are strikingly different from these of other substances.

Water expends when cooled from 0 ˚С. It contracts when heated from 0 to 4˚С. Water reaches its maximum density at 40 ˚С.

Water is life.

Water constitutes 70% of the weight of human body. It’s as necessary to a man as air? Which he breathes, and even more essential to his life than food.

We can live without food for several weeks but we must die in a few days if we have no water to drink.

Water is everywhere. It is one of the most widely spread and important substances known to us. The clouds are nothing but little drops of water. The air we breathe contains water in the form of invisible vapour. Without water there would be neither animal nor vegetable life. The soil under normal conditions contains water, which as we all know is necessary for the growth of plant. The plants themselves contain large quantities of water, in fact, most of the vegetables we use contain much water. Some of them, such as cucumbers and tomatoes contain as much as 95% of water. Milk is about 87% water.

Water is life. All the life on earth depends on water.

Water passes through very interesting natural cycle. The atmosphere, which surrounds the surface of the Earth contains water. It varies in amount in direct proportion to the temperature of it’s gases. Water is also evaporated into atmosphere. Atmosphere, which has become saturated with water, precipitates it’s moisture when the temperature lowers.

This phenomenon is termed rainfall.

The moisture falls to the earth and find it’s way into a number of reservoirs, provided by nature.

From the water surfaces of the earth the sun is constantly evaporating enormous volumes of water. Then they are condensed and precipitated upon the land and water surfaces as rain show or ice. Some of water is evaporated again; some is absorbed by growing vegetation; some flows directly over the ground into the rivers and lakes; some sinks into the soil to become the subterranean water of the earth.

As the descending rain or snow approaches the earth, it washes out of the atmosphere suspended particles of dust and thus purifies the air.

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.