The Earth is made of 0,2 % of water, but its cycle in nature is one of the most important systems in the World. "Water cycle" (well known among scientists as hydrologic cycle) signifies a continual process of water exchange in hydrosphere, between atmosphere, surface and underground water, soil and vegetable world.
22 % of solar radiation, which reaches the Earth, heats the water surface of the oceans, seas, lakes and rivers and in that way transforms water in vapor. Releasing of vapor into the atmosphere is done through the plants transpiration. Free vapor billows, with an air-current in the higher parts of the atmosphere, where vapor condenses and forms the clouds. In this way formed water volumes return down on land as precipitation. This water cycle continues again and again.
One part of water on the Earth is, relatively for a long period, captured in ice and glaciers. This ice covers 10-11 % of the continent. The water level of the sea will increase for 70 m, until all the glaciers of the world have melt. During the last ice age, the sea level was about 122 m lower than today’s level and the glaciers covered almost 1/3 of the continent.
One part of the hydrologic cycle, which we can see every day, also represents fresh water which exists on the Earth as morasses, lakes, accumulations and rivers.
Moreover, some definitions of evapotranspiration also include perspiration from water bodies, evapotranspiration can be defined as water dissipation from the land surface into the atmosphere and transpiration of underground water over plants.
Transpiration is a process of moisture transfer through the plant, from root to little pores on the leave’s bottom, where the water traverses into vapor and goes into the atmosphere. Therefore, transpiration is water evaporation from the leaves of plants. It is pondered that about 10 % of moisture in atmosphere has been released from plants during transpiration.
Condensation, as a contrast to evaporation, represents an air process of transforming the vapor into the liquid form. Condensation is very important for the hydrologic cycle, because it is responsible for the clouds forming and precipitation. Evan though, when there are no clouds in the crystal clear sky, water is still present in a form of a water vapor and drops, which is invisible to our eyes. In the atmosphere, the clouds are formed, because the air, which consists of vapor, raises, cools down and condenses.
Water exists in atmosphere in a shape of vapor, like clouds and humidity. Although atmosphere cannot be a big water reservoir, it is a remarkable "super-highway" for the water moving around the planet. It is beleived that the water volume in atmosphere, in any moment, is around 12 900 km2 and if this water plumps at once on the Earth like rain, it will cover the land area with a 2,5 cm of depth.
Atmosphere circulation on every Earth hemisphere consists of three cells. On equator, the hot air boosts, moving to the north and south and falling in tropic areas. Cold air falls above the poles, and then comes to the mid geographical amplitude
Precipitation represents the water, which is released from the clouds in a shape of rain, ice rain, sleet, snow and ice and this is a basic form of water returning from the atmosphere to the Earth. The clouds consist of vapor and drops, which are too small to be reached like precipitation, but they are big enough to form visible clouds. As far as the precipitation is concerned, it is necessary, in the first place, for water drops to condense into the particle dust, salt or smoke. Then, the drops bump and magnify enough to fall on the earth. Million of drops are necessary to form just one raindrop.
Every year about 40 000 km3 evaporate from the oceans and fall into the land. This annual running is faster than the running of the Amazon and in a theoretical view, this is enough for the water supply of the population that is five times larger than the current one.
LOCAL WATER CIRCULATION
Many connected cycles make the general process of water cycle. Water moving in nature is qualified by many factors such as volume and type of participation, relief, geological composition. Infiltration of water from the terrain surface is done in porosity or permeable rocks, but is not done in impermeable types of soil. Infiltration represents water penetration from the terrain surface into the depth. Anywhere in the world, the water part that is falling as rain infiltrates in underground layers. The volume of infiltrated water stays in a callow of soil, where this water can be discharged into the river, with a penetration through the riverside.
The large volume of water accumulates in soil. Here, water is still moving, but probably very slowly and it is still a part of hydrologic cycle. The big parts of this water originate from precipitation. The top layer of the soil is an unsaturated zone, where the water present in quantities changes with time, but the soil stays unsaturated. Beneath this layer, there is a saturated zone where all pores, fractures and spaces between particles of rocks are filled with water.
Discharge of underground waters represents water projection on the land surface. Participations do not only recharge aquifers - one part ranges to the impermeable rocks and starts to move in horizontal directions. A segment of this water will distil on land surface, in the rivers and oceans. Water which moves beneath the land surface, depends on permeability (how much it is difficult for water to move through the rocks) and porosity (the size of opened pores in materials) of underground rocks. If the rocks enable water to move through them, then the underground waters can move very far per day.