The water cycle or hydrological cycle is the circulation of water on Earth changing between liquid, gaseous and solid states. In this circulatory movement, the water transits between the hydrosphere, the atmosphere, the lithosphere and the cryosphere..
This process is fundamental for life on earth because a large percentage cell is made up of water. In humans, 60% of the body is water, reaching 70% in the brain and 90% in the lungs..
The water cycle encompasses the entire body of planetary water, both surface and underground, in rivers, oceans, the air and in living beings. The most relevant properties of water for the hydrological cycle are its boiling point and freezing point..
The boiling point or temperature at which it goes from liquid to gas is 100ºC at sea level (decreases with height). While the freezing point or temperature at which water passes from a liquid to a solid state is 0 ºC.
Another outstanding property is its character as a universal solvent, since it is the liquid that dissolves the most substances (polar ions and molecules). Water, which is made up of two hydrogen atoms and one oxygen, has a positive pole (hydrogens) and a negative pole (oxygen).
In the water cycle this element goes through six stages: evaporation and transpiration, condensation, precipitation, runoff, infiltration and circulation. The energy that drives the water cycle is solar energy, and another fundamental force is gravity, which enables precipitation, runoff and infiltration..
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The stages of the water cycle are not strictly sequential, that is, not every water molecule necessarily goes through all of them at each turn of the cycle. The combination of all the stages forms a closed flow or cycle that includes the evaporation of water and its atmospheric circulation..
Subsequently, the water condenses and precipitates, circulates through rivers or accumulates in lakes and oceans, where new evaporation occurs. Another part runs off the ground, of this a portion evaporates and another infiltrates, accumulates or circulates underground..
On average, every 8 days all atmospheric water is renewed and every 16 to 180 days the water in rivers is renewed. In contrast, water in a lake or glacier remains for up to 100 years or more..
Evaporation is the transformation of water from a liquid to a gaseous state by increasing its temperature. This increase in temperature is the product of heating caused by solar radiation, mainly ultraviolet.
Likewise, the radiated heat (infrared radiation) by the earth and objects that are on its surface, contributes to the heating of the water..
The water evaporates when it reaches 100ºC or less depending on the atmospheric pressure. This gasification of water consists of the water molecules being charged with kinetic energy, increasing their movement and expanding the water..
As the molecules separate from each other, the water loses the coherence assigned to it by its liquid property and the surface tension is broken. Being lighter, the water transformed into gas rises into the atmosphere as water vapor.
In almost all cases the water in the oceans, rivers and in the soil does not reach 100 ºC, but evaporation occurs, because in a layer of water there are molecules that heat up more than others and break the surface tension , evaporating.
If the air is very dry (low relative humidity), the water molecules that manage to break the surface tension will tend to pass more easily into the air. If on the other hand there is wind, this will drag the layer of water vapor that accumulates on the water.
The highest rate of evaporation occurs in the oceans, where the rate of evaporation is seven times that of the land surface.
Of the water that infiltrates the soil, a part reaches the groundwater layer (saturated zone). While another portion heats up in its transit through the unsaturated zone and evaporates returning to the surface.
Plants need water for their metabolic processes, which they obtain from the soil in most cases. They do this through their roots and when they reach the leaves, and a part is used for the photosynthesis process.
However, about 95% of the water absorbed by plants is released into the environment in the form of water vapor in perspiration. Water vapor is released through the stomata in the foliar epidermis.
It is the passage of a gas to the liquid state, which occurs on a surface due to the decrease in temperature. As the temperature drops, the water molecules decrease their kinetic energy and bond more with each other to condense.
This process requires that there are particles to which the water adheres and the temperature of these particles must be lower than the saturation temperature of the water. Under these conditions, the dew point or dew temperature is reached, that is, the temperature at which water condenses..
The air rises when it is heated and in this process it drags the water vapor that is produced due to evaporation on the earth's surface. When it rises, its temperature decreases until it reaches the dew point and condenses.
Thus, small drops of water are formed that reach between 0.004 and 0.1mm in diameter, which are blown by the wind and end up colliding with each other. The accumulation of these condensation points forms clouds that, upon reaching their water saturation, generate precipitation..
If the temperature is very low, frost is generated, that is, a layer of scales or needles in small pieces of ice. This is produced by direct deposition of water vapor on a surface, not by precipitation.
Precipitation is the fall of condensed water in liquid or solid form from the atmosphere to the Earth's surface. As the condensed water accumulates in the atmosphere in the form of clouds, its weight increases, until it cannot avoid the force of gravity..
Rain is the precipitation of water in a liquid state, being very important since it distributes fresh water over the earth's surface. 91% of the water that precipitates returns directly to the oceans, 9% goes to the continental masses to feed the basins returning to the ocean.
If the temperature in the upper layers of the atmosphere is low enough, the condensed water crystallizes into snowflakes. As they increase in size and accumulate, they end up precipitating by the force of gravity and causing snowfall..
They are ice stones of 5 and 50 millimeters in diameter or even larger, which are formed around suspended material particles. When the ice accumulated around the particle reaches enough weight, it precipitates.
Precipitating water can fall directly onto a body of water (pond, river, lake, or ocean) or onto the ground. Likewise, bodies of water can overflow, that is, part of the water contained escapes from the containment limits..
This process by which a stream of water is generated from the overflow of a container or channel is called runoff. This is generated when the amount of water that precipitates or overflows the container is greater than the infiltration capacity of the soil.
Infiltration is the process by which water penetrates the soil through its pores and cracks. The infiltration rate or amount of water that manages to penetrate the interior of the soil in a given time depends on various factors.
For example, in a sandy soil with coarse particles that leave larger pores in each other, the infiltration will be greater. While in a clay soil, which has finer particles, the infiltration is less.
Soils are made up of different horizons or layers arranged one above the other, each with its own characteristics. There are soils whose surface horizon or horizon A is very permeable, while some of the lower horizons are less so.
If the infiltrated water meets an impermeable layer, it accumulates on it or circulates horizontally. This forms the underground water bodies or aquifers, which are of great importance as a fresh water supply..
The amount of groundwater globally is estimated to be 20 times that of surface water on Earth. This body of water is what maintains the base flow of the rivers and provides water to the plants..
The accumulated water in the subsoil can find ways of exit to the outside and form springs. That is, a natural source of water that springs from the earth forming ponds or rivers..
Much of the water is contained in the oceans, lakes, and underground reservoirs, or frozen at the poles or in high mountains. However, a relevant portion is in permanent circulation, giving dynamics to the water cycle..
Temperature differences between points in the Earth's atmosphere generate air mass displacements. These displacements in turn cause atmospheric pressure differences and winds are produced that carry the water vapor.
Masses of hot air rise from the earth's surface towards the upper layers of the atmosphere. Likewise, the air moves horizontally from areas of high pressure to areas of low pressure..
In the oceans, the water is in constant circulatory movement, forming marine currents. These are determined by the movements of rotation and translation of the Earth..
The water that precipitates on the mountains runs downhill due to gravity following the contour lines of the terrain. In this process a channel is formed by the erosive effect of the water itself and this is channeled through it. In this way, water courses are formed that can be temporary or permanent..
Part of the water that precipitates on the earth does not circulate, because it is immobilized in the form of ice. In seawater the freezing point is below 0 ° C due to the high content of salts (generally -2 ° C).
On the other hand, if there are no particles to which the water clings, its freezing point decreases to - 42 ºC..
Living beings require water to live, in fact living cells are made up of a high proportion of water. Water, being a universal solvent, and being able to dissolve a large amount of solutes, is essential in cellular biochemical reactions.
The water cycle, through precipitation and through rivers, lakes and underground aquifers, supplies the water required for life. Primary production through photosynthesis is the process that guarantees the transformation of solar energy into useful energy for life.
Photosynthesis is not possible without water, both in the case of plankton (aquatic organisms) and in terrestrial plants.
The masses of water existing on Earth, as well as their circulation in the hydrological cycle, are a thermal regulator. The high specific heat of water allows it to gradually absorb heat and also gradually release it.
In the same way, living beings regulate their body heat by transmitting it to body water and losing it through perspiration..
When water evaporates, it frees itself of pollutants and dissolved salts, so that when it precipitates, it is fresh and relatively pure water. However, there are polluting gases and particles in the atmosphere resulting from human activities that can affect its quality..
The water cycle determines or contributes to the existence of a series of climatic phenomena such as rain, snowfall and hailstorms. In the same way, it determines the appearance of fog, the periodic flooding of rivers or temperature variations on the earth's surface.
The water cycle also has certain negative effects for humans, such as leaching, erosion and socio-natural disasters..
It consists of the washing or dragging of the nutrients present in the soil due to the solvent effect of the infiltrating water. In agricultural soils with little capacity to retain nutrients, this phenomenon causes soil impoverishment..
It is the loss of soil or rock wear due to the mechanical action of wind or water. Runoff water has a high erosive power of soil and rocks, depending on the structural and mineralogical characteristics of these.
In bare soils with steep slopes located in areas of high rainfall, erosion is high. The loss of soil due to this cause has a high economic impact on food production.
Torrential rains, as well as heavy snowfalls and heavy hailstorms can cause major negative impacts on human structures and communities. Similarly, the overflowing of rivers and the rise in sea level generate flooding of populated areas and cultivation areas..
The human being, with his actions, alters natural cycles and favors such disasters such as global warming or the construction of facilities in high-risk areas..
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