WATER OR HYDROLOGICAL CYCLE: STAGES AND IMPORTANCE - ENVIRONMENT - 2025

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 of cells 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 mass 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 100 ºC 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 atoms, 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.

Stages of the water cycle

Water cycle. Source: Malama 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.

Later, 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.

1- Evaporation and perspiration

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.

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 a gas rises into the atmosphere as water vapor.

Temperature, relative humidity and wind

In almost all cases, the water in the oceans, rivers and in the ground 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 evaporation rate is seven times that of the earth's surface.

Edaphic evaporation

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.

Perspiration

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. The water vapor is released through the stomata in the foliar epidermis.

2- Condensation

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.

Drops of water due to condensation. Source: Nicole López This process requires that there be 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.

Cloud formation

Cloud formation Source: Arun Kulshreshtha The air rises when it is heated and in this process carries away the water vapor that is produced due to evaporation on the earth's surface. When it rises, its temperature drops to the dew point and condenses.

Thus, small drops of water are formed that reach between 0.004 and 0.1mm in diameter, which are carried 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.

Frost

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.

3- Precipitation

Precipitating rain. Source: Cassini83 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

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.

Nevada

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.

Hail

They are ice stones 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.

4- Runoff

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 as a result of the overflowing 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.

5- Infiltration

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 into 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.

Soil layers

Soils are made up of different horizons or layers arranged one on top of the other, each with its own characteristics. There are soils whose surface horizon or horizon A is highly 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.

Springs

The water accumulated in the subsoil can find ways out to the outside and form springs. In other words, a natural source of water that gushes out of the earth forming ponds or rivers.

6- Circulation

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.

Air currents

The differences in temperatures between points of the Earth's atmosphere generate displacements of air masses. These displacements in turn cause differences in atmospheric pressure 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 high pressure areas to low pressure areas.

Ocean currents

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.

Rivers

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 it is channeled through it. In this way, water courses are formed that can be temporary or permanent.

Water freezing

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 drops to - 42 ºC.

Importance of the water cycle

Vital fluid

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.

Different phases of water. Source: BE 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.

Regulation of temperature

The existing water masses 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.

Similarly, living beings regulate their body heat by transmitting it to body water and losing it through perspiration.

Water treatment

When the water evaporates, it frees itself of pollutants and dissolved salts, so 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.

Climatic events

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 floods of rivers or variations in temperature on the earth's surface.

Negative effects

The water cycle also has certain negative effects for humans, such as leaching, erosion and socio-natural disasters.

Leaching

It consists of the washing or dragging of the nutrients present in the soil due to the solvent effect of the water that infiltrates. In agricultural soils with low nutrient retention capacity, this phenomenon causes soil impoverishment.

Erosion

It is the loss of soil or rock wear as a result of the mechanical action of wind or water. The runoff water has a high erosive power of the 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.

Socio-natural disasters

Torrential rains, as well as heavy snowfalls and heavy hailstorms can cause major negative impacts on human structures and communities. In the same way, the overflowing of rivers and the rise in sea level generate floods in populated areas and cultivation areas.

The human being, with his actions, alters natural cycles and causes such disasters such as global warming or the construction of facilities in high-risk areas.

References

1. Calow, P. (Ed.) (1998). The encyclopedia of ecology and environmental management.
2. Margalef, R. (1974). Ecology. Omega editions.
3. Ordoñez-Gálvez, JJ (2011). Hydrological cycle. Technical primer. Geographical Society of Lima.
4. Sterling, TM and Hernández-Rios, I. (2019). Transpiration - Movement of Water Through Plants. Plant and Soil Sciences eLibrary. Print Lesson.
5. Vera, C. and Camilloni, I. (s / f). The water cycle. Explore. Multimedia training program. Ministry of Education, Science and Technology.