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5.5. The water cycle

The water cycle or hydrological cycle

Although the amount of water present on Earth is practically constant, it is not always in the same medium. Sometimes it rains, then it flows through the rivers, it evaporates, etc.

The water cycle or hydrological cycle describes the path of water from the oceans, its main reservoir, to the atmosphere, and then its accumulation in lakes, rivers and groundwater, returning again to the oceans. In the hydrological cycle, water passes through three states: solid, liquid and gas.

The thermal energy from the Sun is the force that allows the evaporation of water and the cause of the winds. The action of gravity causes water to return from the atmosphere to the ground through precipitation in the form of rain and snow, and to flow into rivers and oceans.

The water is distributed approximately in this way:

  • 97%: in oceans.
  • 2%: on ice.
  • 1%: liquid fresh water.

Distribución del agua en la Tierra

Therefore, most of the evaporated water leaves the oceans, just as it is the medium that receives most of the precipitation, since it occupies 71% of the total surface of the Earth.

Water falling on land can follow three different paths:

  • Some evaporates directly or through transpiration from plants and animals (evapotranspiration). Although water constitutes 70% of living beings, it is a minimal part of the water in the hydrological cycle.
  • Part infiltrates into permeable ground as groundwater, forming aquifers.
  • Part circulates by runoff through streams and rivers towards the sea.

Public Domain,

The water cycle presents the following processes:


The evaporation of water is produced by the heat energy of the Sun. Evaporation occurs in oceans and on continents, passing water from a liquid state to a gaseous state. In this way, the water becomes part of the atmosphere in the form of water vapor.


Condensation occurs when water vapor rises to the upper layers of the troposphere. As the temperature decreases, the air cannot contain as much water in a gaseous state (water vapour) and condensation occurs, forming small drops of water (liquid or even ice) that form clouds.


The small droplets of the clouds grow as the air mass cools until the precipitation of water in the form of rain, snow or hail occurs. The snow and ice deposited in the mountains will slowly melt, providing water that will flow into the rivers in warmer times.


When the precipitation of water on the ground occurs, part of it infiltrates into permeable rocks, becoming part of the aquifers, where large amounts of water accumulate.

An aquifer  is a permeable rocky ground in which groundwater accumulates and circulates.


When the precipitation of water occurs, a large part circulates on the surface as wild waters and is channeled in torrents and rivers until it reaches the sea due to the action of gravity.


Part of the water that has precipitated is absorbed by the roots of the plants and returned to the atmosphere by evaporation through their leaves through transpiration. Perspiration also occurs in animals.

Therefore, the water vapor in the atmosphere (water in a gaseous state) comes from the evaporation of oceanic and continental waters and from the transpiration of plants.

Musical minutes

Listen to the song " My little yellow water ". What do you think of what we have seen in class? Is there an error in what is said?

…A warm and warm yellow water comes out of me

And it goes down a pipe,
it goes under your house,
it goes under your family,
it goes under your workplace,
my little yellow water, my little yellow water.

And it reaches a river,
the shepherd drinks it,
the cows drink it, it
waters the fields,
my little yellow water, my little yellow water.

And go down to the sea,
play with the little fish,
play with the squid,
play with the jellyfish
and the hake
that you eat.
my little yellow water, my little yellow water,
my little yellow water, my little yellow water.

The sun warms up my little yellow water,
puts it at a hundred degrees,
sends it up,
travels through the sky,
arrives in your city
and starts pouring.

Get wet in the streets,
get your father wet,
your mother washes the dishes
with my little yellow water.
My little yellow water wets the schoolyard, my little yellow water
wets the town hall ...

Consequences of the water cycle

The water cycle is essential for life on our planet, since it allows it to maintain its balance as it is related to the four layers of the Earth:

Relationship of the water cycle with the atmosphere

The water in the atmosphere is in the form of vapour. If the atmosphere contains a large amount of water vapor, large temperature variations do not occur, keeping the temperature fairly constant.

In the atmosphere, the condensation of water occurs to form clouds that, later, will give rise to precipitation.

Relationship of the water cycle with the geosphere

Water, together with the rest of external geological agents, is fundamental in the modeling of terrestrial reliefs. For example:

  • Wild or creek waters. Rainwater, before being channeled, is an important erosive agent, especially in areas with scarce vegetation.
  • torrential modeling. Torrents, although of intermittent action, are also important in arid and mountainous areas.
  • River Modelling. The rivers and their fluvial networks are in charge of modeling the land through which they circulate.
  • Karst modeling. Groundwater. Groundwater in contact with soluble rocks (limestone, gypsum) generate caves, other cavities and deposits.
  • Coastal modeling. The action of the sea, through waves and tides, creates characteristic coastal landscapes.
  • Glacier modeling. The ice deposited in the high mountains flows through the valleys dragging rocks and creating a characteristic relief.

Relationship of the water cycle with the biosphere

Living things are mostly made up of water. Therefore, water is essential for them.

Human beings also use water as a resource for numerous activities. We must remember that water is a limited resource and that, if it is not used in a rational way, it can become depleted.