The water cycle or hydrological cycle
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.
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.