ESO 1 Science 4

The Water and the Hydrosphere

Key Information

The Water Cycle
What is it?

The water cycle, also known as the hydrological cycle, is the circulation of water between the different compartments or reservoirs of the Earth's Hydrosphere, involving changes in the physical state of water between liquid, solid, and gaseous phases. The water cycle is powered by the Sun's energy and the Earth's gravity.

The Earth's water cycle involves the following main physical processes:
EvaporationIs the transfer of water from bodies of surface water into the atmosphere. This transfer involves a change in the physical state of water from liquid to gaseous phases, powered mainly by the solar radiation. 90% of atmospheric water comes from evaporation.
EvapotranspirationIs the transfer of water from living beings into the atmosphere. This transfer involves a change in the physical state of water from liquid to gaseous phases, powered mainly by the solar radiation and the heat released by the metabolism of the living beings. 10% of atmospheric water comes from evapotranspiration.
CondensationIt takes place when water vapour in the air accumulates to form liquid water droplets in clouds and fog.
PrecipitationIs atmospheric moisture that has previously condensed (or solidified), falling to the surface of the Earth. This happens mostly as rainfall, but also as snow, hail, or fog.
Surface runoffIncludes the variety of ways by which land surface water moves down slope to the oceans: snowmelt runoff to streams, streamflow, riverflow… Water flowing in streams and rivers may be delayed for a time in lakes. Much of the precipitated water evaporates before reaching the ocean or infiltrates into the soil.
InfiltrationIs the transition of land surface water into the ground. The infiltration rate depends on soil or rock permeability. Infiltrated water may become part of the soil moisture or accumulate in aquifers: in this case it is called groundwater.
Groundwater flowIncludes the movement of groundwater in aquifers. Aquifers tend to move slowly, so the water may return as surface water (into rivers, lagoons, oceans or through springs) after thousands of years in some cases. Water returns to the land surface at lower elevation than where it infiltrated.
Absorption or drinkingAre the ways in which soil moisture or surface water is taken in by living beings.
Volume of water stored in the water cycle's reservoirs:
Volume (106 km3)Percent of total
Seas and oceans137097.25
Ice caps, glaciers and snow covers292
Soil moisture0.0650.005
Streams and rivers0.00170.0001
Living beings0.00060.00004
Average reservoir residence times:
Groundwater: deep10,000 years
Seas and oceans3,200 years
Groundwater: shallow100 to 200 years
Lakes50 to 100 years
Ice caps and glaciers20 to 100 years
Streams and rivers2 to 6 months
Seasonal snow covers2 to 6 months
Soil moisture1 to 2 months
Atmosphere9 days

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