The Water and the Hydrosphere

What to Learn

The water in the Earth: occurrence, water bodies, natural aqueous mixtures vs. distilled water, continental waters vs. marine waters.
The water cycle and its reservoirs.
Seawater movements.
The molecule of water.
The density of water.
Water and life: the roles of water in living beings and the adaptations of living beings with regards to water.

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:

Evaporation	Is 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.
Evapotranspiration	Is 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.
Condensation	It takes place when water vapour in the air accumulates to form liquid water droplets in clouds and fog.
Precipitation	Is 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 runoff	Includes 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.
Infiltration	Is 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 flow	Includes 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 drinking	Are 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 oceans	1370	97.25
Ice caps, glaciers and snow covers	29	2
Groundwater	9.5	0.7
Lakes	0.125	0.01
Soil moisture	0.065	0.005
Atmosphere	0.013	0.001
Streams and rivers	0.0017	0.0001
Living beings	0.0006	0.00004

Average reservoir residence times:

Groundwater: deep	10,000 years
Seas and oceans	3,200 years
Groundwater: shallow	100 to 200 years
Lakes	50 to 100 years
Ice caps and glaciers	20 to 100 years
Streams and rivers	2 to 6 months
Seasonal snow covers	2 to 6 months
Soil moisture	1 to 2 months
Atmosphere	9 days

External Links

Miscellaneous

Wikipedia: water portal.All the information about water in the Wkipedia organized in one single place, with links, news, ideas and more. Also check the "Category:Water" link in the "Categories" section to access all the articles in the Wikipedia related to water.

Interactive questions and answers game.A flash animated game with 25 questions about water. You have to choose from three possible answers, and the game lets you know instantly if you have the right answer.

The Water in the Earth

The Hydrosphere in Geography4kids.com.Freshwater, groundwater, seawater, aquatic biomes and more.

The Water Cycle

Interactive water cycle.A flash animated activity that lets you control the water cycle as you learn.

Seawater Movements

The great oceanic conveyor belt.A very clear introduction to how marine currents work.

The Molecule of Water

5 really weird things about water.The most important molecule for Life is also the one that displays the most strange behaviour and properties. Learn about some of them.