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5.4. Distribution of water on Earth

Distribution of water on Earth

Water, on Earth, can be found in any of its physical states (frozen, liquid, and in the form of water vapor). Water is the basis on which all life on the planet is sustained, since all living beings are made up of water and need it to live. Humans, in addition to living, also need water for our activities, but it is not always available.

The hydrosphere is the discontinuous layer of water that is on the earth's surface or below it. The hydrosphere includes all surface water masses (liquid and solid), groundwater, and water vapor in the atmosphere.

Almost 3/4 of the Earth is covered by water. Although the amount of water in the hydrosphere is very large, only 2.5% is fresh. The rest, 97.5%, is found in seas and oceans, but it is salt water and cannot be used for drinking, agriculture or most human activities. And of the fresh water, much of it is in the polar ice caps and in groundwater, so there is only very little of Earth's water available to humans. 

Distribution of water in the hydrosphere
water situation Volume in km³ Percentage
Sweet water Saltwater fresh water total water
oceans and seas - 1,338,000,000 - 96.5
Polar ice caps and glaciers 24,064,000 - 68.7 1.74
salty groundwater - 12,870,000 - 0.94
fresh groundwater 10,530,000 - 30.1 0.76
continental glaciers and permafrost 300,000 - 0.86 0.022
fresh water lakes 91,000 - 0.26 0.007
salt water lakes - 85,400 - 0.006
Humidity of floor 16,500 - 0.05 0.001
Atmosphere 12,900 - 0.04 0.001
reservoirs 11,470 - 0.03 0.0008
Rivers 2120 - 0.006 0.0002
biological water 1120 - 0.003 0.0001
Total fresh/salt water 35029110km³ (2.5%) 1350955400km³ (97.5%) 100% fresh water
Total water on Earth 1386000000 km³ -

100% full water

Fuente (modificada): Wikipedia

Salt water: the water of the oceans and seas

Although pure water is only made up of hydrogen and oxygen (H2O), in nature we will always find it with some dissolved substance. Generally, the average salinity of ocean water is between 35 and 40 grams of salts per liter. The most abundant salt is sodium chloride (the one we add to food) or common salt, although there are more types of salts. These salts have been carried by rivers from the continents to the sea, and after many years, they have accumulated.

The salinity of the water is not the same in all the seas and oceans. Some sea stands out, such as the "Dead Sea", which has much higher concentrations of salts (almost 10 times more), which makes it more difficult for life to exist in that environment and for it to float much better.

Hombre flotando leyendo el periódico en el Mar Muerto

By Pete [GFDL or CC-BY-SA-3.0], from Wikimedia Commons

The oceans and seas are very important for life on Earth, since:

  • They are the environment in which many aquatic animal and plant species live.
  • They contain most of the existing water on Earth.
  • They regulate the climate of the different regions of the Earth.
  • It is a great tourist resource.
  • They are used as a means of transporting goods that sometimes have catastrophic accidents.
  • The salt water can be used after the desalination process.

Crossword: Salinity of the sea.

Fresh water: the water of the continents

The fresh water of the continents contains mineral salts of calcium, magnesium, etc., but much less concentrated than in salt water , between 0.5 and 3 grams per liter . It is said to be fresh water (it is tasteless) to distinguish it from salt water, which is salty.

Unlike salt water, fresh water can be found in a liquid state (rivers, lakes, groundwater,...) and in a solid state (ice from glaciers and polar ice caps). Therefore, we can find fresh water in:

  • Glaciers and polar ice caps. Most of the fresh water is frozen, forming part of the glaciers and polar ice caps. There are two types of glaciers:
    • Cap or polar or inland glaciers. They are large masses of ice that cover the land surface of the polar areas (GreenlandAntarctica, etc.). Here, when large blocks break off, icebergs are produced.
    • Valley or alpine glaciers. Although they are characteristic of the Alps, they are also found in other mountain ranges, such as the Himalayas or the Andes. They are large accumulations of ice that flow like a river of ice through a valley. The ice accumulates in the glacial cirque, the highest area, and descends through the valley forming the glacial tongue.
  • Groundwater. Rainwater, meltwater, and the water that goes through rivers and lakes, infiltrates into the ground through the pores and cracks forming groundwater. Groundwater constitutes the second largest reserve of fresh water on Earth.
  • Surface water. Although it is the most accessible freshwater, it is the scarcest freshwater reserve. It can be found at:
    • RiversWater currents that run along a fixed channel and a more or less constant flow. 
    • TorrentsWater currents that run through a fixed channel but its flow is intermittent, it only carries water when it rains or in times of thaw.
    • Lakes and lagoonsLakes are masses of water that accumulate in depressions on the earth's surface that are larger than lagoons. When these bodies of water are artificially created by a dam, it is called a swamp or reservoir.
    • WetlandsThey are areas in which groundwater outcrops that usually have great biodiversity.