Water cycle

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Diagram of the water cycle

The term water cycle means the transport and storage of water on a global and regional level. The water changes its state of aggregation several times and passes through the individual spheres such as the hydrosphere , lithosphere , biosphere and atmosphere of the earth . The circulation of water usually takes place between the sea and the mainland . No water is lost in the water cycle , it only changes its condition. These states are represented by the water balance variables and consequently balanced in the water balance.

General scheme of a water cycle

Scheme of the water cycle
Video: water cycle

The oceans are the largest reservoirs of water on earth, they cover most of the earth's surface. Solar energy heats the water. Through evaporation , especially at the sea surface, to a lesser extent on the mainland, creates humidity . The wind transports the moist air to the mainland. When the humid air meets cold air layers, it pushes itself over them and rises ( warm front ), as well as when it hits mountain slopes or is warmed up over warm ground ( convection ). When the air rises, it cools down. Cold air can absorb less water vapor than warmer air, which is why the water vapor contained in the air condenses from a certain height. This creates clouds . If the water droplets are large enough, precipitation occurs and the water falls back to the earth in the form of rain, snow or hail. The form of precipitation depends on the temperature. If the precipitation falls directly into the water, the cycle closes and can start all over again. If the water falls on the ground, it seeps into the groundwater . It then flows into the oceans via the groundwater flow or via springs and rivers . Meltwater from glaciers and snow and rainwater discharged from the surface are also transported into the oceans via rivers. In the polar regions and in high mountains , part of the precipitation is stored in solid form as ice , where it is returned to the oceans via meltwater.

Basic equation of the water cycle

The water cycle can be described in a very simplified way using the following basic equation:

The entry by rainfall N resulting in outflow A and evaporation V . The representation used here is only a simplified example of a water balance equation .

Water on other celestial bodies

According to some scientists, water is very likely to be found on other celestial bodies in the solar system such as Mars .

The water cycle is generally seen as a prerequisite for life .

history

The cycle character of water was recognized or at least guessed at early on. The related principle is panta rhei ” - everything flows . The bible also refers to the water cycle several times or describes extreme rainfall during the flood .

Various theories on the water cycle and its drive:

  • Meteorogenic water cycle: Theory of the water cycle according to today's understanding, which was probably founded by Xenophanes and has Diogenes of Apollonia and Hippocrates of Kos as early representatives.
  • Theory of salt water ascent: reversal of the flow direction of the meteorogenic water cycle after the earth rests on the water of the seas and this feeds all above-ground sources. This theory was founded by Thales and had Hippon and Plato as early representatives.
  • Theory of water formation from the air: The water arises from the air (effect of condensation ) and would feed the sources of the rivers through precipitation. This theory was developed by Aristotle and was the mainstream doctrine well into the early 17th century.

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