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Water cycle

From Tilcayo Cats, the lean encyclopaedia by Tilcayo Knowledge Systems
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The water cycle, also called the hydrological cycle, is the continuous movement of water between the Earth's surface and the atmosphere. As it moves, water changes between gas, liquid and solid.[1][2] The water cycle is powered by energy from the Sun and by the Earth's gravity.[2][2]

Where the Earth's water is

The hydrosphere is all the water on or near the Earth's surface. It includes water vapour in the atmosphere, liquid water on the surface and underground, and ice.[3][3] The oceans cover about 71 per cent of the Earth's surface and hold about 97 per cent of its water. Only about 2.5 per cent is fresh water, and three-quarters of that is ice.[3][4] Only a tiny fraction, about 0.024 per cent of all the Earth's water, is fresh liquid water that is easy to reach.[5]

Main processes

The three basic processes of the water cycle are evaporation, precipitation and runoff.[6]

Evaporation

Evaporation is the change of liquid water into a gas, which then spreads into the atmosphere.[7] Energy from the Sun causes water in the oceans, lakes, rivers, soil and plants to evaporate.[2] Evaporation needs liquid water, a supply of energy, and air that is dry enough to take up more water vapour. It stops when the air is saturated.[8][9] Plants also give off water vapour through openings called stomata. This is called transpiration.[10][11] Most evaporation and precipitation happen over the oceans. The oceans lose more water by evaporation than they gain from precipitation, while over land the reverse is true.[12][6]

Condensation and precipitation

Most water vapour rises into the atmosphere, where it condenses into droplets in clouds.[2] Air can only hold a certain amount of water vapour, and that amount depends on temperature. When moist air cools, some of the vapour condenses, which is why droplets form on the outside of a cold glass.[13][14] As air rises, it expands and cools.[14] Gravity then pulls the water back to the surface as precipitation, such as rain, snow, sleet or hail.[2][11]

Some places get much more precipitation than others. Warm, moist air rises near the Equator, cools and drops its water as rain, so rainfall there is very high. Sinking air makes the regions around 30° north and south of the Equator quite dry.[15][15] When moist winds are forced up over mountains, they cool and drop rain or snow. This leaves a drier "rain shadow" on the far side.[16]

Runoff and groundwater

Most precipitation that falls on land becomes surface runoff. It flows over the land into streams, rivers, lakes, wetlands and the sea.[5][5] Some water soaks into the soil, where plants can use it. Water that seeps deeper is called groundwater.[5][5] It collects in aquifers, underground layers of sand and rock that hold water.[5] Groundwater lies below the water table, where the ground is saturated with water.[17] Eventually runoff and groundwater flow back to the oceans, completing the cycle.[12]

In temperate climates, about a third of precipitation evaporates, about a third becomes surface runoff and about a third soaks down to become groundwater. In dry regions, evaporation takes a much larger share.[6]

Stores

Water is held for a time in stores such as the soil, groundwater, snow and ice, lakes and reservoirs.[18] Most fresh water is stored, mainly as snow, ice and groundwater. It moves slowly and can easily be polluted.[18] Hydrologists keep track of how much water enters and leaves an area using a water balance equation. It relates precipitation, evaporation, runoff and the change in storage.[19]

Human effects

People change the water cycle in three main ways. They take fresh water faster than it is replaced. They clear vegetation and cover land with paving, which increases runoff and reduces the water that soaks into the ground. They also drain wetlands, which help to control flooding.[5] Water running off farmland can also carry fertiliser into rivers and lakes, causing algal blooms.[20]

See also

References

  1. ↑ Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. p. 8. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Miller, G. Tyler; Spoolman, Scott E. (2019). Environmental Science (16th ed.). Boston, MA: Cengage Learning. p. 56. ISBN 978-1-337-56961-3. {{cite book}}: |access-date= requires |url= (help)
  3. ↑ 3.0 3.1 3.2 Miller, G. Tyler; Spoolman, Scott E. (2019). Environmental Science (16th ed.). Boston, MA: Cengage Learning. p. 47. ISBN 978-1-337-56961-3. {{cite book}}: |access-date= requires |url= (help)
  4. ↑ Begon, Michael; Howarth, Robert W.; Townsend, Colin R. (2014). Essentials of Ecology (4th ed.). Hoboken, NJ: Wiley. p. 108. ISBN 978-0-470-90913-3. {{cite book}}: |access-date= requires |url= (help)
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Miller, G. Tyler; Spoolman, Scott E. (2019). Environmental Science (16th ed.). Boston, MA: Cengage Learning. p. 57. ISBN 978-1-337-56961-3. {{cite book}}: |access-date= requires |url= (help)
  6. ↑ 6.0 6.1 6.2 Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. p. 9. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  7. ↑ Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. p. 49. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  8. ↑ Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. pp. 49–50. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  9. ↑ Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. pp. 50–51. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  10. ↑ Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. p. 50. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  11. ↑ 11.0 11.1 Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. p. 11. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  12. ↑ 12.0 12.1 Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. pp. 8–9. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  13. ↑ Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. pp. 20–21. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  14. ↑ 14.0 14.1 Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. p. 21. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  15. ↑ 15.0 15.1 Begon, Michael; Howarth, Robert W.; Townsend, Colin R. (2014). Essentials of Ecology (4th ed.). Hoboken, NJ: Wiley. p. 94. ISBN 978-0-470-90913-3. {{cite book}}: |access-date= requires |url= (help)
  16. ↑ Begon, Michael; Howarth, Robert W.; Townsend, Colin R. (2014). Essentials of Ecology (4th ed.). Hoboken, NJ: Wiley. p. 95. ISBN 978-0-470-90913-3. {{cite book}}: |access-date= requires |url= (help)
  17. ↑ Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. p. 86. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  18. ↑ 18.0 18.1 Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. p. 69. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  19. ↑ Davie, Tim; Quinn, Nevil Wyndham (2019). Fundamentals of Hydrology. Routledge Fundamentals of Physical Geography (3rd ed.). Abingdon: Routledge. pp. 12–13. ISBN 978-0-415-85870-0. {{cite book}}: |access-date= requires |url= (help)
  20. ↑ Miller, G. Tyler; Spoolman, Scott E. (2019). Environmental Science (16th ed.). Boston, MA: Cengage Learning. p. 60. ISBN 978-1-337-56961-3. {{cite book}}: |access-date= requires |url= (help)
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