Evapotranspiration

Figure 1. The water cycle relies on evapotranspiration.[1]

Evapotranspiration is the transfer of energy from the Earth's surface to the atmosphere in the form of latent heat, due to the evaporation of water from the ground and bodies of water, and the transpiration of water from plants. The transpiration aspect of evapotranspiration accounts for about 10% of the moisture in the atmosphere, with evaporation from oceans and other bodies of water accounting for nearly 90% and a tiny amount coming from sublimation (ice changing into water vapour without first becoming liquid).[2]

Transpiration is essentially evaporation of the water from plant leaves.[2] Plants must transpire just like humans have to breathe, and this process depends on many environmental factors.

Evapotranspiration is an important part in the Earth's hydrologic cycle. About 30% of the total ocean volume evaporates each year. Wind carries this water vapour to higher elevations where it cools and condenses, and finally precipitates. About 76% of it goes right back to the ocean, and the rest hits land and drives the life cycles there.[3]

Evapotranspiration is a key factor in Earth's heat balance. The radiant heat coming from the Sun must be rid of by the Earth in order for its temperature to remain constant, and this is partly done by evapotranspiration. This happens because it requires energy to change a substance from liquid to gas, so as the water on the Earth evaporates and transpires it uses some of this solar energy.[4] The contribution of evapotranspiration to Earth's energy budget is displayed in Figure 2 below.

Figure 2. Detailed diagram showing incoming and outgoing energy in a balanced example of Earth's energy budget, with incoming and outgoing values of 341 W/m2. Evapotranspiration accounts for 80 W/m2 of the outgoing energy.[5]


References

  1. Wikimedia Commons [Online], Available: http://en.wikipedia.org/wiki/Evapotranspiration#/media/File:Surface_water_cycle.svg
  2. 2.0 2.1 USGS, Evapotranspiration - The Water Cycle [Online], Available: http://water.usgs.gov/edu/watercycleevapotranspiration.html
  3. S. Marshak, "The Hydrologic Cycle" in Earth Portrait of a Planet, 3rd ed., New York: W.W. Norton & Company, Inc., 2008, ch.F5, pp. 551
  4. R. Wolfson, (May 8, 2015). Energy, Environment and Climate, 2nd ed. New York, U.S.A.: Norton, 2012.
  5. Created internally by a member of the Energy Education team. Adapted from: R. Wolfson, Figure 12.5 in Energy, Environment and Climate, 2nd ed. New York, U.S.A.: Norton, 2012, pp. 331

Authors and Editors

Jordan Hanania, Kailyn Stenhouse, Jason Donev
Last updated: January 8, 2017
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