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Evapotranspiration describes two processes in the hydrologic cycle.

  • Water loss from the soil-plant system, through soil evaporation
  • Plant transpiration is the sum of these processes.

Temperature is a significant factor controlling evaporation and higher temperature speed rate of evaporation. Other factors make an impact on the scale include the amount of water already help by air (humidity) and air movement. Furthermore, high wind speed and low humidity accelerate evaporation. Transpiration is the loss of water vapour primarily through the stomata (stomata is microscopic pores in plants’ surface layers) of green plants and leaves.


Plants over ground have a cover of wax cuticle that helps prevent water loss. It keeps the internal part of the plant almost concentrated with water vapour and is an essential element of a process called photosynthesis. In the daytime, the stomata open to take in carbon dioxide. The water inside the leaves transpires or evaporates and escapes the plant. First, the transpiration cools the plant leaves. Secondly, allowing the photosynthetic apparatus to produce at the optimum energy level. The type of energy is carbohydrates. Decreasing rates of transpiration result of global warming. The local drying affects photosynthesis.

In Cottonwoods near Dallas, Texas, can transpire up to 120 litres of water per tree per day. In the Amazon rainforest, a single large tree can transpire in a day as much as 1,180 litres of water. In a worst-case scenario Amazon, rainforest could be transformed into a Savannah if many trees are cut down and there remains finally little evapotranspiration to sustain regular rainfall. Thorn Thwaite (1937) pioneered modelling of the hydrologic system to derive measures of evapotranspiration and soil moisture and their effects on both plants and crops.

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