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Evapotranspiration is the term used to describe two processes in the hydrologic cycle (see below); Water loss from the soil–plant system, through soil evaporation and plant transpiration, is the sum of these processes. Temperature is a major factor controlling evaporation and higher temperature mostly speed the rate of evaporation. Other factors that make an impact on the rate 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 (all above ground have a cover of wax cuticle that helps prevent water loss. This keeps the internal part of the plant almost concentrated with water vapor and is an important element of a process called photosynthesis. In the daytime, the stomata open to take in carbon dioxide. The water inside the leaves transpires, or simply evaporates and escapes the plant. The transpiration cools the plant leaves; allowing the photosynthetic apparatus to produce at optimum level’s energy in form of carbohydrates. Decreasing rates of transpiration that are due to global warming and local drying also affect photosynthesis.

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


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