What is Evapotranspiration?
Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere.
It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpiration (movement of water from the soil, through roots and bodies of vegetation, on leaves and then into the air).
Evapotranspiration is an important part of the local water cycle and climate, and measurement of it plays a key role in agricultural irrigation and water resource management.
Evapotranspiration is typically measured in millimeters of water per a set unit of time.
Globally, it is estimated that on average between three-fifths and three-quarters of land precipitation is returned to the atmosphere via evapotranspiration.
The word is at least 86 years old, and was first published in hyphenated form.
The American climatologist Charles Warren Thornthwaite later defined it in 1944.
Factors that impact evapotranspiration levels
Primary factors:
The amount of water present;
The amount of energy present in the air and soil (e.g. heat); and
The ability of the atmosphere to take up water (humidity).
Secondary factors:
Vegetation type
Herbaceous plants generally transpire less than woody plants, because they usually have less extensive foliage.
Plants with deep reaching roots can transpire water more constantly, because those roots can pull more water into the plant and leaves.
Conifer forests tend to have higher rates of evapotranspiration than deciduous broadleaf forests, particularly in the dormant winter and early spring seasons, because they are evergreen.
2. Vegetation coverage
Transpiration is a larger component of evapotranspiration (relative to evaporation) in vegetation-abundant areas.
As a result, denser vegetation, like forests, may increase evapotranspiration and reduce water yield.
Two exception to this are cloud forests and rainforests.
3. Soil and irrigation
In areas that are not irrigated, actual evapotranspiration is usually no greater than precipitation, with some buffer and variations in time depending on the soil's ability to hold water.
It will usually be less because some water will be lost due to percolation or surface runoff.
An exception is areas with high water tables, where capillary action can cause water from the groundwater to rise through the soil matrix back to the surface.
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