Evapotranspiration

thumb|right|260px|[[Water cycle of the Earth's surface, showing the individual components of transpiration and evaporation that make up evapotranspiration. Other closely related processes shown are runoff and groundwater recharge.]]

thumb|alt=pet|Global distribution of potential evapotranspiration averaged over the years 1981–2010 from the CHELSA-BIOCLIM+ data set

Evapotranspiration (ET) refers to the combined processes that moves water from the Earth's surface (open water and ice surfaces, bare soil and vegetation) into the atmosphere including both evaporation and transpiration. Evapotranspiration is an important part of the local water cycle and climate, and measurement of it plays a key role in water resource management and agricultural irrigation.

Definition

Evapotranspiration is defined as: "The combined processes through which water is transferred to the atmosphere from open water and ice surfaces, bare soil and vegetation that make up the Earth's surface."

Evapotranspiration is a combination of evaporation and transpiration, measured in order to better understand crop water requirements, irrigation scheduling, and watershed management. The two key components of evapotranspiration are:

Evaporation: the movement of water directly to the air from sources such as the soil and water bodies. It can be affected by factors including heat, humidity, solar radiation and wind speed.
Transpiration: the movement of water from root systems, through a plant, and exit into the air as water vapor. This exit occurs through stomata in the plant. Rate of transpiration can be influenced by factors including plant type, soil type, weather conditions and water content, and also cultivation practices.

Evapotranspiration is typically measured in millimeters of water (i.e. volume of water moved per unit area of the Earth's surface) in 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.

thumb|This map shows the difference in the Eastern and Western United States average evapotranspiration. Notice that where agricultural practices are dominant evapotranspiration is higher than average.

Evapotranspiration does not, in general, account for other mechanisms which are involved in returning water to the atmosphere, though some of these, such as snow and ice sublimation in regions of high elevation or high latitude, can make a large contribution to atmospheric moisture even under standard conditions.

Influencing factors

thumb|Diagram showing impact of ground cover on evapotranspiration and other water usage factors.

Primary factors

Levels of evapotranspiration in a given area are primarily controlled by three factors: Firstly, the amount of water present which includes the amount of available water in both the soil and in larger bodies of water For example, herbaceous plants generally transpire less than woody plants, because they usually have less extensive foliage. Also, plants with deep reaching roots can transpire water more constantly, because those roots can pull more water into the plant and leaves. Another example is that 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. With crop vegetation being widespread, evapotranspiration may also be called "corn sweat".

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. Vegetation coverage also leads to a reduction in the overall surface temperature and the way solar energy effects evapotranspiration rates. Through this, heat transfer can be facilitated faster between the atmosphere and the surface increasing evaporation rates. The density of vegetation coverage can control different environmental conditions of varying ecosystems and the way that water moves through the system. Different vegetation coverages control the conditions on transpiration, but the evapotranspiration process will vary from plant-to-plant base on individual features of the plant. In rainforests, water yield is increased (compared to cleared, unforested land in the same climatic zone) as evapotranspiration increases humidity within the forest (a portion of which condenses and returns quickly as precipitation experienced at ground level as rain). The density of the vegetation blocks sunlight and reduces temperatures at ground level (thereby reducing losses due to surface evaporation), and reduces wind speeds (thereby reducing the loss of airborne moisture). The combined effect results in increased surface stream flows and a higher ground water table whilst the rainforest is preserved. Clearing of rainforests frequently leads to desertification as ground level temperatures and wind speeds increase, vegetation cover is lost or intentionally destroyed by clearing and burning, soil moisture is reduced by wind, and soils are easily eroded by high wind and rainfall events.

Soil and irrigation

Areas that are highly irrigated have a higher amount of moisture that is consistent in the soil. Moisture content within the soil effects total evapotranspiration. Areas with lower moisture levels in the soil result in lower evapotranspiration interaction and will increase the surface temperature of the soil. Areas with higher moisture levels will have more effective evapotranspiration and decrease the surface temperature of the soil.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. If potential evapotranspiration is greater than the actual precipitation, then soil will dry out until conditions stabilize, unless irrigation is used. More information about how evapotranspiration effects overall crop production will lead to the ability to better produce higher quality crops and increase our ability to sustainably irrigate our fields efficiently in large scale agricultural operations. It is shown that where irrigation systems are being implemented, fields and crops are seeing increased levels of evapotranspiration. While on the contrary in places where irrigation methods are not used lower evapotranspiration levels are seen. As the agriculture industry continues to evolve, new agricultural technologies and fertilizers are leading to an increase in evapotranspiration rates.

Estimation from meteorological data

Given meteorological data like wind, temperature, and humidity, reference ET can be calculated. The most general and widely used equation for calculating reference ET is the Penman equation. The Penman–Monteith variation is recommended by the Food and Agriculture Organization and the American Society of Civil Engineers. The simpler Blaney–Criddle equation was popular in the Western United States for many years but it is not as accurate in wet regions with higher humidity. Other equations for estimating evapotranspiration from meteorological data include the Makkink equation, which is simple but must be calibrated to a specific location, and the Hargreaves equations.

To convert the reference evapotranspiration to the actual crop evapotranspiration, a crop coefficient and a stress coefficient must be used. Crop coefficients, as used in many hydrological models, usually change over the year because crops are seasonal and, in general, plant behaviour varies over the year: perennial plants mature over multiple seasons, while annuals do not survive more than a few, so stress responses can significantly depend upon many aspects of plant type and condition.

Potential evapotranspiration

List of remote sensing based evapotranspiration models

alt=Classification of RS-based ET models based on sensible heat flux estimation approaches|thumb|584x584px|Classification of RS-based ET models based on sensible heat flux estimation approaches

ALEXI
BAITSSS
METRIC
Abtew Method
SEBAL
SEBS
SSEBop
PT-JPL
ETMonitor
ETLook
ETWatch