![Surface water cycle](https://upload.wikimedia.org/wikipedia/commons/8/80/Surface_water_cycle.svg)
Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
. It covers both water
evaporation
Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
(movement of water to the air directly from soil,
canopies, and water bodies) and
transpiration
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth a ...
(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
The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly cons ...
and
climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...
, as well as measurement of it plays a key role in
agricultural irrigation and
water resource management
Water resources are natural resources of water that are potentially useful for humans, for example as a source of drinking water supply or irrigation water. 97% of the water on the Earth is Saline water, salt water and only three percent is fresh ...
.
Definition
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, 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 botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bor ...
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 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.
Factors that impact evapotranspiration levels
Primary factors
Because evaporation and transpiration occur when water moves into the air, levels of evapotranspiration in a given area are primarily controlled by:
* 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
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
).
Secondary factors
Vegetation type
Vegetation type can impact levels of evapotranspiration by impacting the factors listed above. For example:
*
Herbaceous plant
Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials.
Definitions of "herb" and "herbaceous"
The fourth edition of t ...
s generally transpire less than
woody plant
A woody plant is a plant that produces wood as its structural tissue and thus has a hard stem. In cold climates, woody plants further survive winter or dry season above ground, as opposite to herbaceous plants that die back to the ground until sp ...
s, because they usually have less extensive foliage.
* Plants with deep reaching roots can transpire water more constantly, because water transpired through leaves comes from the roots.
*
Conifer
Conifers are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single ...
forests tend to have higher rates of evapotranspiration than
deciduous
In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, aft ...
broadleaf forests, particularly in the dormant winter and early spring seasons, because they are
evergreen
In botany, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This also pertains to plants that retain their foliage only in warm climates, and contrasts with deciduous plants, which ...
.
Factors that affect evapotranspiration include the plant's growth stage or level of maturity, it's
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
, percentage of soil cover, soil type,
solar radiation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre (W/m ...
,
humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
,
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
, and
wind
Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...
.
Vegetation coverage
Transpiration
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth a ...
is a larger component of evapotranspiration (relative to evaporation) in vegetation-abundant areas. As a result, denser vegetation like forests may reduce water yield.
Two exception to this are
cloud forest
A cloud forest, also called a water forest, primas forest, or tropical montane cloud forest (TMCF), is a generally tropical or subtropical, evergreen, montane, moist forest characterized by a persistent, frequent or seasonal low-level cloud c ...
s and
rainforests
Rainforests are characterized by a closed and continuous tree canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforest can be classified as tropical rainforest or temperate rainforest ...
. In cloud forests, trees collect the liquid water in fog or low clouds onto their surface, which eventually drips down to the ground. These trees still contribute to evapotranspiration, but often collect more water than they evaporate or transpire. 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
The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated.
T ...
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
In areas that are not irrigated, actual evapotranspiration is usually no greater than
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
, 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
Percolation (from Latin ''percolare'', "to filter" or "trickle through"), in physics, chemistry and materials science, refers to the movement and filtering of fluids through porous materials.
It is described by Darcy's law.
Broader applicatio ...
or
surface runoff
Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when th ...
. An exception is areas with high
water table
The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated.
T ...
s, where
capillary action
Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces li ...
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
Irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow Crop, crops, Landscape plant, landscape plants, and Lawn, lawns. Irrigation has been a key aspect of agriculture for over 5,00 ...
is used.
Measurement
![Lysimeter-design](https://upload.wikimedia.org/wikipedia/commons/e/ea/Lysimeter-design.png)
Evapotranspiration can be measured directly with a weighing or pan
lysimeter
A lysimeter (from Greek λύσις (loosening) and the suffix ''-meter'') is a measuring device which can be used to measure the amount of actual evapotranspiration which is released by plants (usually crops or trees). By recording the amount of p ...
. A lysimeter continuously measures the weight of a plant and associated soil, and any water added by precipitation or irrigation. The change in storage of water in the soil is then modeled by measuring the change in weight. When used properly, this allows for precise measurement of evapotranspiration on small areas.
Indirect estimation
Because atmospheric vapor flux is difficult or time consuming to measure directly,
evapotranspiration is typically estimated by one of several different methods that do not rely on direct measurement.
Catchment water balance
Evapotranspiration may be estimated by evaluating the
water balance
The law of water balance states that the inflows to any water system or area is equal to its outflows plus change in storage during a time interval. In hydrology, a water balance equation can be used to describe the flow of water in and out of ...
of a drainage basin. The water balance equation relates the change in water stored within the basin (''S'') to its input and outputs:
The input is precipitation (''P'') and the outputs are evapotranspiration (which is to be estimated), streamflow (''Q''), and
groundwater recharge
Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs in ...
(''D''). If the change in storage, precipitation, streamflow, and groundwater recharge are all estimated, the missing flux, ET, can be estimated by rearranging the above equation as follows:
Energy balance
A second methodology to estimate the actual evapotranspiration is the use of the energy balance.
where λE is the energy needed to change the phase of water from liquid to gas, R
n is the net radiation, G is the soil heat flux and H is the
sensible heat flux
Sensible heat is heat exchanged by a body or thermodynamic system in which the exchange of heat changes the temperature of the body or system, and some macroscopic variables of the body or system, but leaves unchanged certain other macroscopic vari ...
. Using instruments like a
scintillometer
A scintillometer is a scientific device used to measure turbulent fluctuations of the refractive index of air caused by variations in temperature, humidity, and pressure. It consists of an optical or radio wave transmitter and a receiver at oppos ...
, soil heat flux plates or radiation meters, the components of the energy balance can be calculated and the energy available for actual evapotranspiration can be solved.
The
SEBAL The Surface Energy Balance Algorithm for Land (SEBAL) uses the surface energy balance to estimate aspects of the hydrological cycle. SEBAL maps evapotranspiration, biomass growth, water deficit and soil moisture. Its main creator is Prof. Dr. W.G.M. ...
and
METRIC
Metric or metrical may refer to:
* Metric system, an internationally adopted decimal system of measurement
* An adjective indicating relation to measurement in general, or a noun describing a specific type of measurement
Mathematics
In mathema ...
algorithms solve the energy balance at the earth's surface using satellite imagery. This allows for both actual and potential evapotranspiration to be calculated on a pixel-by-pixel basis. Evapotranspiration is a key indicator for water management and irrigation performance. SEBAL and METRIC can map these key indicators in time and space, for days, weeks or years.
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 equation describes evaporation (''E'') from an open water surface, and was developed by Howard Penman in 1948. Penman's equation requires daily mean temperature, wind speed, air pressure, and solar radiation to predict E. Simpler Hydr ...
. The
Penman–Monteith variation is recommended by the
Food and Agriculture Organization
The Food and Agriculture Organization of the United Nations (FAO)french: link=no, Organisation des Nations unies pour l'alimentation et l'agriculture; it, Organizzazione delle Nazioni Unite per l'Alimentazione e l'Agricoltura is an intern ...
and the
American Society of Civil Engineers
American(s) may refer to:
* American, something of, from, or related to the United States of America, commonly known as the "United States" or "America"
** Americans, citizens and nationals of the United States of America
** American ancestry, pe ...
.
The simpler
Blaney–Criddle equation The Blaney–Criddle equation (named after H. F. Blaney and W. D. Criddle) is a method for estimating reference crop evapotranspiration.
Usage
The Blaney–Criddle equation is a relatively simplistic method for calculating evapotranspiration. Whe ...
was popular in the Western
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
for many years but it is not as accurate in wet regions with higher humidity. Other equations for estimating evapotranspiration from meteorological data includes the
Makkink, which is simple but must be calibrated to a specific location, and lastly the
Hargreaves equations.
To convert the reference evapotranspiration to the actual crop evapotranspiration, a
crop coefficient Crop coefficients are properties of plants used in predicting evapotranspiration (ET). The most basic crop coefficient, ''K''c, is simply the ratio of ET observed for the crop studied over that observed for the well calibrated reference crop under ...
and a
stress coefficient
Stress may refer to:
Science and medicine
* Stress (biology), an organism's response to a stressor such as an environmental condition
* Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
must be used. Crop
coefficient
In mathematics, a coefficient is a multiplicative factor in some term of a polynomial, a series, or an expression; it is usually a number, but may be any expression (including variables such as , and ). When the coefficients are themselves var ...
s as used in many hydrological models usually change along the year to accommodate to the fact that crops are seasonal and, in general, plants behave differently along the seasons:
perennial plants
A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also widel ...
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
![Potential evapotranspiration Hawaii](https://upload.wikimedia.org/wikipedia/commons/d/d3/Potential_evapotranspiration_Hawaii.gif)
Potential evapotranspiration (PET) is the amount of water that would be evaporated and transpired by a specific crop, soil or ecosystem if there were sufficient
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
available. It is a reflection of the energy available to evaporate or transpire water, and of the wind available to transport the
water vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous pha ...
from the ground up into the lower atmosphere and away from the initial location. Often a value for the potential evapotranspiration is calculated at a nearby climatic station on a reference surface, conventionally on land dominated by short grass (though may differ from station to station). This value is called the reference evapotranspiration (ET
0). Actual evapotranspiration is said to equal potential evapotranspiration when there is ample water existent. Evapotranspiration can never be greater than potential evapotranspiration, but can be lower if there is not enough water to be evaporated or plants are unable to transpire maturely and readily.
Some US states utilize a full cover alfalfa reference crop that is in height, rather than the general short green grass reference, due to the higher value of ET from the alfalfa reference.
Potential evapotranspiration is higher in the summer, on clearer and less cloudy days, and closer to the equator, because of the higher levels of solar radiation that provides the energy (heat) for evaporation. Potential evapotranspiration is also higher on windy days because the evaporated moisture can be quickly moved from the ground or plant surface before it precipitates, allowing more evaporation to fill its place.
Potential evapotranspiration is expressed in terms of a depth of water or soil moisture percentage, and can be graphed during the year (see figure).
Potential evapotranspiration is usually measured indirectly, from other climatic factors, but also depends on the surface type, such as free water (for
lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
s and
ocean
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
s), the
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
type for bare soil, and also the density and
diversity
Diversity, diversify, or diverse may refer to:
Business
*Diversity (business), the inclusion of people of different identities (ethnicity, gender, age) in the workforce
*Diversity marketing, marketing communication targeting diverse customers
* ...
of
vegetation
Vegetation is an assemblage of plant species and the ground cover they provide. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic character ...
. Often a value for the potential evapotranspiration is calculated at a nearby climate station on a reference surface, conventionally on short grass (see above). This value is called the reference evapotranspiration, and can be converted to a potential evapotranspiration by multiplying with a surface coefficient. In agriculture, this is called a crop coefficient. The difference between potential evapotranspiration and the actual precipitation is used in
irrigation scheduling Irrigation scheduling is the process used by irrigation system managers to determine the correct frequency and duration of watering.
The following factors may be taken into consideration:
* Precipitation rate of the irrigation equipment – how q ...
.
Average annual potential evapotranspiration is often compared to average annual precipitation, the symbol of which is ''P''. The ratio of the two, P/PET, is the
aridity index
An aridity index (AI) is a numerical indicator of the degree of dryness of the climate at a given location. A number of aridity indices have been proposed (see below); these indicators serve to identify, locate or delimit regions that suffer from ...
. A
humid subtropical climate
A humid subtropical climate is a zone of climate characterized by hot and humid summers, and cool to mild winters. These climates normally lie on the southeast side of all continents (except Antarctica), generally between latitudes 25° and 40° ...
is a zone of climate characterized by hot and humid summers, and cold to mild winters. Subarctic regions have short, mild summers and freezing winters, falling between 50°N and 70°N latitude, depending on local climates. Precipitation and evapotranspiration is low (compared to warmer variants), and vegetation is characteristic of the coniferous/taiga forest.
List of remote sensing based evapotranspiration models
![Classification of RS-based ET models based on sensible heat flux estimation approaches](https://upload.wikimedia.org/wikipedia/commons/3/3f/Classification_of_RS-based_ET_models_based_on_sensible_heat_flux_estimation_approaches..png)
* ALEXI
*
BAITSSS
BAITSSS (Backward-Averaged Iterative Two-Source Surface temperature and energy balance Solution) is biophysical Evapotranspiration (ET) computer model that determines water use, primarily in agriculture landscape, using remote sensing-based inform ...
*
METRIC
Metric or metrical may refer to:
* Metric system, an internationally adopted decimal system of measurement
* An adjective indicating relation to measurement in general, or a noun describing a specific type of measurement
Mathematics
In mathema ...
*
Abtew Method
*
SEBAL The Surface Energy Balance Algorithm for Land (SEBAL) uses the surface energy balance to estimate aspects of the hydrological cycle. SEBAL maps evapotranspiration, biomass growth, water deficit and soil moisture. Its main creator is Prof. Dr. W.G.M. ...
* SEBS
* SSEBop
See also
*
Eddy covariance
The eddy covariance (also known as eddy correlation and eddy flux) is a key atmospheric measurement technique to measure and calculate vertical turbulent fluxes within atmospheric boundary layers. The method analyses high-frequency wind and scala ...
flux (aka eddy correlation, eddy flux)
*
Hydrology (agriculture) Agricultural hydrology is the study of water balance components intervening in farm water, agricultural water management, especially in irrigation and drainage.
Water balance components
The water balance components can be grouped into components ...
*
Hydrologic Evaluation of Landfill Performance The Hydrologic Evaluation of Landfill Performance (HELP) model is a quasi-two-dimensional hydrologic numerical model for conducting water balance analysis of landfills, cover systems, and other solid waste containment facilities; it was developed f ...
(HELP)
*
Latent heat flux
Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition.
Latent heat can be understo ...
*
Water Evaluation And Planning system (WEAP)
*
Soil plant atmosphere continuum
The soil-plant-atmosphere continuum (SPAC) is the pathway for water moving from soil through plants to the atmosphere. Continuum in the description highlights the continuous nature of water connection through the pathway. The low water potential of ...
*
Deficit irrigation Deficit irrigation (DI) is a watering strategy that can be applied by different types of irrigation application methods. The correct application of DI requires thorough understanding of the yield response to water (crop sensitivity to drought stress ...
*
Evaporation
Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
*
Transpiration
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth a ...
*
Precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
References
{{Reflist
External links
New Mexico Eddy Covariance Flux Network (Rio-ET)Texas Evapotranspiration Network*
ttps://wrcc.dri.edu/washoeEt/ Washoe County (NV) Et Projectbr>
US Geological Survey
Hydrology
Agrometeorology
Ecological processes
Irrigation
Water conservation
Water and the environment
Meteorological phenomena