Evapotranspiration#Potential evapotranspiration

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."{{sfn|IPCC|2023a|p=2908}}

Evapotranspiration is a combination of evaporation and transpiration, measured in order to better understand crop water requirements, irrigation scheduling,{{sfn|Goyal|Harmsen|2013|p=xxi}} and watershed management.{{cite journal |last1=Vörösmarty |first1=C.J. |last2=Federer |first2=C.A. |last3=Schloss |first3=A.L. |title=Potential evaporation functions compared on US watersheds: Possible implications for global-scale water balance and terrestrial ecosystem modeling |journal=Journal of Hydrology |date=June 1998 |volume=207 |issue=3–4 |pages=147–169 |doi=10.1016/S0022-1694(98)00109-7 |bibcode=1998JHyd..207..147V }} 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.{{sfn|Allen|Pereira|Raes|Smith|1998|loc=Ch. 1, "Evaporation"}}
• 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.{{sfn|Allen|Pereira|Raes|Smith|1998|loc=Ch. 1, "Transpiration"}}

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.{{sfn|Allen|Pereira|Raes|Smith|1998|loc=Ch. 1, "Units"}} Globally, it is estimated that on average between three-fifths and three-quarters of land precipitation is returned to the atmosphere via evapotranspiration.{{cite journal |last1=Jung |first1=Martin |last2=Reichstein |first2=Markus |last3=Ciais |first3=Philippe |last4=Seneviratne |first4=Sonia I. |last5=Sheffield |first5=Justin |last6=Goulden |first6=Michael L. |last7=Bonan |first7=Gordon |last8=Cescatti |first8=Alessandro |last9=Chen |first9=Jiquan |last10=de Jeu |first10=Richard |last11=Dolman |first11=A. Johannes |last12=Eugster |first12=Werner |last13=Gerten |first13=Dieter |last14=Gianelle |first14=Damiano |last15=Gobron |first15=Nadine |last16=Heinke |first16=Jens |last17=Kimball |first17=John |last18=Law |first18=Beverly E. |last19=Montagnani |first19=Leonardo |last20=Mu |first20=Qiaozhen |last21=Mueller |first21=Brigitte |last22=Oleson |first22=Keith |last23=Papale |first23=Dario |last24=Richardson |first24=Andrew D. |last25=Roupsard |first25=Olivier |last26=Running |first26=Steve |last27=Tomelleri |first27=Enrico |last28=Viovy |first28=Nicolas |last29=Weber |first29=Ulrich |last30=Williams |first30=Christopher |last31=Wood |first31=Eric |last32=Zaehle |first32=Sönke |last33=Zhang |first33=Ke |title=Recent decline in the global land evapotranspiration trend due to limited moisture supply |journal=Nature |date=October 2010 |volume=467 |issue=7318 |pages=951–954 |doi=10.1038/nature09396 |pmid=20935626 |bibcode=2010Natur.467..951J |url=https://cea.hal.science/cea-00906004/file/run.pdf }}{{cite journal |last1=Oki |first1=Taikan |last2=Kanae |first2=Shinjiro |title=Global Hydrological Cycles and World Water Resources |journal=Science |date=25 August 2006 |volume=313 |issue=5790 |pages=1068–1072 |doi=10.1126/science.1128845 |pmid=16931749 |bibcode=2006Sci...313.1068O }}{{cite book |doi=10.5772/52811 |chapter=Water Balance Estimates of Evapotranspiration Rates in Areas with Varying Land Use |title=Evapotranspiration - an Overview |date=2013 |last1=Hasenmueller |first1=Elizabeth A |last2=Criss |first2=Robert E |isbn=978-953-51-1115-3 |editor-last1=Alexandris |editor-first1=Stavros }}{{predatory|date=April 2025}}

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.

{{See also|Water cycle}}

File:Natural & impervious cover diagrams EPA.jpg

Levels of evapotranspiration in a given area are primarily controlled by three factors:{{cite book |doi=10.2134/agronmonogr60.2016.0034 |chapter=A Brief Overview of Approaches for Measuring Evapotranspiration |title=Agroclimatology |series=Agronomy Monographs |date=2018 |last1=Alfieri |first1=J.G. |last2=Kustas |first2=W.P. |last3=Anderson |first3=M.C. |pages=109–127 |isbn=978-0-89118-358-7 }} Firstly, the amount of water present. Secondly, the amount of energy present in the air and soil (e.g. heat, measured by the global surface temperature); and thirdly the ability of the atmosphere to take up water (humidity).

Regarding the second factor (energy and heat): climate change has increased global temperatures (see instrumental temperature record). This global warming has increased evapotranspiration over land.{{sfn|IPCC|2023b|p=1057}} The increased evapotranspiration is one of the effects of climate change on the water cycle.

Vegetation type impacts levels of evapotranspiration.{{cite journal |last1=Giardina |first1=Francesco |last2=Gentine |first2=Pierre |last3=Konings |first3=Alexandra G. |last4=Seneviratne |first4=Sonia I. |last5=Stocker |first5=Benjamin D. |title=Diagnosing evapotranspiration responses to water deficit across biomes using deep learning |journal=New Phytologist |date=25 August 2023 |volume=240 |issue=3 |pages=968–983 |doi=10.1111/nph.19197 |pmid=37621238 |doi-access=free |bibcode=2023NewPh.240..968G |hdl=20.500.11850/628261 |hdl-access=free }} 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.{{cite journal |last1=Swank |first1=Wayne T. |last2=Douglass |first2=James E. |title=Streamflow Greatly Reduced by Converting Deciduous Hardwood Stands to Pine |journal=Science |date=6 September 1974 |volume=185 |issue=4154 |pages=857–859 |doi=10.1126/science.185.4154.857 |pmid=17833698 |bibcode=1974Sci...185..857S }}

Transpiration is a larger component of evapotranspiration (relative to evaporation) in vegetation-abundant areas.{{cite journal |last1=Jasechko |first1=Scott |last2=Sharp |first2=Zachary D. |last3=Gibson |first3=John J. |last4=Birks |first4=S. Jean |last5=Yi |first5=Yi |last6=Fawcett |first6=Peter J. |date=3 April 2013 |title=Terrestrial water fluxes dominated by transpiration |journal=Nature |volume=496 |issue=7445 |pages=347–50 |bibcode=2013Natur.496..347J |doi=10.1038/nature11983 |pmid=23552893 }} As a result, denser vegetation, like forests, may increase evapotranspiration and reduce water yield.

Two exceptions to this are cloud forests and rainforests. 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.{{cite journal |last1=Holder |first1=Curtis D |title=Rainfall interception and fog precipitation in a tropical montane cloud forest of Guatemala |journal=Forest Ecology and Management |date=March 2004 |volume=190 |issue=2–3 |pages=373–384 |doi=10.1016/j.foreco.2003.11.004 |bibcode=2004ForEM.190..373H }}{{Cite web |title=Cloud Forest |url=https://cloudforestconservation.org/knowledge/cloud-forest/ |access-date=2022-05-02 |website=Community Cloud Forest Conservation |language=en-US}} 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.{{Cite web |title=How plants play a vital role for rainfall within the tropical rainforest {{!}} Britannica |url=https://www.britannica.com/video/185619/role-plants-cycle-evaporation-condensation-rainforest-biomes |access-date=2022-05-02 |website=www.britannica.com |language=en}}{{cite journal |last1=Sheil |first1=Douglas |last2=Murdiyarso |first2=Daniel |title=How Forests Attract Rain: An Examination of a New Hypothesis |journal=BioScience |date=April 2009 |volume=59 |issue=4 |pages=341–347 |id={{Gale|A197855789}} |doi=10.1525/bio.2009.59.4.12 |jstor=10.1525/bio.2009.59.4.12 |hdl=10568/20155 }}

{{Unreferenced section|date=August 2022}}

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. If potential evapotranspiration is greater than the actual precipitation, then soil will dry out until conditions stabilize, unless irrigation is used.

{{More citations needed section|date=February 2022}}

File:Lysimeter-design.png

{{Main|Lysimeter}}

Evapotranspiration can be measured directly with a weighing or pan lysimeter. 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 over small areas.

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.

Evapotranspiration may be estimated by evaluating the water balance equation for a given area:. The water balance equation relates the change in water stored within the basin (S) to its input and outputs:

$\backslash Delta\; S\; =\; P\; -\; ET\; -\; Q\; -\; D\; \backslash ,\backslash !$

In the equation, the change in water stored within the basin (ΔS) is related to precipitation (P) (water going into the basin), and evapotranspiration (ET), streamflow (Q), and groundwater recharge (D) (water leaving the basin). By rearranging the equation, ET can be estimated if values for the other variables are known:

$ET\; =\; P\; -\backslash Delta\; S\; -\; Q\; -\; D\; \backslash ,\backslash !$

A second methodology for estimation is by calculating the energy balance.

$\backslash lambda\; E\; =\; R\_n\; -\; G\; -\; H\; \backslash ,\backslash !$

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. Using instruments like a scintillometer, 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 and METRIC algorithms solve for 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.{{cite web|url= http://www.waterwatch.nl/tools0/sebal.html|title= SEBAL_ WaterWatch|url-status= live|archive-url= https://web.archive.org/web/20110713133722/http://www.waterwatch.nl/tools0/sebal.html|archive-date= 2011-07-13}}

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{{sfn|Allen|Pereira|Raes|Smith|1998|p={{pn|date=April 2025}}}} and the American Society of Civil Engineers.{{cite journal |last1=Rojas |first1=Jose P. |last2=Sheffield |first2=Ronald E. |title=Evaluation of Daily Reference Evapotranspiration Methods as Compared with the ASCE-EWRI Penman-Monteith Equation Using Limited Weather Data in Northeast Louisiana |journal=Journal of Irrigation and Drainage Engineering |date=April 2013 |volume=139 |issue=4 |pages=285–292 |doi=10.1061/(ASCE)IR.1943-4774.0000523 |bibcode=2013JIDE..139..285R }} 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{{clarify|date=September 2023}}, so stress responses can significantly depend upon many aspects of plant type and condition.

{{excerpt|Potential evaporation|paragraphs=1-4}}

File:Classification of RS-based ET models based on sensible heat flux estimation approaches..png

• ALEXI{{cite journal |last1=Anderson |first1=M. C. |last2=Kustas |first2=W. P. |last3=Norman |first3=J. M. |last4=Hain |first4=C. R. |last5=Mecikalski |first5=J. R. |last6=Schultz |first6=L. |last7=González-Dugo |first7=M. P. |last8=Cammalleri |first8=C. |last9=d'Urso |first9=G. |last10=Pimstein |first10=A. |last11=Gao |first11=F. |title=Mapping daily evapotranspiration at field to continental scales using geostationary and polar orbiting satellite imagery |journal=Hydrology and Earth System Sciences |date=21 January 2011 |volume=15 |issue=1 |pages=223–239 |doi=10.5194/hess-15-223-2011 |bibcode=2011HESS...15..223A |doi-access=free |hdl=10447/53094 |hdl-access=free }}
• BAITSSS{{cite journal |last1=Dhungel |first1=Ramesh |last2=Aiken |first2=Robert |last3=Colaizzi |first3=Paul D. |last4=Lin |first4=Xiaomao |last5=O'Brien |first5=Dan |last6=Baumhardt |first6=R. Louis |last7=Brauer |first7=David K. |last8=Marek |first8=Gary W. |title=Evaluation of uncalibrated energy balance model (BAITSSS) for estimating evapotranspiration in a semiarid, advective climate |journal=Hydrological Processes |date=15 July 2019 |volume=33 |issue=15 |pages=2110–2130 |doi=10.1002/hyp.13458 |bibcode=2019HyPr...33.2110D }}{{cite journal |last1=Dhungel |first1=Ramesh |last2=Allen |first2=Richard G. |last3=Trezza |first3=Ricardo |last4=Robison |first4=Clarence W. |title=Evapotranspiration between satellite overpasses: methodology and case study in agricultural dominant semi-arid areas: Time integration of evapotranspiration |journal=Meteorological Applications |date=October 2016 |volume=23 |issue=4 |pages=714–730 |doi=10.1002/met.1596 |bibcode=2016MeApp..23..714D |doi-access=free }}
• METRIC{{cite journal |last1=Allen |first1=Richard G. |last2=Tasumi |first2=Masahiro |last3=Trezza |first3=Ricardo |title=Satellite-Based Energy Balance for Mapping Evapotranspiration with Internalized Calibration (METRIC)—Model |journal=Journal of Irrigation and Drainage Engineering |date=August 2007 |volume=133 |issue=4 |pages=380–394 |doi=10.1061/(ASCE)0733-9437(2007)133:4(380) |bibcode=2007JIDE..133..380A }}
• Abtew Method{{cite journal |last1=Abtew |first1=Wossenu |title=Evapotranspiration measurements and modeling for three wetland systems in south Florida |journal=JAWRA Journal of the American Water Resources Association |date=June 1996 |volume=32 |issue=3 |pages=465–473 |doi=10.1111/j.1752-1688.1996.tb04044.x |bibcode=1996JAWRA..32..465A }}
• SEBAL{{cite journal |last1=Bastiaanssen |first1=W.G.M. |last2=Menenti |first2=M. |last3=Feddes |first3=R.A. |last4=Holtslag |first4=A.A.M. |title=A remote sensing surface energy balance algorithm for land (SEBAL). 1. Formulation |journal=Journal of Hydrology |date=December 1998 |volume=212-213 |pages=198–212 |doi=10.1016/S0022-1694(98)00253-4 |bibcode=1998JHyd..212..198B }}
• SEBS{{cite journal |last1=Su |first1=Z. |title=The Surface Energy Balance System (SEBS) for estimation of turbulent heat fluxes |journal=Hydrology and Earth System Sciences |date=28 February 2002 |volume=6 |issue=1 |pages=85–100 |doi=10.5194/hess-6-85-2002 |bibcode=2002HESS....6...85S |doi-access=free }}
• SSEBop{{cite journal |last1=Senay |first1=Gabriel B. |last2=Bohms |first2=Stefanie |last3=Singh |first3=Ramesh K. |last4=Gowda |first4=Prasanna H. |last5=Velpuri |first5=Naga M. |last6=Alemu |first6=Henok |last7=Verdin |first7=James P. |title=Operational Evapotranspiration Mapping Using Remote Sensing and Weather Datasets: A New Parameterization for the SSEB Approach |journal=JAWRA Journal of the American Water Resources Association |date=June 2013 |volume=49 |issue=3 |pages=577–591 |doi=10.1111/jawr.12057 |bibcode=2013JAWRA..49..577S |doi-access=free }}
• PT-JPL{{sfn|FAO|2023|p={{pn|date=April 2025}}}}{{cite journal |last1=Fisher |first1=Joshua B. |last2=Tu |first2=Kevin P. |last3=Baldocchi |first3=Dennis D. |title=Global estimates of the land–atmosphere water flux based on monthly AVHRR and ISLSCP-II data, validated at 16 FLUXNET sites |journal=Remote Sensing of Environment |date=March 2008 |volume=112 |issue=3 |pages=901–919 |doi=10.1016/j.rse.2007.06.025 |bibcode=2008RSEnv.112..901F }}
• ETMonitor{{sfn|FAO|2023|p={{pn|date=April 2025}}}}{{cite journal |last1=Hu |first1=Guangcheng |last2=Jia |first2=Li |title=Monitoring of Evapotranspiration in a Semi-Arid Inland River Basin by Combining Microwave and Optical Remote Sensing Observations |journal=Remote Sensing |date=16 March 2015 |volume=7 |issue=3 |pages=3056–3087 |doi=10.3390/rs70303056 |doi-access=free |bibcode=2015RemS....7.3056H }}
• ETLook{{sfn|FAO|2023|p={{pn|date=April 2025}}}}
• ETWatch{{sfn|FAO|2023|p={{pn|date=April 2025}}}}{{cite journal |last1=Wu |first1=Bingfang |last2=Zhu |first2=Weiwei |last3=Yan |first3=Nana |last4=Xing |first4=Qiang |last5=Xu |first5=Jiaming |last6=Ma |first6=Zonghan |last7=Wang |first7=Linjiang |title=Regional Actual Evapotranspiration Estimation with Land and Meteorological Variables Derived from Multi-Source Satellite Data |journal=Remote Sensing |date=20 January 2020 |volume=12 |issue=2 |pages=332 |doi=10.3390/rs12020332 |doi-access=free |bibcode=2020RemS...12..332W }}

• Eddy covariance flux (aka eddy correlation, eddy flux)
• Effects of climate change on the water cycle
• Hydrology (agriculture)
• Hydrologic Evaluation of Landfill Performance (HELP)
• Latent heat flux
• Water Evaluation And Planning system (WEAP)
• Soil plant atmosphere continuum
• Deficit irrigation
• Biotic pump

{{Reflist}}

• {{cite book |ref={{Sfnref|FAO|2023}} |doi=10.4060/cc8150en |title=Remote sensing determination of evapotranspiration |date=2023 |isbn=978-92-5-138242-4 |publisher=FAO }}
• {{cite book |doi=10.1201/b15779 |title=Evapotranspiration |date=2013 |isbn=978-0-429-16135-3 |editor-last1=Goyal |editor-last2=Harmsen |editor-first1=Megh R. |editor-first2=Eric W. }}
• {{cite book |ref={{Sfnref|IPCC|2023a}} |doi=10.1017/9781009325844.029 |chapter=Glossary |title=Climate Change 2022 – Impacts, Adaptation and Vulnerability |date=2023a |pages=2897–2930 |isbn=978-1-009-32584-4 |author1=Intergovernmental Panel on Climate Change }}
• {{cite book |ref={{Sfnref|IPCC|2023b}} |doi=10.1017/9781009157896.010 |chapter=Water Cycle Changes |title=Climate Change 2021 – the Physical Science Basis |date=2023b |pages=1055–1210 |isbn=978-1-009-15789-6 |author1=Intergovernmental Panel on Climate Change }}
• {{cite book |first1=Richard G |last1=Allen |first2=Luis S |last2=Pereira |first3=Dirk |last3=Raes |first4=Martin |last4=Smith |url=https://www.fao.org/4/x0490e/x0490e00.htm |title=Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements |publisher=Food and Agriculture Organization of the United Nations |date=1998 |isbn=978-92-5-104219-9 |series=FAO Irrigation and drainage paper 56 |location=Rome, Italy }}

• [https://web.archive.org/web/20090625040657/http://bosque.unm.edu/~cleverly/index.html New Mexico Eddy Covariance Flux Network (Rio-ET)]
• [https://texaset.tamu.edu/ Texas Evapotranspiration Network]
• [http://www.llansadwrn-wx.co.uk/evap/lysim.html Use and Construction of a Lysimeter to Measure Evapotranspiration]
• [https://wrcc.dri.edu/washoeEt/ Washoe County (NV) Et Project]
• [https://www.usgs.gov/special-topics/water-science-school/science/evapotranspiration-and-water-cycle US Geological Survey]

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