What Is Evapotranspiration?
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What is evapotranspiration? It is the combined transfer of water from the land surface to the atmosphere as water vapor. The University of Wyoming Extension defines it as the sum of two processes, evaporation and transpiration. Evaporation is the loss from the soil surface, and transpiration is the loss from plants. This explainer explains each process, the difference between the three common ET terms, and why the number matters for watering. Everything here comes from one extension bulletin, and its limits are stated at the end.
Quick answer
- University of Wyoming Extension defines evapotranspiration as the combined transfer of water from land surface to atmosphere by evaporation and transpiration.
- In the field, the two processes are usually considered together as ET, because they are hard to calculate separately.
- The bulletin separates three terms: PET, ETref, and ETc, with ETc the actual crop value under field conditions.
- ETc is commonly calculated by taking ETref and adjusting it with a crop coefficient, Kc.
The two processes behind the term
The bulletin’s definition has two parts. Evapotranspiration is the transfer of water from land surface to atmosphere in the form of water vapor, by evaporation and by transpiration. Evaporation is the loss from the soil surface. Transpiration is the loss from plants.
The bulletin also explains why the two are treated as one number. Transpiration and evaporation are difficult to calculate independently. Under field settings, the two processes are therefore considered together as ET in many applications, including irrigation scheduling. The practical quantity for a grower or gardener is the combined loss.
Plants control part of the transpiration. The bulletin gives an example from dry soil: under dryland conditions, the leaf stomata close when the available soil moisture is depleted to meet the atmospheric demand. Once the stomata close, less water leaves the plant. The bulletin then says that under these conditions plants will experience wilting, and ET will be reduced.
Why ET matters for water planning
The bulletin calls ET one of the largest fractions of an ecosystem’s water cycle. That size is the reason it matters for planning.
The bulletin argues that an accurate assessment is critical. It names water resources management and environmental assessments on field, watershed, and regional scales. The stated reason is that the assessment of ET supports those decisions.
In agricultural settings, the bulletin says that reliable estimates of ET are vital. It lists several uses in irrigated agriculture: scheduling irrigation, predicting soil water status, water allocation, efficient use, long-term estimates of water supply, demand and use, and the design and management of water management infrastructure. Irrigation scheduling is the use most relevant to a home garden.
On dryland, the bulletin gives a simple bound. Seasonal ET of a crop under dryland conditions will usually be close or equal to the sum of the available soil water and precipitation. When soil water and rain are the only sources, the season’s total loss cannot exceed what the soil and sky supplied.
For practical watering habits during dry spells, see how to water a garden in drought. For signs that a plant has too much or too little water, see how to tell if you are overwatering or underwatering plants. Those articles cover the water side, while this one covers the loss term behind it.
The three terms: PET, ETref, and ETc
The bulletin’s main job is to separate three terms that are often used interchangeably. The first is potential evapotranspiration, PET. The bulletin states that the term potential is equivalent to the maximum possible level of ET under unlimited soil water supply and actual climatic conditions. The definition assumes unlimited soil water, so PET does not depend on what the soil holds.
The second term is reference evapotranspiration, ETref. The bulletin defines it as the rate of water loss by evaporation and transpiration from a healthy, hypothetical reference crop, which is grass. The definition fixes the grass height at 4.72 inches, and it describes the reference as closely resembling the evapotranspiration from an extensive surface of green grass of uniform height.
The third term is actual crop evapotranspiration, ETc. The bulletin defines it as the amount of water actually transpired from plants and evaporated from the soil surface under actual climatic conditions, and under non-optimal soil, biological, management, and environmental conditions. The bulletin explains the difference from PET directly: in actual field conditions the crop may encounter soil water shortage or waterlogging, diseases, or soil salinity.
The bulletin states that it describes the basic concept of evapotranspiration and defines the major difference between commonly used ET terms, mainly PET, ETref, and ETc. PET and ETref describe the reference conditions, while ETc describes the crop in its actual conditions.
How ETc is calculated
The bulletin describes a common method. It requires first the determination of ETref, and then an adjustment of that value by a specific crop coefficient to calculate ETc. The crop coefficient is written as Kc. The bulletin states that the crop coefficient incorporates the crop canopy characteristics and management practices.
The coefficient is not fixed for a crop. Each crop has different sets of specific Kc values, which depend on the growth stages of the crop and on field or crop management practices.
The bulletin also points to measurement tools. It lists approaches such as eddy covariance systems, Bowen ratio energy balance systems, and lysimeters for quantifying ET. The bulletin presents the ETref and Kc method as a commonly used route, not as the only one.
How we chose
We picked one source, a University of Wyoming Extension bulletin on evapotranspiration, because it defines the terms in plain language and states the calculation method. A second candidate, a University of Nebraska PDF bulletin, returned HTTP 200, but its text extraction is garbled across columns, so its wording could not be quoted verbatim and no claim was taken from it. Retail and product pages were not used. The Wyoming page returned HTTP 200 when checked directly on 2026-10-27.
What we could not verify
The bulletin is written for agricultural and irrigated settings. It does not give ET values for home lawns, garden beds, or containers. Its examples come from field crops and a semi-arid region, so the numbers in its discussion do not carry over to a yard without a separate local source.
What the sources do not say
The bulletin does not give a daily or weekly ET value for a garden in any region. It does not give a crop coefficient for a home vegetable bed or a lawn. It does not say how much water a container plant needs in a given week. Each of these points is outside the bulletin, so this article leaves them open.
Frequently asked questions
Is evapotranspiration the same as evaporation?
No. The Wyoming bulletin defines evapotranspiration as the combined transfer of water from land surface to atmosphere by evaporation and transpiration. Evaporation is one of the two processes it combines.
What is the difference between PET and ETc?
PET is the maximum possible level of ET under unlimited soil water supply and actual climatic conditions. The bulletin says ETc is different from PET because the crop in real field conditions may face soil water shortage, waterlogging, diseases, or soil salinity.
Why do plants lose less water in dry soil?
The bulletin says that under dryland conditions the leaf stomata close when the available soil moisture is depleted to meet the atmospheric demand. When that happens, plants experience wilting and ET is reduced.
What is a crop coefficient?
The bulletin says the crop coefficient, Kc, incorporates the crop canopy characteristics and management practices. Each crop has different Kc values depending on its growth stages and field or crop management.
Why do irrigators need an ET estimate?
The bulletin says reliable ET estimates are vital for scheduling irrigation, predicting soil water status, and water allocation. It also calls ET one of the largest fractions of an ecosystem’s water cycle.
Sources
- University of Wyoming Extension, “Evapotranspiration: Basics, Terminology and its Importance,” Bulletin B-1293, https://www.wyoextension.org/publications/html/B1293/ (HTTP 200, opened 2026-10-27)
