In the attached link you have all what you need about potential and reference evapotranspiration and their calculation methods.

http://www.fao.org/docrep/X0490E/x0490e00.htm#Contents

 Thanks so much for your great help.

I'm happy to help.

Potential evapotranspiration (ET) is the maximum achievable ET for a given crop. Reference ET is also the maximum ET for reference crop, which is either grass or alfalfa. You can find many articles online for the calculation method.

Thanks Abhinaya. I am reading related articles.

Abhinaya Subedi gave a good answer. Moreover, the reference evapotranspiration could be converted to potential evapotranspiration multiplying its value to a crop coefficient (Kc).

I'd like to ask for help too. Please check my doubt: https://www.researchgate.net/post/Hargreaves-Samani_equation_calibration

Potential evaporation is defined as the amount of evaporation that would occur if a sufficient water source were available.  This is only the evaporation component.  The transpiration component is not part of this number according to the definition.  PET is crop independent. ETcrop is crop specific and can be used to calculate water amounts for agriculture.  PET has already been calculated for many areas.  It is a static number.  It is a potential for evaporation but does not include crop water needs.  ETcrop is a real time crop specific measurement converted into needed water amount from ETref and a crop coefficient curve.  ETref is the water need of mowed grass.  ETref can be calculated from a weather station having Temperature ,Relative Humidity, Wind speed, and Solar radiation according to the above FAO document.  A crop coefficient curve is a table usually generated by researchers or area agronomists.  See FAO also for more details on Crop Coefficients.  I suggest also using maturity modelling if ETcrop is to be used.  

Thanks Jeff for kindly and sagacious response.

Reference evapotranspiration means exactly the same thing as potential evapotranspiration. FAO decided on this subject and recommended that reference evapotranspiration be used to avoid any encroachment (see first paragraph on page 3 of FAO bulletin 56). now for the evapotraspiration of the culture it is necessary to multiply the evapotranspiration of reference by the cultural coefficient.

"The evapotranspiration rate is a reference to the reference field, and is referred to as the reference crop of the reference field or to the reference field. AND is strongly discouraged due to ambiguities in their definitions. " (Allen et al 1998)

Allen, Richard G., Luis S. Pereira, Dirk Raes, Martin Smith, and others. 1998. Crop evapotranspiration-Guidelines for Computing Crop Water Requirements-FAO Irrigation and drainage paper 56. FAO, Rome 300 (9): D05109.

The “potential evapotranspiration” concept was first introduced in the late 1940s, which is defined as “the amount of evapotranspiration in a given time by a large vegetation of short green crop, completely shading the ground, of uniform height and with adequate moisture at all the times in the soil”. The objective of defining “potential” evapotranspiration is to eliminate the crop specific changes in the evapotranspiration process. The concept of PET received instant approval, and it became the central theme in estimating crop water use from then onwards. However, this concept has certain limitations. In the definition of PET, the evapotranspiration rate is not related to a specific crop. The main confusion with PET is that there are many types of crops that fit into the description of a short green crop, and we may be confused as to which crop should be selected as a short green crop! Having considered the ambiguities and limitations of PET, by the early 1980s, the concept of “reference crop evapotranspiration” (ETo) was introduced by irrigation scientists.

In reference evapotransipration, the reference surface closely resembles an extensive surface of green grass of uniform height, actively growing, completely shading the ground, and with adequate water. The FAO Expert Consultation on Revision of FAO Methodologies for Crop Water Requirements (1990) accepted the following unambiguous definition for the reference surface: “A hypothetical reference crop with an assumed crop height of 0.12 m, a fixed surface resistance of 70 s/m and an albedo of 0.23.” The fixed surface resistance of 70 s/m implies a moderately dry soil surface resulting from about a weekly irrigation.

Reference evapotranspiration is defined as “the rate of evapotranspiration from a hypothetical reference crop with an assumed crop height of 0.12m, a fixed surface resistance of 70 s/m and an albedo of 0.23, closely resembling the evapotranspiration from an extensive surface of green grass of uniform height, actively growing, well-watered, and completely shading the ground.” Using the concept of ETo, the evaporative demand of the atmosphere can be considered independent of crop type, crop development, and management practices.

Please note that use of other concepts such as “potential evapotranspiration” is strongly discouraged due to ambiguities in their definitions, and for all practical purposes, reference evapotranspiration is same as the potential evapotranspiration mentioned in classical literature.

For determining ETo, many techniques are available including those that use pan evaporation data and those that use calculated reference crop evapotranspiration rates by the use of empirical formulae. After trying various empirical formulae and observing their limitations, the FAO Expert Consultation on Revision of FAO Methodologies for Crop Water Requirements held in May 1990 recommended the FAO Penman-Monteith method as the sole and standard method for the definition and computation of the reference evapotranspiration, ETo. The new method has global validity, and was recognized by the International Commission for Irrigation and Drainage and by the World Meteorological Organization.

Just like PET and ETo, there are several concepts, which may confuse you. For example, water use efficiency and water productivity; crop factor and crop coefficient; water foot print and virtual water; diffusion pressure deficit(DPD) and cell water potential; and so on. I have tried to discuss these kinds of issues in my latest book "Irrigation and Water Management" Published by Ane Books, New Delhi (2021). I invite all of to have a review of this book written in a contemporary style. Please visit

https://www.researchgate.net/publication/354616406_Irrigation_and_Water_Managementor

both terms mean the same

One term is crop independent and one term is crop dependent.

Reference evapotranspiration is that from a grass surface that is well-watered. Potential evapotranspiration is that from a surface that has unlimited water (such as a lake). potential evapotranspiration (PET) is defined as the amount of evaporation that would occur if a sufficient water source were available. If the actual evapotranspiration is considered the net result of atmospheric demand for moisture from a surface and the ability of the surface to supply moisture, then PET is a measure of the demand side. Surface and air temperatures, insolation, and wind all affect this. A dryland is a place where annual potential evaporation exceeds annual precipitation.(Penman–Monteith equation (1965) - equation or method to estimate PET.Reference crop evapotranspiration (ETo) plays a major role in estimating crop water requirements and hence, it is an important factor in the hydrological studies of the region.

The calculation methods implemented in the MABIA Method are those of the FAO Penman-Monteith equation as outlined in the FAO Irrigation and Drainage Paper 56. This equation can be written as follows:

where

ETo = reference evapotranspiration [mm/day], Rn = net radiation at the crop surface [MJ/m^2/day], G = soil heat flux density [MJ/m^2/day] which can be neglected (G=0), Tmean = mean air temperature [°C], u2 = wind speed measured at 2 m height [m/s], es = saturation vapor pressure [kPa], ea = actual vapor pressure [kPa], es-ea = saturation vapor pressure deficit [kPa], D = slope vapor pressure curve [kPa/°C], g = psychrometric constant [kPa/°C].

Hello,

to facilitate the reasoning, the ET0 is the ETP of a reference crop (Grass) under reference conditions (no water stress, length of the lawn, etc.).

ETP remains a rather theoretical notion that is increasingly abounded, while ET0 is a notion that allowed us to move from "measuring" to "calculating" to facilitate the modeling of water needs.

best regards!

Dr. Nabil EL JAOUHARI

Qin Liu came across the same question with few answers on Research Gate and felt like sharing with you and the experts above

https://www.researchgate.net/post/Difference_between_the_Actual_Evapotranspiration_AET_and_Potential_Evapotranspiration_PET

Hope so this will be helpfull

Dear Dr Qin Liu . See the following useful RG link: Article Similarity and difference of potential evapotranspiration an...

Kindly see also the following useful RG link : Preprint Methods of Evapotranspiration

Also check please the following good RG link: Article Estimating reference crop evapotranspiration under limited c...

The following RG link is also very useful: Chapter A Parametric Model for Potential Evapotranspiration Estimati...