You can use the ETo Calculator as provided by the FAO.

Check this link for guide

http://www.fao.org/land-water/databases-and-software/eto-calculator/en/

Reference evapotranspiration is that from a grass surface. Potential evapotranspiration is from water surface (such as a lake). The methodology used to compute is described by www.usgs

Dear Swadhina Koley, Reference evapotranspiration (ETo) is the maximum water loss in vapour form from an exposed surface covered with short (10-20 cm ), green, healthy grass with no short of supply of water whereas PET is maximum water loss from an exposed surfaces (not included any reference point) covered by short grass with no short supply of water. Both are different thing. Next thing is that, you can also compute ETo from PET with use of empirical methods that depend upon your crop, location and other parameters

Dear Dr Yogesh please refere the link https://www.hydrol-earth-syst-sci.net/17/1331/2013/hess-17-1331-2013.pdf

Thank you so much @Dr Anil Kumar....for reference, kindly follow the above referenced link for more details about ET.

The concept of potential evapotranspiration (PET) is no longer used now, as it has been replaced by reference crop evapotranspiration (ETo) because of certain reasons. In old books, you will see only PET and not ETo.

The potential evapotranspiration concept was first introduced in the late 1940s. It was 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 major objective in bringing this “potential” evapotranspiration concept was to remove the effects specific to crops in the evapotranspiration process.

The concept of PET was well accepted, and it became the focal idea in estimating crop water use for a long time. After some time, however, certain limitations cropped up. In the definition of PET, the ET rate was not related to a specific crop. The main confusion with PET was that there were many types of crops that fitted into the description of a ‘short green crop’, and one might be confused as to which short crop should be selected! Having considered the ambiguities and limitations of PET, by the early 1980s, the concept of “reference crop evapotranspiration” (ETo) was introduced.

The major objective of introducing ETo was to assess the evapotranspiration demand of the atmosphere independently without considering the effects of crops, cultivar differences, crop development, and management practices. In reference evapotranspiration, 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) after specifying the reference surface explicitly, defined reference evapotranspiration 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.”

Hi Swadhina,

Potential evapotranspiration (ETp): The potential evapotranspiration concept was first introduced in the late 1940s and 50s by Penman and it is defined as "the amount of water transpired in a given time by a short green crop, completely shading the ground, of uniform height and with adequate water status in the soil profile". Note that in the definition of potential evapotranspiration, the evapotranspiration rate is not related to a specific crop.

Reference evapotranspiration (ETo): Reference evapotranspiration is defined as "the rate of evapotranspiration from a hypothetical reference crop with an assumed crop height of 0.12 m (4.72 in), a fixed surface resistance of 70 sec m-1 (70 sec 3.2ft-1) 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".

For more details, you can also refer to:

Article Evaluation of Variable-Infiltration Capacity Model and MODIS...

https://edis.ifas.ufl.edu/publication/ae256

I hope it helps,

Cheers

Ankur

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)

C George Thomas Thanks for drawing the possible areas of making mistakes. Since you have mentioned the term crop coefficient, I would like to ask you about the term "area-averaged crop coefficients". I really do not have any concept about that. If you have the knowledge, please do share as it would help me to understand and implement the parameter in a part of my research. Any idea, like what is actually meant by the term "area-averaged", how to calculate one, or if there is any readily available database out there, from where I can obtain the values, would help.

Thanks in advance.

Regards.

I also didn't come across area-averaged crop coefficients. But you can have area-averaged evapotranspiration. I am providing a link to a paper.

https://d-nb.info/1143427750/34

Thank you so much, Sir C George Thomas

I invite all of to have a review of my latest book “Irrigation and Water Management”, where these kinds of issues are discussed in a contemporary style. Please visit

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

www.anebooks.com