do you have any field data of evapotranspiration? this is the best for your calibration of the H-S equation. P-M equation may also estimate the ET with significant bias.

Thanks Yi Luo. I agree with, but my doubt is in the calibration process.

Dear Lucas,

You can refer to the following article for your asked question on calibration of Hargreaves method.

https://link.springer.com/article/10.1007/s11629-014-3238-3

Hope it helps!

Dear Lucas, Refer following help you to decide suitable calibration of the H-S equation

http://onlinelibrary.wiley.com/doi/10.1002/ird.1874/abstract

     the goal of Hargreaves-Samani (http://www.zohrabsamani.com/research_material/files/Hargreaves-samani.pdf)  is to validate methods which can estimate reference evapotranspiration with limited data; specifically without direct measurement of solar radiation.  I would suggest using this estimated solar radiation directly in the ETref calculation as Watts per meter^2.  Then, using a crop coefficient curve, the ETref can be converted to ETcrop for the exact crop and the exact maturity level.

      Finding a good and valid crop coefficient curve for a crop in a soil type is difficult. One must consider creating the curve through on site through experimentation.  

     My preferred method for solar radiation value is to use a solar radiation sensor.  This is the most expensive sensor in an ET weather station.  But since one ET weather station can serve a large area, the cost of this sensor is paid for by potential water savings.  I use the 6450 Davis Solar Radiation Sensor

https://www.zohrabsamani.com/research_material/files/Hargreaves-samani.pdf)

Effectively the calibration is done as you describe it. However, I suggest you read the article that we have published with several colleagues that describes this process:

Luis Morales-Salinas, Samuel Ortega-Farías, Camilo Riveros-Burgos, José Neira-Román, Marcos Carrasco-Benavides & Rafael López-Olivari. 2017. MONTHLY CALIBRATION OF HARGREAVES–SAMANI EQUATION USING REMOTE SENSING AND TOPOCLIMATOLOGY IN CENTRAL-SOUTHERN CHILE. International Journal of Remote Sensing. https://doi.org/10.1080/01431161.2017.1323287.

I have two years data of AET , can able to do such validation ? If yes, anybody can give me detail procedure about it's computation or ready to coloration. please.

Hey Pralhad, check my publication regarding different calibration methods for Hargreaves-Samani Equation.

Article Calibration methods for the Hargreaves-Samani equation

Thanks very much for valuable guidance