Allen et al. (1998) (FAO 56) suggested that Hargreaves-Samani (HS) equation could be calibrated by linear regression using Penman Monteith as reference, according to the following model: ET0 = a + b(ET0HS). To perform this calibration, I just need to plot the Penman-Monteith in the Y axis and Hargreaves-Samani in X axis and obtain the parameters a (intercept) and b (slope) of the best fitted line and use them as calibration parameters for the above cited model?
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
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