The performance of Hargreaves (HG) method is improved by adjusting the parameter of the original HG method according to regional conditions. The HG method was modified by resulting new value of constant by applying simple mathematical logic.
In the above equation, ETo HG was set equal to ETo PM and the constant ‘0.0023’ was set as ‘M’ to be determined. Hence, the adjusted equation was written as
ETo HG= ETo PM = M Ra (Tmax – T min) 1/2 T + 17.8
The calibrated HG equation is in the form
Y = MX
Where, Y= ETo PM; X= Ra (Tmax – T min) 1/2 T + 17.8. By the determined set of values of Y and X, the constant M was calculated. The Hargreaves method attained by means of above logic i-e by varying the value of constant has been written as HG mod equation. The value of Ra (extraterrestrial radiation) used in Hargreaves method was determined in research conducted by Samani and Hargreaves.
Statistical evaluation
In this study, the roots mean square error (RMSE), percentage error of estimate (PE), standard error of estimation (SEE) and coefficient of determination (R2) were used for the evaluation of ETo methods.
(3)
(4) (5)
Where Pi= ETo estimated by the PM method and Oi= ETo estimated by different forms of HG method. P and ? are the average of Pi and Oi, and n is the total number of data.

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