The ET equation is considered calibrated when the SEE < 20% of the average ET0 estimate.
For these regional validation or calibration studies, it is recommended that one use weather data observed above a green vegetation cover. Because the sensitivity of ET0 estimates to changes in T and ea are generally nonlinear, the ratios of sensitivities of two ET0 equations will rarely equal the ratios of ET0 estimates by the two equations. Therefore, use of nonreference weather data will introduce a bias in linear calibration coefficients.
When the vegetation becomes dry during any period, it is advisable to use only weather data from months during which precipitation or irrigation is sufficient (greater than ET0) to ensure that the stations were surrounded by green, evaporating vegetation and that weather measurements were synchronized with the reference ET energy exchange.
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