As presented above, the FAO-PM equation has been selected together with the updated definition of reference ET. All calculation procedures have been standardized according to the weather data observations and the timescale of computations to improve the accuracy of estimation. Therefore, it is highly recommended that the FAO-PM method be applied when full data are available and when appropriate estimation procedures are applied to missing data. Therefore, if a different method is to be used, it should first be validated and/or calibrated using the FAO-PM.
The validation or calibration of the method should be on a regional basis because different trends and ranges in the variation of meteorological variables induce different behavior of the ET equations. These are never linear and may respond with more sensitivity to one variable than to another. Thus, regional validation or calibration should be performed in large regions, where those trends and ranges in the variation of weather variables are similar. However, there is less need for climatic homogeneity as is required when variables have to be imported from one weather station to another to complete data sets.
For regional calibration of the selected equation, simple calibration factors can be computed on a monthly basis through linear regression as
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