It was expected that the pan method would give acceptable estimates, depending on the location of the pan. The modified Penman method was considered to offer the best results with minimum possible error in relation to a living grass reference crop. To accommodate users with different data availability, four methods were presented to calculate the reference crop evapotranspiration (ET o): the Blaney-Criddle, radiation, modified Penman and pan evaporation methods.
To meet this need, guidelines were developed and published in the FAO Irrigation and Drainage Paper No. Testing the accuracy of the methods under a new set of conditions is laborious, time-consuming and costly, and yet evapotranspiration data are frequently needed at short notice for project planning or irrigation scheduling design.
Relationships were often subject to rigorous local calibrations and proved to have limited global validity. Need for a standard ET o methodĪ large number of more or less empirical methods have been developed over the last 50 years by numerous scientists and specialists worldwide to estimate evapotranspiration from different climatic variables. The method, its derivation, the required meteorological data and the corresponding definition of the reference surface are described in this chapter. The FAO Penman-Monteith method is recommended as the sole ET o method for determining reference evapotranspiration.
This chapter introduces the user to the need to standardize one method to compute reference evapotranspiration (ET o) from meteorological data.
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