The other FAO recommended equations showed variable adherence to the reference crop evapotranspiration standard of grass. The modified Penman was frequently found to overestimate ET o, even by up to 20% for low evaporative conditions. Deviations from computed to observed values were often found to exceed ranges indicated by FAO. Although the results of such analyses could have been influenced by site or measurement conditions or by bias in weather data collection, it became evident that the proposed methods do not behave the same way in different locations around the world. Numerous researchers analysed the performance of the four methods for different locations. Users have not always respected these conditions and calculations have often been done on daily time steps.Īdvances in research and the more accurate assessment of crop water use have revealed weaknesses in the methodologies. For the pan method it was suggested that calculations should be done for periods of ten days or longer. The Blaney-Criddle method was recommended for periods of one month or longer. These climatic methods to calculate ET o were all calibrated for ten-day or monthly calculations, not for daily or hourly calculations. Finally, the publication proposed the use of the Blaney-Criddle method for areas where available climatic data cover air temperature data only. The radiation method was suggested for areas where available climatic data include measured air temperature and sunshine, cloudiness or radiation, but not measured wind speed and air humidity. 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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