Summary
The FAO modified Penman equation has gained acceptance as a standard method of estimating reference crop evapotranspiration. Although theoretically sound the Penman equation becomes increasingly empirical when parameters or variables have to be estimated. When evapotranspiration estimates are being used for practical purposes the uncertainties introduced by these empirical factors and relationships should not be neglected. Evapotranspiration estimates for north-east Sri Lanka are used to illustrate the importance of the empiricisms in the FAO modified Penman equation. It is shown that the different empirical relationships used to estimate net radiation and the wind function in the FAO modified Penman equation and in the Penman (1963) equation produce a 23% difference in the estimate of annual reference crop evapotranspiration.
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Batchelor, C.H. The accuracy of evapotranspiration estimated with the FAO modified penman equation. Irrig Sci 5, 223–233 (1984). https://doi.org/10.1007/BF00258176
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DOI: https://doi.org/10.1007/BF00258176