Computation of ETo under Nonideal Conditions
Publication: Journal of Irrigation and Drainage Engineering
Volume 123, Issue 5
Abstract
The Food and Agriculture Organization of the United Nations (FAO) version of the Penman-Monteith method (FAO-PM) has recently been proposed as the standard for estimating reference evapotranspiration (ETo). A standard, well-watered reference environment for the data used for the ETo estimate has also been proposed. At those sites providing a reference environment the average minimum air temperature (Tn) is approximately the same as average dew point temperature (Td). For this reason and because the computation of Td is plagued with errors, the FAO-PM can be modified to use Td=Tn. This modification is referred to as FAO-MOD. However, most weather stations lack the intensive input required by the FAO-PM. Few weather stations provide a standard environment. Errors in measuring the required weather variables are common. Under these conditions, equations with few weather parameters are recommended. The Hargreaves et al. (1985) method (ETH) is one of the simplest and most accurate empirical equations. The ETH method and ETH corrected for site aridity (ETH-MOD) are compared with the FAO-PM and FAO-MOD. Comparisons are made for many different locations and conditions. The results indicate that the differences in ETo values computed by the different methods are minor when compared with the uncertainties in estimating actual crop evapotranspiration.
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Information
Published In
Journal of Irrigation and Drainage Engineering
Volume 123 • Issue 5 • September 1997
Pages: 394 - 400
Copyright
Copyright © 1997 American Society of Civil Engineers.
History
Published online: Sep 19, 1997
Published in print: Sep 19, 1997
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