Computation of ETo under Nonideal Conditions

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.