COMPUTATION OF ETO UNDER NONIDEAL CONDITIONS

The Food and Agriculture Organization of the United Nations (FAG) vers ion of the Penman-Monteith method (FAG-PM) has recently been proposed as the standard for estimating reference evapotranspiration (ETo). A s tandard, well-watered reference environment for the data used for the ETo estimate has also been proposed. At those sites providing a refere nce environment the average minimum air temperature (T-n) is approxima tely the same as average dew point temperature (T-d). For this reason and because the computation of T-d is plagued with errors, the FAG-PM can be modified to use T-d = T-n. This modification is referred to as FA8-MOD. However, most weather stations lack the intensive input requi red by the FAG-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 recommend ed. The Hargreaves et al. (1985) method (ETH) is one of the simplest a nd most accurate empirical equations. The ETH method and ETH corrected for site aridity (ETH-MOD) ate compared with the FAG-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 di fferent methods are minor when compared with the uncertainties in esti mating actual crop evapotranspiration.