Effects of vapor-pressure deficit and net-irradiance calculation methods on accuracy of standardized Penman-Monteith equation in a humid climate

The effects of some common vapor pressure deficit (VPD) and net irradiance (R-n) calculation methods on the accuracy of ETo values estimated by using the standardized ASCE Penman-Monteith (ASCE-PM) equation for short grass were examined by comparing the estimated ETo values with measured ETo values in a humid climate. Sensitivity analysis showed 17% and 84% change in the estimated daily ET, values per unit change in the calculated VPD and R-n values, respectively. A total of 12 VPD and 27 R-n calculation methods were examined. Analyses of variance indicated lack of equality in the means of estimated ET, values obtained by different VPD and R-n methods. The percent mean error in the estimated ETo values ranged from -0.9 to -8.4% for VPD methods and from -0.3 to -19.7% for R-n methods. On the basis of the coefficient of determination (r(2)) and the standard error of the estimated (S-y/x) values, the VPD calculated from saturation vapor pressure (e(s)), estimated by averaging the e(s) at the maximum and minimum daily air temperatures, and actual vapor pressure (e(a)), estimated by using either the average of minimum and maximum relative humidity or the dew-point temperature, gave more accurate results. Net irradiance (R-n) estimated by using a regression of relative short-wave solar irradiance, as well as a linear regression on the square root of e(a), resulted in relatively more accurate estimates of ETo than that obtained by methods based on e(a) or clear-sky data alone. These results indicate that in a humid climate, some of the VPD and R-n methods have a significant effect on the accuracy of the ETo estimated by using the standardized ASCE-PM equation.