An Empirical Simplification of the Temperature Penman-Monteith Model for the Tropics

A simple empirical equation (EPM) is presented to considerably shorten the computational steps required to estimatereference grass evapotranspiration (ETo) in the tropics using the FAO-56 Penman-Monteith equation (TPM) when theonly available weather data are those of temperature. Generally EPM predicted TPM ETo with very high efficiencies, achieving statistical performance measures as high as r2 = 1.00 , E1 = 0.98 , E2 = 1.00 , MAE=0.01 mm/day in testson data from six locations in four countries in West Africa. EPM was of general form , = b / /17000ETo EPM T Rs k Ra− χ , where ETo = reference grass ETo (MJ m−2 d −1 ), T = daily average air temperature ( o C), Rs = estimated solarradiation (MJ m−2 d −1 ), Ra = computed extraterrestrial radiation (MJ m−2 d −1 ); b , k , and χ , were parameterscomputed from local latitude and temperature data. The simplicity of EPM is expected to encourage wider usage ofTPM ETo estimates which are more accurate than estimates obtained by using locally-uncalibrated versions of simplerETo models where only temperature data are available