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

Abstract

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