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 r2 =1.00 , E1 = 0.98 , E2 =1.00, MAE=0.01 mm/day in tests
on data from six locations in four countries in West Africa. EPM was of general form , = b / /17000
ETo EPM T Rs k ? Ra? ,
where ETo = reference grass ETo (MJ m?2 d ?1 ), T = daily average air temperature ( o C), Rs = estimated solar
radiation (MJ m ?2 d ?1 ), Ra = computed extraterrestrial radiation (MJ m ?2 d ?1 ); b , k , and ? , 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.

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