Evapotranspiration estimation from infrared surface temperature. I: The performance of the flux equation

In many conditions, there is a need for simple yet accurate methods to estimate crop evapotranspiration. There is interest in the energy balance approach based on infrared surface temperature (T-s). However the accuracy of the method relies on how closely T-s approximates the true aerodynamic temperature (T-0). Several authors have reported a reasonable agreement between the two temperatures, allowing good estimates of latent heat flux. In this study, errors in T-0 were minimized by measuring the sensible heat flux independently from the other components of the energy balance, and by completing the aerodynamic resistance from the top of the canopy: the level sensed by the infrared thermometer; lip to the reference level. Only data gathered in neutral conditions were retained. Results show that T-s, especially In dry conditions, can greatly depart from T-0. The implications of this difference are that (1) the aerodynamic resistance cannot be corrected for stability conditions based on the difference between radiometric surface and air temperatures; and (2) estimates of sensible heat flux using T-s are subjected to considerable errors, either in magnitude and specially in sign. This indicates the need for a different interpretation of the meaning of T-s.