EVAPOTRANSPIRATION ESTIMATION FROM INFRARED SURFACE TEMPERATURE. I: THE PERFORMANCE OF THE FLUX EQUATION

摘要

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 o ). Several authors have reported a reasonable agreement between the two temperatures, allowing good estimates of latent heat flux. In this study, errors in T o were minimized by measuring the sensible heat flux independently from the other components of the energy balance, and by computing the aerodynamic resistance from the top of the canopy, the level sensed by the infrared thermometer, up 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 o . 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 .