Comparison of Canopy Emissivity Parametric Models With TES Emissivity Measurements

Abstract

Canopy temperature is a key factor in many studies, such as evapotranspiration and heat fluxes estimation. To retrieve it accurately, it is needed a precise characterization of the emissivity in the thermal infrared spectral range. Several parametric models are proposed to retrieved effective emissivity at different observation angles, from the previous knowledge of the vegetation and soil emissivities. The present work compares some of these models with emissivity measurements obtained with Temperature-Emissivity Separation (TES) method. For that, FR97, Mod3 and Rmod3 parametric models have been compared with radiometric measurements. Emissivity measurements were done for 7 different observation angles, from 0o to 60o in steps of 10o, and were repeated for six values of leaf area index (LAI) varying from 0.5 m2/m2 to 2.8 m2/m2.For high LAI values (more than 1.5 m2/m2), differences between TES and models emissivities are from +0.011 to −0.009, being positive for the FR97 model and negative for the Mod3 and Rmod3 models. Very little variation in emissivity is shown at high observation angles in all models at high LAIs. The measured emissivities are shown constant against observation angles, without any decreasing for high angles.For low LAI values (less than 1.5 m2/m2), these differences vary from −0.002 to −0.024. In this case, the FR97 model values show a good agreement with the TES emissivity measurements.Therefore, for high LAI values, the TES emissivity measurements are closer to those of the Mod3 and Rmod3 models. However, the TES emissivity measurements are closer to those given by the FR97 model at lower LAI values.