Measurement and spatial variation of thermal infrared surface emissivity in a savanna environment

Abstract

Knowledge of the surface thermal emissivity is necessary for the application of thermal infrared remote sensing, which forms the basis for water and surface energy balance monitoring from space. Thermal emissivity was measured in the wavelength band 8–14 μm for a series of representative surfaces within a natural bush‐savanna environment intermixed with cultivated fields in Botswana. The measurements were performed with an emissivity box, which may be constructed with different dimensions to meet the general requirements of the user. The device is well‐suited for bare soil, grass, and other types of soil and low‐vegetation cover combinations. The measured emissivity varied between 0.914 for bare soil (loamy sand) and 0.986 for a surface completely covered with savanna shrub (Euclea undulata). Calculations are based on the measurements of the radiative surface temperature of three different reflecting configurations of the box. Repetitive estimates of the emissivity were found to be very reproducible for the same surface types and showed the large spatial variability of surface emissivity within the study area. This paper describes the emissivity box together with other versions of the box concept found in the literature, and it describes the operation and calibration procedure as well as the results and consequences of the emissivity measurements for the application of thermal infrared remote sensing.