Thermal Infrared Emissivity Dependence on Soil Moisture in Field Conditions

An accurate estimate of land surface temperature, which is a key parameter in surface energy balance models, requires knowledge of surface emissivity. Emissivity dependence on soil water content has been already reported and modeled under controlled conditions at the laboratory. This paper completes and extends that previous work by providing emissivity measurements under field conditions without elimination of impurities, local heterogeneities, or soil cracks appearing in the drying process. The multispectral radiometer CE312-2, with five narrow bands and a broad band in the 8-13-μm range, was used, and surface emissivity values were determined through a temperature-emissivity separation algorithm. A bare soil plot of 10 ×17 m² was selected for this study in the framework of a camelina 2010 experiment. This experiment was carried out during March and April 2010 at The University of Arizona Maricopa Agricultural Center in central Arizona, USA. The soil plot was flood irrigated every two to three days and left to dry. Field emissivity measurements were collected under cloud-free skies, around noon, for different values of soil water content. Soil samples were collected to estimate the soil moisture (SM) using the gravimetric method. An overall increase of emissivity with SM was obtained in all channels. However, when wetted soils subsequently dried, the final minimum emissivity was greater than the initial minimum emissivity. This hysteresis could be due to cavity effects produced by soil cracks not originally present. Thus, the deterioration of soil surface tends to reduce the emissivity spectral contrast. Soil-specific and general relationships obtained by Mira et al. were tested and compared with the field measurements. Field emissivities agree within 2% with the modeled values for all bands under noncracked surface conditions, whereas differences reach 5% in the 8-9- μm range when cracks are present.