|Hunsaker, Douglas - Doug|
Submitted to: IEEE Transactions on Geoscience and Remote Sensing
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/9/2010
Publication Date: 5/27/2011
Citation: Sanchez, J.M., French, A.N., Mira, M., Hunsaker, D.J., Thorp, K.R., Valor, E., Caselles, V. 2011. Field thermal infrared emissivity dependence on soil moisture. IEEE Transactions on Geoscience and Remote Sensing. 99:1-8. DOI:10.1109/TGRS.2011.2142000. Interpretive Summary: Land surface temperature is closely related to soil moisture and therefore to water transport between soils, plants and the atmosphere. These temperatures can be used to help measure plant water use, detect plant water stress, or to monitor effects due to climate change. To improve the accuracy of surface temperature estimates over irrigated lands, a study was undertaken to quantify the effects of soil moisture upon a thermal infrared emissivity, a radiative temperature efficiency factor. The objective was to observe changes in emissivity relationships over time after repeated irrigation events. In a spring 2010 experiment at Maricopa, Arizona, emissivities increased logarithmically with increased surface soil moisture. Emissivities also increased with soil cracking density, a commonplace feature observed after repetitive irrigations. The results of this study are important for remote sensing research scientists and also for agricultural engineers utilizing land surface temperature observations to schedule irrigations.
Technical Abstract: Accurate estimate of land surface temperature, a key parameter in surface energy balance models, requires knowledge of the surface emissivity. Emissivity dependence on soil water content has been already reported and modeled under controlled conditions at the laboratory. This study 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 5 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 2-3 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 using the gravimetric method. An overall increase of emissivity with soil moisture 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, deterioration of the soil surface tends to reduce the emissivity spectral contrast. Soil-specific and general relationships obtained by Mira et al. (2010) were tested and compared with the field measurements. Field emissivities agree within 2% with the modeled values for all bands under non-cracked surface conditions, whereas differences reach 5% in the 8-9 µm range when cracks are present.