Effect of Soil Moisture on the Angular Variation of Thermal Infrared Emissivity of Inorganic Soils

Emissivity is influenced by different factors. This study deals with the effect of the soil moisture (SM) content on the zenithal (θ) variation of ratio-to-nadir emissivity (ε_r), for a wide variety of inorganic bare soils. To retrieve ε_r, a goniometer assembly was used, together with two identical CIMEL Electronique CE312-2 radiometers working at six spectral bands within 7.7-14.3 μm, performing simultaneous radiance measurements at different combinations of zenith and azimuth angles. The results showed that the effect of SM upon ε_r(θ) is different depending on the spectral range and textural composition of the sample. Sandy soils showed a decrease of ε_r(θ) from nadir up to 0.132 for θ ≥ 40^° at 8-9.4 μm under dry conditions, but this decrease was reduced to 0.093 with the increase of SM. Clayey samples did not present dependence of ε_r(θ) on SM. Loamy texture samples presented a more sharp decrease of ε_r(θ) with the increase of SM, reaching differences between nadir values and 70 ^° up to 6%, at all spectral ranges studied. Finally, a parameterization of ε_r with SM and θ was derived allowing to obtain ratio-to-nadir emissivities with an accuracy of ± 0.011.