Mid-infrared laser reflectance of moist soils

The effect of varying moisture content on the mid-infrared (9-11-µm) laser backscatter reflectance of different soils was investigated experimentally. Total reflectance was separated into a diffuse and a coherent component. Consistent with observations in the visible and near-infrared spectral regimes, the diffuse mid-infrared reflectance values decrease with increasing gravimetric moisture content. At low moisture contents, this phenomenon is attributed to increased forward scattering, while total internal reflection at the water-air interface results in lower reflectance values at higher moisture contents. The coherent component, however, increases initially for low moisture content as a result of the reduction in surface roughness on moistening, decreases subsequently, and then tends to increase again as the moisture content nears the field capacity of the soil. An empirical model based on the above physical mechanisms is developed for the diffuse component and tested by using the measured data. It is conjectured that the laser reflectance is sensitive to moisture in the surface layer (≃ 0.25-mm depth) and thus may have the potential to monitor near-surface soil moisture.