Spectral Reflectance of Wetted Soils

Soil reflectance spectra are responsive to a number of soil characteristics, including water content, mineral content, the presence/absence of organic material and the roughness or coarseness of the soil. To some extent, these components alter the spectral reflectance of soils independently, but there also appears to be some interaction among these effects. The model presented in this paper is a preliminary and relatively simple representation using the absorption and refractive properties of water along with absorption due to dissolved organic material to modifying the reflectance spectrum of a dry soil. The model is an attempt to capture the variability in reflectivity in a way that will lend itself to inversion. The model represents the effect of water in two ways: a thin layer of water on the soil particles alters the reflectance of the soil largely by reducing the magnitude of the reflectance; second, where water provides a more significant optical path (presumably with interstitial soil water) the spectral absorption characteristics of water dominate with some modification by the presumed presence of organic material dissolved in the water. Organic material can be expected to introduce absorption effects in the ultraviolet and blue portions of the spectrum. Taken together, these properties appear to account for a major portion of the spectral changes in soil reflectance; however, there are interesting and consistent discrepancies, at least in the current formulation of the model. The model and its characterization of soil reflectance is described and discussed.


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