Scientists discover why wet soil is dark

In a major breakthrough Associate Profession William Philpot of Cornell School of Civil & Environmental Engineering has cracked the secret of wet soil. He has now produced a simple theoretical model describing the reflectance from wetted soils which describes the major variations of changes in the spectral reflectance which lead to soil getting darker when wet.

This is hugely encouraging as previous cutting edge research on the darkening of soil have suggested very distinct theoretical hippotheses. Whilst Ångström (1925) attributed the reduction to total internal reflection within the film of water coating soil particles, later authors have expanded on this idea in a variety of ways, modifying Ångström’s formula to account for light that is not totally internally reflected (Lekner and Dorf, 1988) and incorporating the effect of spectral absorption by the water itself (Bach and Mauser, 1994), a necessity when considering spectral reflectance through the short wave infrared (SWIR).

Philpot’s work whilst theoretical, is grounded in good basic experimentation. However Philpot is strangely modest over his groundbreaking discovery:

the model should be regarded with skepticism and used with great care, if at all. On the other hand, while the conceptual design of this model leaves much to be desired, its success suggests that there is some truth captured by the model structure.”

For those of a scientific mind, there are at least three general conclusions that can be drawn from this research:

Emitted light becomes darker due to multiple reflections from water film surface back to soil particles where each pass absorbs more light

  1. The darkening effect appears to be spectrally bland and largely independent of absorption effects of the water.
  2. Absorption by water is significant in the infrared suggesting that there is a substantial optical path in the pore water.
  3. The absorption spectrum of water is modified, probably due to absorption by substances dissolved in the water and/or as a result of water being partially bound to the soil.

http://www.asdi.com/documents/spectral-reflectance-of-wetted-soils.pdf

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