MOISTURE EFFECTS ON VISIBLE SPECTRAL CHARACTERISTICS OF LATERITIC SOILS

Samples from ferralitic soils of similar texture were immersed in water, placed in a pressure chamber and subjected to different soil suction pressures (pF: 2.0, 2.5, 2.8, 3.0, and 4.2). The Cie color values (intensity, hue and saturation) were taken as the spectral signatures of these soils and calculated from the diffuse reflectance spectra. The dominant wavelength lambda(d) (hue) increases with moisture content; as the soil becomes more "colored" by iron oxi-hydroxides, this increase becomes more pronounced. The purity Pe (saturation) decreases systematically with increasing moisture content. When the moisture content increases, i.e., from oven-dried to air-dried and then to moist samples, there is a change of the refractive index, N, of the immersion medium from N = 1 (air) to N = 1.33 (water). This change causes an increase in the volumetric reflectance, especially in the weak absorption regions, and a decrease in the specular reflectance. Contrary to the generally accepted assumption, diffuse reflectance spectra of soils at different moisture contents undergo nonlinear changes leading to displacements in the spectral extrema of the absorption bands. Therefore, spectral signatures of moist soils cannot be derived simply from those of their dry equivalents.