Relationship between surface soil water content, evaporation rate, and water absorption band depths in SWIR reflectance spectra

Abstract

Abstract The spectral reflectance of three soil samples – selected to represent a range of particle size distribution, texture, and drying characteristics – was monitored as the samples progressed from fully saturated to air dry. Band depths of prominent water absorption features in the near and shortwave infrared, centered at 970 nm, 1160 nm, 1440 nm, and 1930 nm, were then compared to the surface water content, with the expectation that this metric would be characteristic of the soil moisture content and be largely independent of soil properties. As expected, the band depths at 970 nm and 1160 nm decreased monotonically as the water content decreased. Unfortunately, these bands were only detectable in the sample consisting of large, reflective particles and large pore spaces (quartz sand). That they were not apparent in the darker soils with smaller particles and pore spaces suggests that these water absorption bands will not be generally useful for observing surface soil moisture. In contrast, the band depths of the 1440 nm and 1930 nm absorption features were very responsive to the water content over the entire drying cycle, providing a strong, but more complex signal, with the band depth initially increasing with decreasing water content before reaching a maximum and finally decreasing late in the drying cycle. The change in band depth at 1930 nm was particularly interesting since it reached a maximum coincident with the transition from constant evaporation rate (stage-1 drying) to a much slower evaporation rate (stage-2 drying) for all three soils. A simple mathematical model suggests that the pattern of change in the band depth is related to the near extinction of light at the band center relative to the band shoulders, which can be related to a specific water optical path length. The maximum absorption band depth at 1930 nm (and the associated water optical path length) appears to coincide with a trace amount of water present in the pore spaces for all three soil samples, which is consistent with the transition in drying rates.