The effect of moisture content and humification on the hyperspectral reflectance of peat

Work using HyMap has shown a relationship between hyperspectral reflectance and the degree of humification of in situ upland peats in the south Pennines, especially in the shortwave infrared. The contribution of moisture content, however, could not be excluded. In order to decouple moisture and humification effects, a controlled drying experiment was carried out in the laboratory on three peat blocks of low, intermediate and poor humification. The NERC ASD FieldSpec Pro was used in contact probe mode to measure peat reflectance spectra at different gravimetric moisture contents. Analysis has concentrated on water absorptions centred on 1400 and 1940 nm and ligno-cellulose absorption at 2100 nm. As the peat dried out, albedo increased and changes occurred in the position of the shoulders, the slope between them and the position, depth, width and asymmetry of features below the continuum line. Spectral indices were used to express these changes. Humification signals became more pronounced in dry peat, and were stronger for poorly humified peat. The 1200 nm feature associated with water, cellulose and lignin disappeared on drying, even for the poorly humified peat, suggesting that it was primarily due to water. Absorptions at 1400 and 1900 nm remain even in dry peat. Early results suggest that the albedo of the shoulders of the 1900 nm feature, and to a lesser extent the 1400 nm feature, are the most sensitive to moisture, while the shape of the features below the continuum line is also sensitive to humification. On-going work is testing the value of combinations of candidate indices to separate peat into moisture-humification classes.