Can Orthogonal Projections of Visnir Spectra Yield Useful Information On Soil Water?.

VisNIR diffuse reflectance spectroscopy has been used to rapidly measure physical and chemical properties of the soil, including clay and organic matter content.  Differences in soil water cause non-linear changes the reflectance signal, particularly in the near-infrared range, leading to inaccuracies in the models developed to predict soil properties using VisNIR spectra.  Typically, to prevent interference by soil water, spectra are collect on air dried samples.  Recently however, external parameter orthogonalization (EPO) has emerged as a technique to remove the effect of the water content from VisNIR spectra, without knowing the water content of the soil at the time it was scanned.  This correction allows spectra collected from soils at a variable and unknown water contents to be used in prediction models developed from soils scanned in the dried and ground condition.  The EPO algorithm used to remove the effect of water content from soil spectra, creates two a projection matrices, one that only has the useful soil spectra with information on a soil property of interest, e.g. soil Organic C, and one matrix with the effect of water content on the soil spectra.  Results from this study will present investigations into the matrix with information on the effect soil water.