A Modified Technique for Measuring the Unsaturated Hydraulic Conductivity in Moss and Peat Soils

English Abstract:

Currently, there are two methods for measuring unsaturated hydraulic conductivity (K_unsat) in Sphagnum mosses: the original method developed by Price et al., (2008) and a modification of this method by McCarter and Price (2014) (herein referred to as the “original” and “modified” methods, respectively). The original method has a soil-water pressure (ψ) gradient across the sample and requires 1-2 days to complete, whereas the modified method maintains a constant ψ within the sample but requires ~3 weeks to parameterize. The concern is that the gradient in original method may result in a distribution of K_unsat in the sample that is not represented by the mid-depth pressure. Twenty samples (10 cm diameter, 5 cm height) of horticultural peat were compressed to a target bulk density (0.19 ± 0.01 g cm³). Measurements of K_unsat were conducted at ψ = -5, -10, -15, and -25 mb using both the original and modified method. The volumetric soil water content (VWC) was lower (t-test: df = 19, p<0.001) at both -10 and -15 cm using the modified method, while no statistical difference was found (t-test: df = 19, p>0.1) at -5 and -25 cm. However, the K_unsat was lower with the modified method at all ψ tested (Wilcoxon signed rank test: V = 210, p<0.001). A general linear model used to elucidate factors controlling K_unsat found VWC (p<0.001) and the method (p<0.01) were the only significant relationships; thus, the ψ distribution in the original method was partly driving the difference in K_unsat. Although the original method is appropriate for representation of field conditions where a ψ gradient is typically imposed, the modified method appears to be representative of the actual K_unsat of the sample.