Modified technique for measuring unsaturated hydraulic conductivity in sphagnum moss and peat

Highly compressible soil, such as Sphagnum moss and peat, undergo volume change with varying volumetric soil water content (q) and pressures (y), so typical methods for determining unsaturated hydraulic conductivity (K_unsat) in non-compressible mineral soils can be problematic. However, characterizing these relationships are essential for modeling ecohydrological processes. Two methods have been developed for determining K_unsat of these highly compressible soils using “floating” tension disks; the original method imposes a y gradient across a sample, while a modified method imposes no y gradient and flow is driven by gravity. However, it is unknown if they produce comparable results. Milled horticultural peat was compressed (n = 34) to a bulk density of 0.19 ± 0.01 g cm^-3 and K_unsat and q were measured for each sample at variable y-steps (-5, -10, -15, and -25 cm) for both methods. In the modified method average q was found to be lower (p < 0.001, df = 19) at y-steps -10 and -15 cm, while average K_unsat was lower (p < 0.001, W = 210) at all y-steps. Numerical modeling (Hydrus-1D) of each y-step identified nonlinear distributions of y, q, and K_unsat within a sample in the original method (verified with tensiometer measurements), whereas a uniform distribution of these parameters was observed in the modified method. We conclude the modified method produces a more precise measurement of the K(y) function. Although these methods were developed for Sphagnum moss and peat soils, the method can likely be used for other compressible or delicate media.