Estimation of the Flow Conducting Soil Pore Spectra Using Borehole Permeameter and Soil Core Samples

Abstract

Aims: Hydraulic conductivity plays an important role in the evaluation of various soil functions, which in turn may become decisive in urban development plans. Different approaches to measure hydraulic conductivity often inhere in methodological artefacts. To address the subjected soil volume as well as different measurement approaches, in situ measurements using a constant-head permeameter were compared to lab-determined hydraulic conductivities of soil cores at adjacent soil profiles. Methodology: At a forest, an arable, and a grassland site in a catchment in Northern Bavaria retention characteristics as well as hydraulic conductivities were quantified for 100 cm³- and 250 cm³-soil cores and constant-head permeameter measurements conducted. Using the van-Genuchten RETC-program, unsaturated hydraulic conductivities of the soil core samples were calculated. Results: No volume-related differences between hydraulic conductivities of the 100 cm³- and the 250 cm³-soil cores became evident. Hydraulic conductivities of the soil cores markedly exceeded the values of the borehole permeameter approach. Contrasting the former with the hydraulic conductivities measured with the borehole permeameter, soil pore threshold diameters were derived that might be excluded from the infiltration process. While lab-determined hydraulic conductivities of soil cores were based on almost the whole soil pore spectrum, in-situ measurements mostly reflected matrix flow. This effect was most pronounced at the forest and least pronounced at the arable-land site, pointing to the respective soil pore spectra. Conclusion: Borehole permeameter measurements should be chosen to illustrate percolation under standard conditions, while the soil-core approach is favourable under wet boundary conditions, when macropores are active, e.g. in the scope of flow and contaminant transport studies.