An investigation into the impact of soil particle conductivity and percolation threshold on the Hilhorst model to estimate pore water conductivity in soils

Abstract

The aim of this work has been to assess the Hilhorst model, used for estimating in-soil pore water conductivity, against soil properties of percolation threshold and soil particle conductivity. The Hilhorst model has the benefit of requiring a single soil-specific parameter which makes this model easy to apply from soil permittivity and bulk conductivity measurements. However, the Hilhorst model requires that the bulk conductivity measurement is dominated by pore water conductivity, which is not always the case in many ‘‘real-world’’ settings. This work examines a mathematical framework derived from combining the Hilhorst and Ewing and Hunt models which allows the Hilhorst soil parameter to be derived for a range of soil particle conductivities (0 to 10 mS m⁻¹) and percolation thresholds (0 to 0.1 m³ m⁻³). The analysis in this work indicates that the Hilhorst parameter is highly sensitive to both soil properties with respect to the default value of 4.1 that is often employed. This assessment indicates that: (i) soil particle conductivity can result in the Hilhorst model overestimating pore water conductivity, which becomes more significant for lower bulk conductivities, and, (ii) percolation threshold can result in the Hilhorst model underestimating pore water conductivity, which becomes more significant for lower soil moisture contents. The results from this analysis suggest that the application of the Hilhorst model should be considered against the context of soil particle conductivity and percolation threshold for a given soil under test.