Integrating Muli-Step-Flux-Method for full range soil hydraulic characterisation: From saturation to oven-dryness

Abstract. Soil hydraulic properties (SHP), defined by the water retention curve (WRC) and the hydraulic conductivity curve (HCC), are crucial to describe water storage and flow in soils. Several methods have been developed and combined to measure these two fundamental curves across the full range from saturation to oven dryness. However, for the HCC, there is still a data gap between approximately -1 to -100 hPa, which is expected to be affected by soil structure. We present an experimental workflow in which the multi-step-flux (MSF) method is integrated into the well-established combination of methods for measuring SHP, namely the falling head method, the simplified evaporation method, and the dew point method, specially designed to be applied to the same sample. The MSF is an adaptive, direct measurement of the HCC based on applying series of steady-state water flows to a soil sample characterised by unit hydraulic gradient. Once equilibrium is achieved, the sample is characterised at each step by a constant pressure head, constant water content and constant unsaturated hydraulic conductivity which is equal to the applied flux. We tested the method for three different soil columns: a repacked sand with a very well-defined air-entry pressure and two undisturbed structured silt loams. For the sand, the MSF results coincide with the saturated hydraulic conductivity value, measured with the falling head method. For the undisturbed loam samples, the structural effect on the HCC is clearly visible. Integrating the MSF into the common lab-workflow to characterise SHP will help future studies to investigate soil structure effect on SHP.