| 01 Apr 2026
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.
In this technical note the authors demonstrate how different methods covering different ranges in water potential can be combined to get a consistent description of soil hydraulic properties. The key point is how the results of the Multi-Step-Flux methods to measure hydraulic conductivity at high water potentials coincides with the more traditional methods. The results look promising, which is good news. This is true for the three materials used in this study were only drainage was considered. The paper is well structured and largely well written. I have one general remark and some technical details that should be considered prior to publication.
General remark:
The focus is more on the technical aspects and less on the potential of practical applications. Nonetheless, I miss at least a short discussion of the limitations of the applicability of these lab-measured SHPs for modeling water dynamics in natural soil (e.g. using Richards equation). These limitations come from well known features such as hysteresis and non-equilibrium effects (only shortly mentioned in the introduction) but also due to the boundary conditions (i.e. natural atmospheric boundary condition vs well define flux rates and water potentials adjusted in the lab). The MSF method is an excellent tool to study these important features which could be mentioned as well. I am a little worried that this paper sends the message that with MSF we now filled a critical gap and that we can now carry on with our traditional beloved concepts - it is not like that!
Technical details:
L30 right reference? falling head method is much older.
L37ff What about tension disc infiltrometers that covering at least a part of this gap?
L78: How deep was the sample taken in this subsoil?
L87 Conductivity was only measured for decreasing water contents (i.e. decreasing flux rates?)
L89 Split this sentence in two?
L105-112: You could mention that this MSF method was introduced by Weller et al (2011)
L109: hydraulic heat?
L122: Why do you need a Mariotte bottle in addition to a peristaltic pump?
L124: Please provide units for the delay values, I suppose they are in cm
L132: Could you justify the duration the flux steps of „at least one hour“ based on data (for example tensiometer readings during this hour)? When looking at Weller et.al. (2011) the water potential was still far from equilibrium after one hour. I think perfect equilibrium cannot be reached but may be some range of uncertainty for the water potential in K(h) could be given?
L174: S-sharp?
L208ff: The MSF flux rates are all in the range above the air entry point while it should be easy to adjust lower fluxes in MSF (which is an important strength of the method). Why did you not do this?
In Fig. 2 I see only two data points while from what is written in line 199 there should be four. Could you explain?
L216: Again, tension infiltrometers are an established method in this range close to saturation. However, this is rather a field measurement but it would be interesting how it compares (in another paper).
L222ff: A note on the presentation in Fig.2: It is a bit difficult to assess curve shapes and “small step drops” when the symbols are plotted so large that such details are barely visible (but the figure probably looks much more convincing this way).
L263: I have difficulties to follow this statement. Why is a sharp drop in conductivity near saturation physically unrealistic? That is exactly what we would expect (and what is frequently observed) if a relatively dense material is permeated by macropores, such as earthworm burrows (as expected for Loam 2). And besides, that is exactly what was measured! The physical problem that Ippisch rightly pointed out, is that the conductivity curve cannot be derived from the WRC using Mualem’s concept when the van Genuchten parameter n is significantly less than 2 - but this is a different story.
L280: I am wondering if the well known dual porosity models are not suitable to address exactly this problem? It is obvious that additional MSF measurement are highly valuable, but do we really need new models to cover this phenomenon?