| 05 Dec 2025
Thermohydraulic Experiments on Water Infiltration in Frozen Slopes: The Role of Macropores and Initial Water Content
Abstract. Infiltration of rainwater or snowmelt into frozen soil is strongly constrained by ice-blocked pore spaces, depending on the thermal and hydrological state. The resulting reduction in permeability promotes surface runoff, which can trigger erosion or debris flows. Preferential pathways such as macropores can locally bypass this barrier, yet their quantitative role has remained poorly constrained by experiments. Here, we present nine large-scale rainfall experiments in a tiltable soil box inside a controlled climate chamber, systematically varying initial water content and the presence or absence of an interconnected macropore network. The coarse textured soil was instrumented with a dense three-dimensional grid of temperature and volumetric water content sensors, complemented by continuous outflow monitoring of drainage and surface runoff. Frost depth was governed primarily by the antecedent thermal state and only weakly by the macropore network or initial water content. In contrast, infiltration/runoff partitioning depended strongly on initial water content and secondarily on the macropore network. Under low initial water content conditions, infiltration was dominated by matrix flow, whereas at high initial water content the frozen matrix became effectively impermeable and the macropore network enabled rapid bypass infiltration. Progressive refreezing and particle-assisted clogging reduced macropore functionality over time, shifting flow towards surface runoff. These results reveal the transient, non-linear role of macropore networks in frozen soils and provide a benchmark for testing dual-domain and non-equilibrium models relevant to process representation in alpine hydrology and slope stability.
The manuscript presents a carefully designed set of large-scale laboratory experiments investigating infiltration, runoff, and drainage in frozen slopes, with particular emphasis on the role of macropore networks and antecedent water content. The experimental setup is ambitious, the dataset is rich, and the effort to move beyond traditional column experiments toward a slope-scale configuration is appreciated. The topic is timely and relevant to cold-region hydrology and slope processes, and the paper has the potential to become a useful benchmark dataset for model testing. The manuscript is well-organized and prepared with clarity. Most of the methodological limitations are clearly outlined.
However, in its current form, the manuscript systematically over-interprets its results in favor of its hypothesis. Where the observations are robust, the conclusions are largely intuitive and confirm existing understanding; where the authors advance more interesting or non-intuitive interpretations, the supporting evidence is insufficiently constrained. Several claims conflate what is observed with what is inferred, and in some cases the causal chain between measurements and conclusions is not convincingly established.
For these reasons, I do not recommend acceptance in the current form. At the same time, I do not consider the work fundamentally flawed. A major revision is appropriate, provided that the authors substantially revise the interpretation, tighten the logic, and clearly delimit what can and cannot be concluded from the data.
Major comments
1) A recurring issue throughout the manuscript is the insufficient separation between “direct observations” and “interpretive statements”. This is particularly evident in claims regarding preferential bypass flow at intermediate initial water content, “threshold-type” behaviour around a specific volumetric water content, and macropore controlled advective heat transport.
In several places, statements are phrased as if the experiments "demonstrate" specific mechanisms, whereas the data are at best "consistent" with those mechanisms. Given the absence of direct diagnostics of flow paths (e.g. tracer tests, dye experiments, imaging), preferential flow remains an inference, not an observation. This distinction must be made explicit throughout the manuscript. Of course, some degree of interpretive uncertainty is inevitable in experiments of this type. my concern here is not ambiguity per se, but overstatement relative to that ambiguity.
I strongly encourage the authors to revise the text so that:
Related to this point, several of the key interpretations rely on transient behaviour (e.g. early drainage onset, switching between drainage- and runoff-dominated regimes, and progressive loss of macropore effectiveness). These are inherently rate-related phenomena, yet the analysis is presented almost exclusively in cumulative (volume) form. While cumulative fluxes are informative for overall partitioning, complementary rate-based representations (e.g. time-resolved inflow, runoff, and drainage rates, and their balance) could provide a more quantitative basis for comparing scenarios and for assessing whether observed differences reflect transient dynamics or simply integrated effects. I present this as a suggestion rather than a requirement, but such analyses could help clarify several of the interpretations advanced in the manuscript.
2) The manuscript repeatedly refers to a “threshold” initial volumetric water content (around 12–13%) above which macropores become hydraulically important. While the qualitative distinction between low, intermediate, and high initial water content regimes is evident in the data, the use of the term threshold is a bit too strong.
Only a small number of discrete initial conditions are tested, and the inferred transition depends on soil texture, porosity, macropore geometry, freezing history, and experimental boundary conditions. As such, the data do not justify the identification of a sharp or general threshold, nor the presentation of a specific numerical value as physically meaningful beyond this setup.
The authors should:
More generally, the manuscript would benefit from a clearer discussion of transferability. The experiments are necessarily highly controlled, with a specific soil texture, porosity, macropore geometry, and freezing history. While such idealization is appropriate for process understanding, the current discussion does not sufficiently articulate which aspects of the observed behaviour are expected to be transferable to other soils, macropore configurations, or thermal regimes, and which are strictly setup-specific. Clarifying this distinction would help readers assess how the results should be interpreted beyond the particular experimental configuration studied here, and would strengthen the contribution of the paper.
3) The interpretation of earlier drainage onset in the intermediate water content macropore experiment as evidence of preferential bypass flow is not fully convincing in its current form. While the observation itself (earlier drainage relative to the non-macropore case) is clear, alternative explanations cannot be ruled out, including packing heterogeneity introduced during soil preparation, localised shrinkage or cracking during freezing and wetting, preferential flow along probe–soil interfaces, or differences in frost geometry not fully captured by averaged profiles. Specially given how small sometimes the differences are.
Given that the authors acknowledge artefacts in other experiments (e.g. near-surface sensor exposure and structural heterogeneity), the manuscript should explicitly discuss these alternative explanations and justify why preferential flow through the artificial macropore network is the most plausible interpretation. At minimum, the language should be softened to reflect the inferential nature of this conclusion.
4) The macropore network is central to the study, yet its mechanical and hydraulic integrity is not sufficiently explored. Important questions remain insufficiently addressed, including whether macropores remain open and hydraulically connected at the onset of irrigation (specifically relevant for such a coarse material), the extent to which macropore collapse or partial closure may occur during freezing and wetting, and how representative the chosen macropore diameter, orientation, and connectivity are relative to natural systems.
While the authors describe the network as a simplified analogue, the manuscript should more explicitly acknowledge that this configuration represents an upper-bound scenario for macropore influence and discuss the implications for transferability to natural soils.
5) Changes in volumetric water content during irrigation are interpreted in several places as evidence of infiltration, bypass flow, or macropore-driven transport. However, under partially frozen conditions, ΔVWC may reflect multiple processes, including liquid redistribution, phase change (melting or refreezing), and measurement artefacts near the freezing point.
Given the strong sensitivity of dielectric measurements near 0 °C and the reliance on relative rather than absolute changes, the manuscript should be more cautious in attributing ΔVWC patterns to specific hydraulic processes. Statements that rely heavily on ΔVWC fields should be revisited and, where necessary, qualified.
6) Several passages attribute downward migration of the freezing front during irrigation to advective heat transport associated with infiltration. As currently written, this interpretation is not always physically clear. Infiltrating water is warmer than the frozen soil, and advective heat transport would intuitively promote thawing rather than deeper freezing unless the coupled effects of phase change and latent heat release are explicitly considered. The authors should clarify the underlying energy balance and ensure that the explanation of freezing-front movement is physically consistent and clearly articulated.
Minor comments
In the end, I think the experimental work is solid and the dataset is valuable. However, substantial revision is required to bring the interpretation, framing, and strength of the claims into alignment with the evidence. If the authors substantially narrow their conclusions, clarify limitations, and better distinguish observation from inference, the manuscript could become a meaningful contribution suitable for publication in The Cryosphere.