Thermohydraulic experiments on water infiltration into 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 early and enhanced drainage responses, which we interpret as preferential bypass flow through the interconnected macropore network. 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.