When Frozen Slopes Switch Regimes: Moisture-Controlled Runoff Generation and the Transient Role of Macropores

Rainfall on frozen slopes represents a critical hydrological control on landslide and debris-flow initiation in cold and alpine environments, as frozen soil layers can strongly limit infiltration and favour surface runoff. Depending on the thermal and hydraulic state of the subsurface, precipitation may either infiltrate through partially unfrozen pathways or be rapidly converted into runoff, with important implications for erosion and slope destabilisation. Freeze-thaw dynamics and preferential flow through macropores can further complicate this partitioning by transiently modifying soil permeability and infiltration pathways, while their interaction with pre event soil moisture conditions remains poorly constrained under event-scale conditions.

We present nine large-scale rainfall experiments conducted on an inclined frozen soil body inside a controlled climate chamber. The experiments systematically varied initial volumetric water content and the presence or absence of an interconnected macropore network, while continuously monitoring soil temperature, liquid water content, subsurface drainage, and surface runoff. Our results show that hydrological responses of frozen slopes are primarily controlled by initial water content, with macropores exerting a secondary but highly non-linear influence. At low initial water content, infiltration was dominated by matrix flow despite frozen conditions, resulting in limited surface runoff. At intermediate water content, macropores enabled rapid bypass infiltration through the partially frozen profile, promoting early drainage and subsurface water transfer. At high initial water content, the frozen matrix became effectively impermeable and infiltration depended almost entirely on macropore flow. However, macropore functionality was transient: progressive refreezing and particle-assisted clogging reduced hydraulic connectivity during ongoing infiltration, causing a rapid shift from bypass infiltration to runoff-dominated conditions.

These results demonstrate that macropores in frozen slopes act as dynamic flow pathways whose hydraulic effectiveness depends on pre-event moisture conditions. While open macropores can enable subsurface infiltration under otherwise restrictive frozen conditions, progressive refreezing or clogging can reduce their functionality during infiltration events, causing a shift from infiltration-dominated responses toward surface runoff. The observed regime shifts highlight the need to explicitly represent transient preferential flow and refreezing processes in landslide hydrology and slope stability models, as they critically control hydrological preconditioning and the timing and magnitude of runoff-driven erosion and slope instability in seasonally frozen terrain.