Freezing of Subarctic Hillslopes, Wolf Creek Basin, Yukon, Canada

Freezing processes were monitored at five sites within the Wolf Creek basin, Yukon, Canada during the winter of 1998–1999. Ground temperatures were measured using thermocouples in hillslopes that had frost status ranging from permanent to seasonal. The timing of freezing and ground thermal regimes varied among the five sites and was controlled by (1) the variation in surface soil temperature, (2) frost status (seasonal vs. permanent), (3) moisture content of the active layer, (4) properties of the soil profile, and (5) the presence/absence of subsurface drainage. On slopes with permafrost, cooling was rapid and two-sided freezing closed the active layer several months after the onset of freezing. On a slope with seasonal frost only, dry soil conditions allowed frost to penetrate to depth. In contrast, a slope with seasonal frost that had continuous drainage, frost depths were shallow due to heat advected from flowing water. A simple one-dimensional conduction model with latent heat was used to simulate freezing processes. Model performance varied among the slopes, and results indicate that (1) conduction is the predominant heat transfer mechanism during freezing, (2) latent heat is the principal factor controlling frost front descent, and (3) lateral flow significantly retards frost penetration because of heat advection. This information is valuable in assessing spatial variability within tile-based models and in predicting freezing, which defines an effective end-of-season on lateral hydrological processes.