Modeling Aspect‐Controlled Evolution of Ground Thermal Regimes on Montane Hillslopes

The seasonal evolution of the ground thermal regime in cold regions influences hydrologic flow paths, soil biogeochemistry, and hillslope geomorphology. In mountain environments, steep topography produces strong gradients in solar insolation, vegetation, and snowpack dynamics that lead to large differences in soil temperature over short distances, suggesting a need for high‐resolution, process‐based models that quantify the influence of topography. We present soil temperature and snow depth results from a coupled thermo‐hydrologic model compared to field observations from Gordon Gulch, a seasonally snow‐covered montane catchment in the Colorado Front Range in the Boulder Creek Critical Zone Observatory. The field site features two instrumented hillslopes with opposing aspects: Despite the persistent snowpack on the north‐facing slope, seasonally frozen ground is more prevalent there than the south‐facing slope, which experiences significantly higher incoming radiation that prevents the persistence of frozen ground. A novel modeling framework is developed by coupling a surface energy balance model incorporating solar radiation and snowpack processes to an existing subsurface model (PFLOTRAN‐ICE). The coupled model is used to reproduce strong aspect‐controlled differences in soil temperature and snow depth evident from observations during water years 2013–2016, including a higher incidence of frozen ground under the north‐facing slope. Representation of the snowpack and its insulating effects significantly improves soil temperature estimates on the north‐facing slope, particularly the duration of soil freezing in the spring, which is underestimated by 1–2 months without including the snowpack.