Projections of Permafrost and Seasonally Frozen Ground Along an Elevation Gradient at Niwot Ridge, Colorado, USA

ABSTRACT The intensity, duration, and spatial distribution of frozen soil influences hydrologic flow paths, soil biogeochemistry, and slope geomorphology. In cold regions, where the ground thermal regime is controlled by the seasonal snowpack, representation of the snowpack in models simulating seasonally frozen ground is required and leads to significant improvements in soil temperature estimates. With long‐term climate and ground temperature observations, Niwot Ridge, a seasonally snow‐covered alpine catchment in the headwaters of the Boulder Creek watershed, serves as an ideal location for analyzing frozen ground under a changing climate. In this study, we use a coupled thermo‐hydrologic model to provide novel perspectives on cryosphere research at Niwot Ridge. We project how end‐of‐21st‐century changes in Front Range air temperature, snowfall, and snowpack cold content will influence the ground thermal regime, including seasonally frozen ground and permafrost, in comparison to the 1952–1970 period. In projections of seasonally frozen ground, the model predicts two additional months of unfrozen soils by the end of the 21st century compared with the 1952–1970 time period, which is expected to lead to an increase in the number of days favorable for microbial respiration. Our permafrost analysis supports the occurrence of permafrost above 3800 m with active layer thickness 1.8 m (1952–1970), 2.2 m (2001–2013), and 38 m by end of 21st century. The simulated deep soil thaw over the last several decades (1970–2020) is small compared with projected deep soil thaw through the current century, which is expected to lead to reductions in frost cracking.