Influence of Snowpack Cold Content on Seasonally Frozen Ground and Its Hydrologic Consequences: A Case Study From Niwot Ridge, CO

Abstract

AbstractFrozen ground influences subsurface hydrology by reducing soil permeability, impeding infiltration, and inducing surface runoff. As the seasonal snowpack strongly controls the evolution of the ground thermal regime, it is necessary to represent the snowpack in models simulating frozen ground and its hydrologic consequences. Conventional understanding of the relationship between snowcover and frozen ground considers the snowpack as an insulator, shielding the ground from cold winter temperatures and thus inhibiting frozen ground. However, observations and modeling at Niwot Ridge, a seasonally snow‐covered alpine catchment in the headwaters of the Boulder Creek watershed, illustrate that snowpack cold content can promote and preserve frozen ground beneath the snowpack during the snowmelt season, unlike bare ground patches that thaw relatively rapidly due to exposure to solar radiation and warm spring temperatures. We present results from a coupled snowpack and subsurface thermo‐hydrologic model including freeze‐thaw processes that captures these contrasts reasonably well. Our results suggest that the cold snowpack at this site acts as a heat sink and promotes frozen ground, in contrast to the conventional understanding of the snowpack as an insulator. In the subalpine zone, infiltration simulations show that shallow freezing beneath snow‐covered ground has a much stronger effect on infiltration than shallow freezing beneath bare ground because the soil beneath the snow remained frozen during snowmelt, whereas bare patches thawed by the time they received excess snowmelt run‐on.