Friday, May 29, 2020, 9:30AM - 10:30AM
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Numerical modeling of hillslope thermo-hydrology to understand spatial and temporal trends in soil ice formation and implications for hydrologic partitioning
The intensity, duration, and spatial distribution of frozen soil influences hydrologic flow paths, soil biogeochemistry, and slope 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. Surface energy balance calculations and a physical snowpack model based on the Utah Energy Balance have been coupled with PFLOTRAN-ICE, a subsurface thermo-hydrologic model that simulates water and energy transport in the subsurface, including freeze-thaw processes. A thermo-hydrologic modeling study is presented against the backdrop of field observations from Gordon Gulch and Niwot Ridge, seasonally snow-covered catchments in the headwaters of the Boulder Creek watershed.
Despite a persistent snowpack on the north-facing slope at Gordon Gulch, seasonally frozen ground is more prevalent and persistent there because of low solar insolation and a relatively thin snowpack. The south-facing slope experiences significantly higher incoming solar radiation that prevents the persistence of frozen ground. Representation of the snowpack and surface energy balance significantly improves soil temperature estimates compared to model forcing based on air temperature alone. At Niwot Ridge, deep (>1m depth) frozen soil underlying bare ground impeded groundwater recharge, and shallow frozen ground (<1m depth) beneath seasonal snow limited infiltration. Modeled alpine and subalpine snow cover exerted a positive effect on soil temperatures, but did not prevent or eliminate frozen ground completely. Shallow freezing beneath snow-covered ground exerted a much stronger effect on infiltration than shallow freezing beneath bare ground because the soil beneath the snow remained frozen while the snowpack was melting, whereas solar insolation thawed bare patches by the time they received excess snowmelt “run-on”. In projections of seasonally frozen ground, simulations forecast two additional months of unfrozen soils by the end of the 21st century compared to the 1952-1970 time period. End-of-21st-century projections including the snowpack suggest that low early-season snow accumulation will lead to December soil freezing episodes that are more intense and persistent than in the past. A permafrost analysis provides support for the occurrence of permafrost above 3800m, and suggests that the deep soil thawing that has taken place over the last several decades is small compared to deep soil thaw that should be expected throughout the current century.
Free and open to the public.