Modeling Permafrost Aggradation and Thaw in a Peat Plateau Wetland Complex

Permafrost degradation strongly influences the hydrology and ecology of northern watersheds. In wetland-dominated zones of discontinuous permafrost, permafrost occurs below tree-covered peat plateaus where the tree-canopy and vadose zone insulate and preserve the permafrost. As the permafrost thaws, peat plateaus shrink, the landscape transitions into unforested wetlands, and hydrologic surface and subsurface connectivity increases. The resultant permafrost-degraded areas comprise large portions of the drainage density of sub-arctic basins and alter the region’s water and energy balances. To better understand the fundamental processes affecting permafrost thaw, a three-dimensional finite element numerical model is constructed for a peat plateau wetland complex in Northwest Territories, Canada (61°18'N, 121°18'W). FEFLOW groundwater flow and heat transport modeling software is used in conjunction with the piFreeze plug-in, to account for phase changes between ice and water. Representative permafrost is developed with both steady-state and transient approaches. The steady-state approach uses a ground temperature of -2.5 °C and 1.3 °C applied over the modern-day permafrost plateau and wetlands, respectively. Actual permafrost evolution, to current conditions, would require 100’s of years of climate variations over an evolving landscape. To properly develop a subsurface environment which matches field data collected since 1994, the model simulations are conducted in transient mode. The transient approach applies daily climatic data over the plateau between 1875 and 2015; the Simultaneous Heat and Water model (SHAW) is used to calculate ground temperatures and infiltration rates. Transient simulations are computationally demanding for the 2.4 million node model. Simulations advance at 2 to 3 years per 24 CPU hours on advanced workstations. Modeling permafrost aggradation / degradation will allow for the testing of remedial measures, such as mulching and borehole heat exchangers, to stabilize permafrost in high value infrastructure environments.

Keywords: permafrost degradation, peat plateau, heat and water movement, ecohydrology