HEAT TRANSPORT IN NORTHERN PEATLANDS: NUMERICAL SIMULATIONS, BENCHMARKS, AND APPLICATIONS WITH FREEZING

The subsurface temperature regime is an important factor controlling the carbon cycle in Northern Peatlands. Warmer temperatures favor the consumption of carbon by anaerobes, and lead to increased methane production. To understand the processes controlling heat transport in these systems, temperature was measured over two years in a vertical profile in the Red Lake Bog, Minnesota. Air temperature and temperature at 12 depths from 0 to 400 cm were recorded at the crest of the peat bog at sub-daily intervals. Analysis of temperature dissipation indicates that the thermal conductivity of the peat system is 0.5 W m^-1 C^-1.

The measured heat data were simulated using SUTRA, a U.S. Geological Survey code for coupled porewater flow and heat transport systems. In order to incorporate the observed subsurface freezing, the code was modified to include freezing and melting of porewater, and includes proportional heat capacity and thermal conductivity of water and ice, decreasing matrix permeability due to ice formation, and latent heat of freezing. The model was verified by correctly simulating a Lunardini analytical solution for ice formation in a porous medium with a mixed ice water zone. Two possible benchmark problems for groundwater and energy transport with ice formation and melting are proposed that may be used by other researchers for code comparison. The model results suggest that advective porewater flow is not necessary to transport heat within the peat profile and that most heat is transferred by thermal conduction through the water-logged soils. The modified SUTRA model correctly simulates the temperature and ice distributions in the peat bog, and shows the importance of ice formation and melting in northern peat systems. Changes in local climate and associated changes in the thermal regime may cause non-linear feedbacks in the peat-carbon dynamics.