Three-phase numerical model for subsurface hydrology in permafrost-affected regions

Abstract

Abstract. Degradation of near-surface permafrost due to changes in the climate is expected to impact the hydrological, ecological and biogeochemical responses of the Arctic tundra. From a hydrological perspective, it is important to understand the movement of the various phases of water (gas, liquid and ice) during the freezing and thawing of near surface soils. We present a new non-isothermal, single-component (water), three-phase formulation that treats air as an inactive component. The new formulation is implemented in the massively parallel subsurface flow and reactive transport code PFLOTRAN. Parallel performance for this implementation is demonstrated, and validation studies using previously published experimental data are performed. A comparison between the new model and a more complete two-component (air–water) multiphase approach shows only minor differences. When water vapor diffusion is considered, a large effect on soil moisture dynamics is seen, which is due to dependence of thermal conductivity on ice content. A large three-dimensional simulation (with around 6 million degrees of freedom) of seasonal freezing and thawing is also presented.