GROUNDWATER-PERMAFROST INTERACTIONS FOLLOWING FIRE: WATER AND ENERGY BALANCE EFFECTS

The frequency and severity of fire in the Arctic is anticipated to increase due to climate change. While it is known that fire in cold regions can enhance permafrost thaw and increase active layer thickness (the depth to which soil annually freezes and thaws), interactions between groundwater and permafrost in burned regions are not well understood. Here, we investigate the degree to which groundwater flow both drives and responds to changes in active layer thickness using the Anaktuvuk River Fire (Alaska, USA, 2007) as an example. Using a numerical groundwater flow model with freeze-thaw capabilities driven by field observations, we quantify the combined and separate impacts of fire-induced changes in the water and energy balance on active layer thickness and groundwater flow dynamics.

We find that changes in the energy balance (increased soil temperature) are the dominant driver of post-fire permafrost thaw, but that hydrological properties of the subsurface (organic and mineral soil permeability) are the key control over the propagation of fire effects both vertically and laterally. Furthermore, we find that fire effects vary as a function of landscape position, with greater thaw in lowland settings due to advective heat transport from upland areas. These results indicate that the effects of fire are not necessarily confined only to burned regions, but can propagate laterally through groundwater flow to influence landscape-scale permafrost and hydrogeological processes. These results have implications for diverse high-latitude biogeochemical cycles, particularly in the context of future warming which is anticipated to both thaw permafrost and intensify fire regimes.