Groundwater Controls on Postfire Permafrost Thaw: Water and Energy Balance Effects

Fire frequency and severity are increasing in high-latitude regions, but the degree to which groundwater flow impacts the response of permafrost to fire remains poorly understood. Here we use the Anaktuvuk River Fire (Alaska, USA) as an example for simulating groundwater-permafrost interactions following fire. We identify key thermal and hydrologic parameters controlling permafrost response to fire both with and without groundwater flow, and separate the relative influence of changes to the water and energy balances on active layer thickness. Our results show that mineral soil porosity, which influences the bulk subsurface thermal conductivity, is a key parameter controlling active layer response to fire in both the absence and presence of groundwater flow. However, including groundwater flow in models increases the perceived importance of subsurface hydrologic properties, such as the soil permeability, and decreases the perceived importance of subsurface thermal properties, such as the thermal conductivity of soil solids. Furthermore, we demonstrate that changes to the energy balance (increased soil temperature) drive increased active layer thickness following fire, while changes to the water balance (decreased groundwater recharge) lead to reduced landscape-scale variability in active layer thickness and groundwater discharge to surface water features such as streams. These results indicate that explicit consideration of groundwater flow is critical to understanding how permafrost environments respond to fire. While scientists know permafrost (permanently frozen ground) often thaws following fire, it is not well understood if groundwater movement enhances or reduces this thawing process. In this study, we simulate the response of permafrost to fire using models that both include and ignore groundwater flow with many different model input data sets. Our results show that when groundwater flow is ignored, the relative importance of soil properties associated with heat movement may be overestimated, and the importance of soil properties associated with water movement are likely to be underestimated. Additionally, we show that increased soil temperature is the most important factor leading to deeper permafrost thaw following fire. However, lower groundwater recharge rates at burned locations decreased permafrost thaw differences between upland and lowland regions of a watershed, as well as groundwater flow into streams and rivers.