The impacts of ongoing climate warming on cold‐regions hydrogeology and groundwater resources have created a need to develop groundwater models adapted to these environments. Although permafrost is considered relatively impermeable to groundwater flow, permafrost thaw may result in potential increases in surface water infiltration, groundwater recharge, and hydrogeologic connectivity that can impact northern water resources. To account for these feedbacks, groundwater models that include the dynamic effects of freezing and thawing on ground properties and thermal regimes have been recently developed. However, these models are more complex than traditional hydrogeology numerical models due to the inclusion of nonlinear freeze–thaw processes and complex thermal boundary conditions. As such, their use to date has been limited to a small community of modeling experts. This article aims to provide guidelines and tips on cold‐regions groundwater modeling for those with previous modeling experience.This article is categorized under: Engineering Water > Methods Science of Water > Hydrological Processes

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.

The presence of taliks (perennially unfrozen zones in permafrost areas) adversely affects the thermal stability of infrastructure in cold regions, including roads. The role of heat advection on talik development and feedback on permafrost degradation has not been quantified methodically in this context. We incorporate a surface energy balance model into a coupled groundwater flow and energy transport numerical model (SUTRA‐ice). The model, calibrated with long‐term observations (1997–2018 on the Alaska Highway), is used to investigate and quantify the role of heat advection on talik initiation and development under a road embankment. Over the 25‐year simulation period, the new model is driven by reconstructed meteorological data and has a good agreement with near surface soil temperatures. The model successfully reproduces the increasing depth to the permafrost table (mean absolute error <0.2 m), and talik development. The results demonstrate that heat advection provides an additional energy source that expedites the rate of permafrost thaw and roughly doubles the rate of permafrost table deepening, compared to purely conductive thawing. Talik initially formed and grew over time under the combined effect of water flow, snow insulation, road construction and climate warming. Talik formation creates a new thermal state under the road embankment, resulting in acceleration of underlying permafrost degradation, due to the positive feedback of heat accumulation created by trapped unfrozen water. In a changing climate, mobile water flow will play a more important role in permafrost thaw and talik development under road embankments, and is likely to significantly increase maintenance costs and reduce the long‐term stability of the infrastructure.

Actively heated fiber-optic distributed temperature sensing (aFO-DTS) measures soil moisture content at sub-meter intervals across kilometres of fiber-optic cable. The technology has great potential for environmental monitoring but calibration at field scales with variable soil conditions is challenging. To better understand and quantify the errors associated with aFO-DTS soil moisture measurements, we use a parametric numerical modeling approach to evaluate different error factors for uniform soil. A thermo-hydrogeologic, unsaturated numerical model is used to simulate a 0.01 m by 0.01 m two-dimensional domain, including soil and a fiber-optic cable. Results from the model are compared to soil moisture values calculated using the commonly used T_cum calibration method for aFO-DTS. The model is found to have high accuracy between measured and observed saturations for static hydrologic conditions but shows discrepancies for more realistic settings with active recharge. We evaluate the performance of aFO-DTS soil moisture calculations for various scenarios, including varying recharge duration and heterogeneous soils. The aFO-DTS accuracy decreases as the variability in soil properties and intensity of recharge events increases. Further, we show that the burial of the fiber-optic cable within soil may adversely affect calculated results. The results demonstrate the need for careful selection of calibration data for this emerging method of measuring soil moisture content.