Groundwater controls on post-fire permafrost thaw: Water and energy balance effects

Abstract

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, neglecting groundwater flow increases the perceived importance of subsurface thermal properties, such as the thermal conductivity of soil solids, and decreases the perceived importance of hydrologic properties, such as the soil permeability. 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.

Rapid and Accurate Estimates of Streamflow Depletion Caused by Groundwater Pumping Using Analytical Depletion Functions

“…Most importantly, archetypal numerical models eliminate site‐specific complexity unrelated to our research questions to develop generalizable understanding of the importance of process‐based representations of streamflow depletion via a comparison between the two modeling approaches. Given these advantages, archetypal models have a long history of use in hydrogeology (Bredehoeft & Kendy, ; Kendy & Bredehoeft, ; Lamontagne‐Hallé et al, ; Tóth, ; Zipper et al, , Zipper, Lamontagne‐Hallé, et al, ).…”

“…We used a multistage spin‐up to ensure the groundwater and surface water components of our models had reached a dynamic equilibrium prior to beginning our pumping experiments (Somers et al, ; Zipper, Lamontagne‐Hallé, et al, ). First, we ran a steady state simulation with no pumping and recharge rates defined as the long‐term average annual baseflow (150 mm/year).…”

Changing groundwater discharge dynamics in permafrost regions

“…It has been successfully tested against numerical and analytical solutions (Kurylyk et al 2014b), including the international benchmarking initiative, InterFrost (Rühaak et al 2015, Grenier et al 2018, and used in many permafrost studies to model groundwater flow dynamics (e.g. Ge et al 2011, McKenzie and Voss 2013, Briggs et al 2014, Evans and Ge 2017, Zipper et al 2018. The present code version allows for unsaturated flow with freeze/thaw as described in Briggs et al (2014).…”

Comparing Streamflow Depletion Estimation Approaches in a Heavily Stressed, Conjunctively Managed Aquifer

“…The distribution of these properties among our experimental sample is shown in Figure 2 and the distribution among all wells is shown in Figure . To span each of these characteristics as uniformly as possible, we used the Latin Hypercube Sampling method (McKay et al., 1979) to randomly sample 250 parameter combinations spanning these six characteristics (Zipper, Lamontagne‐Hallé, et al., 2018). We then selected the pumping well that had the shortest euclidean distance in parameter space to each parameter sample, resulting in a total of 166 unique pumping wells in our final evaluation because some wells were closest to multiple samples.…”