The Thermal Response of Permafrost to Coastal Floodplain Flooding
Flooding of low-lying Arctic regions has the potential to warm and thaw permafrost by changing the surface reflectance of solar insolation, increasing subsurface soil moisture, and increasing soil thermal conductivity. However, the impact of flooding on permafrost in the continuous permafrost environment remains poorly understood. To address this knowledge gap, we used a combination of available flooding data on the Ikpikpuk delta and a numerical model to simulate the hydro-thermal responses of permafrost to coastal flooding.
Arctic coastal flooding events have increased and are expected to continue to increase in both frequency and intensity. Our model-based study provides the first investigation of the impact of floods on low-lying coastal permafrost hydro-thermal dynamics. The change in active layer thickness due to floods may have important biogeochemical feedback on carbon and nutrients presently frozen in permafrost. This model-based exploration of the flooding effect on permafrost landscapes helps advance our understanding of the vulnerability of coastal permafrost to flood inundation and aid in the assessment of flooding risks of coastal landscapes.
In this study, we investigated how coastal freshwater flooding would alter the hydro-thermal processes of permafrost by using a physically-based hydro-thermal dynamic model, ATS (Advanced Terrestrial Simulator). We simulated the changes in subsurface temperature, soil moisture, and ice content of a synthetic coastal permafrost landscape under three flooding scenarios, including the late spring and early summer floods, mid to late summer floods, and all floods throughout the spring and summer. We found that floods have an important impact on coastal permafrost dynamics. Flooding leads to cooler surficial soil due to the increase of latent heat associated with the thawing and evaporation of the wetter soil. Conversely, floods facilitate heat penetration to deeper soil zones due to the increased thermal conductivity of wetter soils. The cooling effect of floods may be intensified in the second year because the increased ice content and soil moisture in the first year can continue affecting the subsurface temperature in the second year. Although the cooling effect may reduce the heat input and weaken the warming effect, the subsurface warming continues with repeated floods year after year. The timing of floods is a key factor controlling the flooding effect. The floods occurring in the middle or late summer are more likely to cause warmer permafrost and a deeper thaw depth relative to floods from the late spring and early summer.