Are Water Tracks Wetting or Drying as Arctic Permafrost Thaws?

Abstract:

The Arctic has warmed at twice the rate of the global average, resulting in permafrost thaw and an intensified hydrologic cycle. In the Arctic, the hydrology of continuous permafrost hillslopes is largely controlled by seasonally saturated zero-order channels known as water tracks. Water tracks exist on upland hillslopes that cover as much as a third of the Arctic landscape. When the soil above continuous permafrost thaws in the summer, water tracks fill with and route water in response to snowmelt and rainfall. Carbon from thawing permafrost is released approximately three times more readily when soils are unsaturated than saturated; thus, understanding whether water tracks are wetting or drying as permafrost thaws in a warming Arctic is fundamental for predicting the rate and timing of soil carbon release. Here, we employ a calibrated three‐dimensional, physically-based subsurface flow model with variable saturation and freeze and thaw capabilities and Landsat-derived wetness indices to quantify the role of ground surface temperatures on soil saturation for water tracks in the Upper Kuparuk River watershed on the North Slope of Alaska, USA. Model analyses indicate that higher ground surface temperatures along water tracks promote increases in groundwater discharge to the surface where water tracks act as conduits for large‐recharge events and continue to discharge groundwater into the autumn after the adjacent hillslope has frozen. Preliminary remote sensing analyses corroborate these modeling results; from 2011 to 2019, as ground surface temperatures increased, the surface wetness (Modified Normalized Difference Water Index, MNDWI) of water tracks also increased. These findings suggest that water tracks play a critical role in soil saturation states and consequently, biogeochemical responses, across warming upland Arctic landscapes.