Seasonal variation of hydrologic connectivity for High Arctic stream segments

In the High Arctic, new channel networks are developing relatively rapidly within 10s of years, typically attributed to exteneded thaw seasons under climate change and late season permafrost loss. During the 2024 field season on Tallurutit (Devon Island), we visited a developing channel network where relatively flat pools are connected by steeper channelised segments which exhibit evidence of recent gravel and sand transport. However, despite visiting during a storm event, we did not observe active sediment transport and only limited surface-water runoff. This brings up the question: How does the hydrologic connectivity between channel segments and surface runoff fraction change across a thaw season?

We developed a new box model that couples surface and subsurface water flow between pools conserving enthalpy to explore the hydrologic response of an analogue landscape segment across a thaw season. Using this model, we first use synthetic rainevents to identify the key factors modulating the hydrologic response and then use new weather data from Tallurutit and historic climate data to explore when surface water erosion is likely to occur under typical climate conditions and for future climate scenarios.

Subjecting the landscape to the same magnitude-duration rainevent in the early versus late thaw season shows that surface water runoff fraction is greatest early in the thaw season as the shallow thaw front limits subsurface water storage. This results in successive overspill events that rapidly transport water across pools and promote erosion. In the late thaw season, subsurface water storage dominates and pools are connected via subsurface water flow. As a result surface water flow and resultant erosion is minimised. We summarise a key implication of our work through a reservoir response time that depends on the bed permeability, pool length, and thaw depth which modulates whether a pool will dampen a hydrologic signal or transmit it downstream.

Combined with grain-scale laboratory experiments which show that bed erodibility is inversely proportional to thaw depth, this work suggests that the formation and evolution of channel networks in the High Arcitc is primarily driven by early season discharge events. Under climate change, the frequency and magnitude of early season rainstorms and heatwaves, resulting in rapid snowmelt, are becoming more common. This suggests that the observed rapid channelisation in the High Arctic is related to early season climate extremes, instead of long-term warming averages and late season permafrost loss.