Is flow declining in headwater streams of Arctic Alaska?

Asik Creek in Noatak National Preserve.

NPS/Jon O'Donnell

Arctic landscapes are undergoing rapid changes due to climate warming at high latitudes. As air temperatures warm, the boreal forest is expanding north into tundra, and permafrost soils are thawing. These shifts in terrestrial ecosystems can dramatically alter what happens in aquatic ecosystems. In a new study published in Environmental Research Letters, researchers from the U.S. Geological Survey, the University of Alaska Anchorage, and the National Park Service show that streamflow may be declining in headwater reaches due to the combined effects of vegetation change and permafrost thaw.

In this collaborative study, scientists examined hydrological conditions across 11 headwater catchments in Noatak National Preserve and Kobuk Valley National Park in northwest Alaska. Study catchments varied with the amount of forest cover and the degree of permafrost thaw. They used this gradient in vegetation and permafrost to explore how these factors might control streamflow. Findings from this study suggest that there are two main processes contributing to lower streamflow in headwater reaches. First, boreal tree line expansion causes an increase in evapotranspiration, or the physical and biological transport of water to the atmosphere. Second, permafrost thaw reroutes water to deep groundwater aquifers, often bypassing the stream channel. Together, these processes can cause streamflow to decline and may contribute channel drying. Headwater streams serve as critical habitat for native fish species in the Arctic and this decline in flow may contribute to a loss of habitat.

Sensitivity of headwater streamflow to thawing permafrost and vegetation change in a warming Arctic

Abstract

Climate change has the potential to impact headwater streams in the Arctic by thawing permafrost and subsequently altering hydrologic regimes and vegetation distribution, physiognomy, and productivity. Permafrost thaw and increased subsurface flow have been inferred from the chemistry of large rivers, but there is limited empirical evidence of the impacts to headwater streams. Here we demonstrate how changing vegetation cover and soil thaw may alter headwater catchment hydrology using water budgets, stream discharge trends, and chemistry across a gradient of ground temperature in northwestern Alaska. Colder, tundra-dominated catchments shed precipitation through stream discharge, whereas in warmer catchments with greater forest extent, evapotranspiration (ET) and infiltration are substantial fluxes. Forest soils thaw earlier, remain thawed longer, and display seasonal water content declines, consistent with greater ET and infiltration. Streambed infiltration and water chemistry indicate that even minor warming can lead to increased infiltration and subsurface flow. Additional warming, permafrost loss, and vegetation shifts in the Arctic will deliver water back to the atmosphere and to subsurface aquifers in many regions, with the potential to substantially reduce discharge in headwater streams, if not compensated by increasing precipitation. Decreasing discharge in headwater streams will have important implications for aquatic and riparian ecosystems.

Koch, J. C. Y. Sjoberg, J. O’Donnell, M. Carey, P. F. Sullivan and A. Terskaia. 2022. Sensitivity of headwater streamflow to thawing permafrost and vegetation change in a warming Arctic. Environmental Research Letters 17(4): 044074.