Water flow in the active layer along an arctic slope – An investigation based on a field campaign and model simulations

Abstract

Abstract. As climate conditions change, the hydrological regime in the active layer is subject to change too. This influences the transport of solutes and the availability of nutrients, e.g. nitrogen particularly, along slopes. There is a lack of understanding the pathways and travel times of water and nutrients along slopes in discontinuous permafrost regions and 10 how to scale changes along transects to the rest of the landscape. This study presents a comprehensive data set of a field site in Disko Island in Greenland aiming at constructing a hydrological model of the area. Data from automated weather stations, geophysical surveys, soil samples and soil sensors and tracer experiments are combined to describe the spatial variability in the field and to serve as input to a two-dimensional model (SUTRA) for simulating water and solute transport in the summer period. The model is calibrated and validated against volumetric water content and breakthrough curves of the applied 15 tracers. Observed and simulated results suggest that the flow velocity in the active layer is directly influenced by annual precipitation patterns leading to water flow during the summer and rapid movement at the end of summer. Yearly travel times for the specific field site are simulated to be approximately 14 m/a and the highest peak velocities are most likely caused by preferential flow paths. The spatial heterogeneities linked to the frost topography seem to control the direction and velocity of flow. The observed discontinuous movement of a conservative tracer suggests that the movement of dissolved 20 nitrogen compounds such as nitrate, being released along the slope in consequence of permafrost thawing, could possibly quickly influence nitrogen cycling at the end of the slope. This may trigger a feedback of climate changes in terms of increasing carbon sequestration due to additional plant growth in these otherwise nitrogen-limited Arctic ecosystems. 25The Cryosphere Discuss., https://doi

Dissolved Organic Matter Chemistry and Transport Along an Arctic Tundra Hillslope

“…Within similar lowgradient Arctic watersheds, permafrost losses are expected to deepen hydrologic flow paths and increase groundwater flows (Blaen et al, 2014;Vonk et al, 2015;Walvoord & Kurylyk, 2016;White et al, 2007). Relatively fast DOM mobilization through mineral soils has been observed within thin saturated layers that can develop at the base of the active layer as a result of infiltration, and via fill-spill mechanisms that are controlled by permafrost microtopography (Zastruzny et al, 2017). Relatively fast DOM mobilization through mineral soils has been observed within thin saturated layers that can develop at the base of the active layer as a result of infiltration, and via fill-spill mechanisms that are controlled by permafrost microtopography (Zastruzny et al, 2017).…”

“…the residence time of water within the mineral matrix (Angermann et al, 2017;Atchley et al, 2015;Rodhe & Seibert, 2011;Zastruzny et al, 2017). Conceptual model depicting the downslope transport and transformation of dissolved organic matter (DOM) through Arctic hillslopes.…”

“…While the vertical decrease in soil bulk density was a significant predictor of modal tracer velocities, our results suggest deepening downslope flow paths will not substantially retard DOM delivery to surface waters. Relatively fast DOM mobilization through mineral soils has been observed within thin saturated layers that can develop at the base of the active layer as a result of infiltration, and via fill-spill mechanisms that are controlled by permafrost microtopography (Zastruzny et al, 2017). Similar work by Neilson et al (2018) suggests substantial surface-subsurface (up to 30 cm deep) exchanges of water occur during downslope transport and are driven by local topography and vertical decreases in subsurface permeability.…”

“…However, shifts in vegetation and root exudate deposition, as well as chromatographic fractionation (coiled arrow), influence DOM structural diversity, which increases during downslope export and is greatest along the thawing mineral-permafrost interface. the residence time of water within the mineral matrix (Angermann et al, 2017;Atchley et al, 2015;Rodhe & Seibert, 2011;Zastruzny et al, 2017). As a result, sustained, lateral dispersion may partially offset rapid export along preferential flow paths in low-gradient catchments and help shape the molecular composition of materials delivered to surface waters (Harms & Jones, 2012;Kawahigashi et al, 2004;Vonk et al, 2015;Ward & Cory, 2015).…”

“…However, preferential flow mechanisms such as pipe networks (Carey & Woo, 2000), water tracks (Bowden et al, 2008), desiccation and contraction cracks (Lachenbruch, 1962;Woo, 2012), and macropore flow (Weiler & McDonnell, 2006) can increase DOM exchange between soil horizons and lead to the mass loading of DOM to recipient streams (Covino, 2017), particularly as permafrost thaws. While antecedent soil moisture conditions typically control when, and how extensively, preferential flow networks are activated (Weiler & McDonnell, 2006), water in saturated mineral soils can be discharged to porous organic horizons and released directly to streams (Hinton et al, 1994;Kirchner, 2003;Neilson et al, 2018;Zastruzny et al, 2017). Evidence that DOC exported from permafrost soils is susceptible to photodegradation and microbial consumption (Ward et al, 2017) suggests increasing connectivity of soil flow paths to sunlit waterways could amplify C-climate forcing (Cory et al, 2013).…”