Arctic hillslope hydrologic response to changing water storage conditions

Solute transport from terrestrial to aquatic environments depends on dynamics of water storage and flux. In the arctic, these dynamics are related to changes in permafrost and hydrological conditions that vary with climate across multiple scales. In order to predict the continued trajectory of arctic landscape and ecosystem evolution, observed changes to the hydrologic regime and riverine nutrient fluxes require properly scaled, mechanistic explanations. We address this issue at the hillslope scale by quantifying hydrologic response to changing storage as part of a collaborative effort to understand the coupled hydrology and biogeochemistry of arctic hillslopes. Hillslopes underlain by continuous permafrost experience gradual, summer-season increases in potential water storage through active layer thaw, as well as stochastic changes in available water storage as soil moisture conditions change due to storm events, evapotranspiration, and subsurface flow. Preferential flowpaths called water tracks are ubiquitous features draining arctic hillslopes and are the focus of our study. We predict that water track hydrologic response to precipitation is a function of snowmelt or storm characteristics and available storage. We hypothesize that ¬the ratio of runoff to precipitation will decrease as available storage increases, whether due to the seasonal increase in active layer thaw, or an extended dry period. Intensive snow and thaw depth surveys on a water track on the hillslopes of the Upper Kuparuk River watershed in northern Alaska during May to June 2013 reveal that snow persisted one week longer in a water track than the adjacent hillslope, and thus active layer thaw initiated earlier on the adjacent hillslope. Despite this earlier thaw timing, thaw depth in the water track exceeded that on the non-track hillslope within five days of being uncovered. Thaw, and thus subsurface storage, in water tracks remained greater than the rest of the hillslope for at least the subsequent two months. Deeper thaw coupled with a slight topographic depression in the water tracks relative to the adjacent hillslopes generates a hydraulic gradient that directed water not only downslope, but also across slope into the water tracks. We expected that steeper hydraulic gradients across slope and into water tracks would increase hillslope soil water contributions and increase the specific conductivity of water flowing through the water track. We also expect hillslope contributions to scale with water track catchment characteristics such as drainage area and slope. We test these hypotheses by monitoring water table fluctuations in gridded wells on the hillslope and in our six intensive study sites throughout the summer. Our results provide direct evidence that active layer thaw and the timing and amount of precipitation are important controls on water and solute flux from arctic hillslopes. Depending on the magnitude of climate-induced changes to these controls, there will likely be important consequences for downslope ecosystems.