The average temperature of permafrost has increased globally by about 0.4°C in the last century. This is partly due to the Arctic amplification of an increase in air temperature in the Northern Hemisphere, but also due to increased snow thickness, especially in areas of discontinuous permafrost (Biskaborn et al., 2019). This change in temperature changes the physical properties of the subsurface, with impacts on infrastructure (Hjort et al., 2018), groundwater resources, vegetation distribution (Jorgenson et al., 2013; Lloyd et al., 2003), carbon and nitrogen cycling (Petrone et al., 2006), and greenhouse gas emissions (Jansson & Taş, 2014), leading to further acceleration of climate change. Jorgenson et al. (2010) show that complex feedbacks exist between permafrost dynamics and topography, vegetation distribution, snow pack, ground temperature, and subsurface hydrological properties. Rising permafrost temperatures cause increasing hydraulic conductivities in the soil to bedrock column, enabling or enhancing surface water-groundwater interactions, changes to the groundwater residence times, and eventual alterations to groundwater and stream water temperature and compositional dynamics (Hinzman et al., 2005; Ireson et al., 2013). These changes in hydrology enhance the importance of the deeper subsurface for carbon and nutrient cycling (Koch et al., 2013; Lyon et al., 2010), and are particularly important for discontinuous permafrost, which accounts for 19% of the Northern Hemisphere's land surface that is covered by permafrost, that is 4.4 × 10 6 km 2 (Zhang et al., 2003). This region is characterized by a Abstract Climate change is causing rapid changes of Arctic ecosystems. Yet, data needed to unravel complex subsurface processes are very rare. Using geophysical and in situ sensing, this study closes an observational gap associated with thermohydrological dynamics in discontinuous permafrost systems. It highlights the impact of vegetation and snow thickness distribution on subsurface thermohydrological properties and processes. Large snow accumulation near tall shrubs insulates the ground and allows for rapid and downward heat flow. Thinner snow pack above graminoid results in surficial freezing and prevents water from infiltrating into the subsurface. Analyzing short-term disturbances, we found that lateral flow could be a driving factor in talik formation. Interannual measurements show that deep permafrost temperatures increased by about 0.2°C over 2 years. The results, which suggest that snowvegetation-subsurface processes are tightly coupled, will be useful for improving predictions of Arctic feedback to climate change, including how subsurface thermohydrology influences CO 2 and CH 4 fluxes. Plain Language Summary When permafrost thaws, water can flow quicker through the ground, creating a very complex subsurface flow system. In this study, we gain detailed insight into these complex processes by measuring the electrical resistivity of the ground daily. Our results show that the type of vegetation and the ...

Abstract. Measuring soil and snow temperature with high vertical and lateral resolution is critical for advancing the predictive understanding of thermal and hydro-biogeochemical processes that govern the behavior of environmental systems. Vertically resolved soil temperature measurements enable the estimation of soil thermal regimes, frozen-/thawed-layer thickness, thermal parameters, and heat and/or water fluxes. Similarly, they can be used to capture the snow depth and the snowpack thermal parameters and fluxes. However, these measurements are challenging to acquire using conventional approaches due to their total cost, their limited vertical resolution, and their large installation footprint. This study presents the development and validation of a novel distributed temperature profiling (DTP) system that addresses these challenges. The system leverages digital temperature sensors to provide unprecedented, finely resolved depth profiles of temperature measurements with flexibility in system geometry and vertical resolution. The integrated miniaturized logger enables automated data acquisition, management, and wireless transfer. A novel calibration approach adapted to the DTP system confirms the factory-assured sensor accuracy of ±0.1 ∘C and enables improving it to ±0.015 ∘C. Numerical experiments indicate that, under normal environmental conditions, an additional error of 0.01 % in amplitude and 70 s time delay in amplitude for a diurnal period can be expected, owing to the DTP housing. We demonstrate the DTP systems capability at two field sites, one focused on understanding how snow dynamics influence mountainous water resources and the other focused on understanding how soil properties influence carbon cycling. Results indicate that the DTP system reliably captures the dynamics in snow depth and soil freezing and thawing depth, enabling advances in understanding the intensity and timing in surface processes and their impact on subsurface thermohydrological regimes. Overall, the DTP system fulfills the needs for data accuracy, minimal power consumption, and low total cost, enabling advances in the multiscale understanding of various cryospheric and hydro-biogeochemical processes.

The bentonite buffer long-term integrity is of significant interest in the performance assessment (PA) of nuclear waste disposal. This study aims at understanding how the initial geochemical parameters affect long-term chemical properties within the buffer, which will subsequently affect the transport. Using coupled thermal-hydrological-chemical (THC) models for migration of U(VI) in a generic repository, we performed a global sensitivity analysis (GSA) to identify the influence of each parameter on the temporal evolution of a spatially averaged distribution coefficient for the entire buffer. Such an analysis can be used in a repository-scale PA. In this work, we used the TOUGHREACT software to model coupled THC processes in a generic clay repository with bentonite buffer. In this model, U(VI) is released from a canister via schoepite dissolution, which is assumed to occur 1000 years after closure. U(VI) migrates through the bentonite buffer affected by two-site protolysis non-electrostatic surface complexation and cation exchange. GSA results showed that adsorption density on smectite, pH, volume fractions of smectite, calcite, Ca2+ aqueous concentration all play a significant role in U(VI) transport, since roughly 80% of adsorbed U(VI) is absorbed by smectite, and Ca2+ affects the aqueous complexation with U(VI). This work demonstrates the complex process models potential usefulness that can be transferred to the PA model. It also provides information needed to proceed with the development of a reduced-order model, which has the potential to optimize repository designs, site characterization, performance confirmation.