The overall spatial and temporal influence of shrub expansion on permafrost is largely unknown due to uncertainty in estimating the magnitude of many counteracting processes. For example, shrubs shade the ground during the snow-free season, which can reduce active layer thickness. At the same time, shrubs advance the timing of snowmelt when they protrude through the snow surface, thereby exposing the active layer to thawing earlier in spring. Here, we compare 3056 in situ frost table depth measurements split between mineral earth hummocks and organic inter-hummock zones across four dominant shrub–tundra vegetation types. Snow-free date, snow depth, hummock development, topography, and vegetation cover were compared to frost table depth measurements using a structural equation modeling approach that quantifies the direct and combined interacting influence of these variables. Areas of birch shrubs became snow free earlier regardless of snow depth or hillslope aspect because they protruded through the snow surface, leading to deeper hummock frost table depths. Projected increases in shrub height and extent combined with projected decreases in snowfall would lead to increased shrub protrusion across the Arctic, potentially deepening the active layer in areas where shrub protrusion advances the snow-free date.

3The Cretaceous Chalk in Northern Europe and other similar fractured rock aquifers frequently 4 have very thick unsaturated (vadose) zones which control both their hydraulic response to 5 rainfall and the extent to which pollutants are delayed or attenuated before reaching groundwater. 6Understanding their hydraulic responses is a pre-requisite for prediction of future trends in 7 groundwater recharge and quality. Accurate characterization of these responses remain elusive 8 because of difficulties in both obtaining in-situ measurements and in devising appropriate 9 conceptual models of flow processes in unsaturated fractured rock. In this study we addressed 10 both issues by simultaneously monitoring soil water dynamics through continuously logged 11 matric potential and moisture content and measuring discharge into a subsurface tunnel at up to 12 45 m depth within the unsaturated zone of Cretaceous Chalk in Northern England. Winter 13 drainage fluxes from the base of the soil zone were estimated using the HYDRUS code for one-14 dimensional variably saturated media. Comparison of soil zone drainage representing the 15 hydraulic input into the Chalk unsaturated zone with tunnel discharge provides insights into the 16 flow dynamics of the unsaturated zone. The relative magnitudes of the soil drainage and deeper 17 unsaturated zone discharge show that flow pathways converge resulting in increased flow 18 focussing with depth in the unsaturated zone. The observed short lag times between the soil 19 surface and the inflow sites in the subsurface tunnel suggest that contaminants from the surface 20 could rapidly reach the water where flow and solutes can be rapidly moved through the system via preferential fracture 34 pathways or more slowly through the porous matrix. The quantification of recharge and 35 characterisation of flow and transport in the Chalk unsaturated zone is not an easy task and 36 despite several decades of study considerable uncertainty remains regarding the dominant flow 37 regime (e.g. Smith, 1970;Headworth, 1972;Wellings, 1984;Geake and Foster, 1989; Price et 38 al., 1993;Ireson et al., 2006;Lee et al., 2006;Ireson and Butler, 2011). 39 40 Uncertainty regarding the response of the deeper unsaturated zone arises partly due to difficulties 41 in quantifying drainage water leaving the base of the soil zone. The soil buffers and attenuates 42 precipitation events, dependent on the soil moisture content versus depth profile at the time of a 43 given rainfall event. Establishing the water release characteristics allows assessment of the 44 movement of water in the soil zone and can be achieved by monitoring moisture content and soil 45 tension, and through the application of numerical modelling. However, monitoring depth is 46 testing that produced high tritium concentrations in rainfall during 1963-1964 and 1958-1959 65 respectively. The position of two peaks corresponding to these events was used to calculate a 66 mean downward solute transport rate of less than 0.1 mmhr -1 (2.4 x...