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Abstract How does hillslope structure (e.g., hillslope shape and permeability variation) regulate its hydro‐geochemical functioning (flow paths, solute export, chemical weathering)? Numerical reactive transport experiments and particle tracking were used to answer this question. Results underscore the first‐order control of permeability variations (with depth) on vertical connectivity (VC), defined as the fraction of water flowing into streams from below the soil zone. Where permeability decreases sharply and VC is low, >95% of water flows through the top 6 m of the subsurface, barely interacting with reactive rock at depth. High VC also elongates mean transit times (MTTs) and weathering rates. VC however is less of an influence under arid climates where long transit times drive weathering to equilibrium. The results lead to three working hypotheses that can be further tested.H1:The permeability variations with depth influence MTTs of stream water more strongly than hillslope shapes; hillslope shapes instead influence the younger fraction of stream water more.H2:High VC arising from high permeability at depths enhances weathering by promoting deeper water penetration and water‐rock interactions; the influence of VC weakens under arid climates and larger hillslopes with longer MTTs.H3:VC regulates chemical contrasts between shallow and deep waters (C_ratio) and solute export patterns encapsulated in the power law slope b of concentration‐discharge (CQ) relationships.Higher VC leads to similar shallow versus deep water chemistry (C_ratio∼1) and more chemostatic CQ patterns. Although supporting data already exist, these hypotheses can be further tested with carefully designed, co‐located modeling and measurements of soil, rock, and waters. Broadly, the importance of hillslope subsurface structure (e.g., permeability variation) indicate it is essential in regulating earth surface hydrogeochemical response to changing climate and human activities. 

Abstract Soils derived from different lithologies and their controls on preferential flow remain underexplored in forested landscapes. In the same lithology, the propensity for preferential flow occurrence at different hillslope positions also remains largely elusive. By utilizing a soil moisture response time method, we compared preferential flow occurrence between a shale site (Shale Hills, silt loam soils) and a sandstone site (Garner Run, sandy loam soils) at four hillslope positions: ridge‐top, North‐ and South‐facing mid‐slopes and toe slope, for over 2 years. The catchments are neighbouring and covered by temperate forest. For the four hillslope positions, Shale Hills had higher preferential flow frequencies compared to Garner Run. Between these two catchments, the South‐facing mid‐slope sites showed the highest contrasts in preferential flow frequency (33.5% of events at Shale Hills vs. 8.8% at Garner Run) while the ridge‐top sites showed the lowest contrasts (18.7 vs. 13.2%). Additionally, over the unfrozen period, for seven out of eight monitoring sites, drier antecedent conditions tended to be more favourable for preferential flows to occur, with significant (p < .01) relationships at two sites. Except for the South‐facing mid‐slope sites, both Shale Hills and Garner Run had two preferential flow pathways. The characteristic preferential flow pathways at Shale Hills were the B_wand C horizons, and for Garner Run, preferential flow moved from the E/AE horizon to the B_whorizon. This study shows that shale‐derived soils tended to have higher preferential flow occurrence than sandstone soils, but hillslope positions exhibit different levels of contrasts. More effort should be paid to study the impact of lithology on preferential flows in the context of land surface modelling and biogeochemical reactions to improve ecosystem services of headwater catchments.