How does hillslope structure (e.g., shape and permeability variation) regulate its hydro-geochemical functioning (flow paths, solute export, chemical weathering)? Results from numerical reactive transport experiments and particle tracking from this work underscore the first-order control of permeability variations (with depth) on vertical connectivity (VC). VC is defined as the fraction of water fluxes below the soil zone. Where permeability decreases sharply and VC is low, most of the water flows through the top 6 meters of the subsurface, barely interacting with reactive rock at depth. At similar discharge, weathering rates increase with mean transit time (MTT). VC however is less of an influence under arid climates where long transit times drive weathering to equilibrium. The results lead to several working hypotheses that can be further tested. H1: The permeability variations with depth more strongly influence MTTs of stream water; hillslope shapes instead influence the younger water fraction of stream water. H2: High VC arising from high permeability at depths enhances weathering because it promotes deeper water penetration and water-rock interactions; the influence of VC weakens under arid climates and larger hills 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 and deep water chemistry (C_ratio ~ 1), leading to more chemostatic CQ patterns. With carefully designed, co-located measurements of soil, rock, and waters, these hypotheses can be further tested . Broadly, the hillslope subsurface structure (permeability variation) is essential in regulating earth surface hydrogeochemical response to changing climate and human activities.

Susan Brantley

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