Vertical Connectivity Regulates Water Transit Time and Chemical Weathering at the Hillslope Scale

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 similar to 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.

Automated summary

The study shows that permeability variations in hillslope structure have significant effects on vertical connectivity, mean transit times, and chemical weathering rates. However, in arid climates, the influence of long transit times on weathering is less significant. Three hypotheses were proposed and it was highlighted that hillslope structure plays a crucial role in regulating the hydrogeochemical response of surface water to changing climate and human activities.