TY - JOUR
T1 - Vertical Connectivity Regulates Water Transit Time and Chemical Weathering at the Hillslope Scale
AU - Xiao, Dacheng
AU - Brantley, Susan L.
AU - Li, Li
N1 - Funding Information:
This work was financially supported by funds from the National Science Foundation for the Susquehanna Shale Hillslopes Critical Zone Observatory (SSHCZO) to S. L. Brantley (EAR 13‐31726). D. Xiao was additionally supported by funds from the Penn State Earth and Environmental Systems Institute. This research was conducted in Penn State's Stone Valley Forest, which is funded by the Penn State College of Agriculture Sciences and the Department of Ecosystem Science and Management and managed by the staff of the Forestlands Management Office. We acknowledge discussions with members in the SSHCZO and Brantley and Li Reactive Water research group. We are deeply grateful for the help from Brandon Forsythe for data organization and management. We are thankful for the careful text editing by Bryn Stewart from the Li Reactive Water Group. Constructive comments from two anonymous reviewers and the associate editor has significantly improved the manuscript. We particularly acknowledge Dr. Daniel Ibarra for his suggestion of framing the discussion into hypotheses, which has significantly sharpened the paper.
Funding Information:
This work was financially supported by funds from the National Science Foundation for the Susquehanna Shale Hillslopes Critical Zone Observatory (SSHCZO) to S. L. Brantley (EAR 13-31726). D. Xiao was additionally supported by funds from the Penn State Earth and Environmental Systems Institute. This research was conducted in Penn State's Stone Valley Forest, which is funded by the Penn State College of Agriculture Sciences and the Department of Ecosystem Science and Management and managed by the staff of the Forestlands Management Office. We acknowledge discussions with members in the SSHCZO and Brantley and Li Reactive Water research group. We are deeply grateful for the help from Brandon Forsythe for data organization and management. We are thankful for the careful text editing by Bryn Stewart from the Li Reactive Water Group. Constructive comments from two anonymous reviewers and the associate editor has significantly improved the manuscript. We particularly acknowledge Dr. Daniel Ibarra for his suggestion of framing the discussion into hypotheses, which has significantly sharpened the paper.
Publisher Copyright:
© 2021. The Authors.
PY - 2021/8
Y1 - 2021/8
N2 - 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 (Cratio) 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 (Cratio ∼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.
AB - 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 (Cratio) 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 (Cratio ∼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.
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U2 - 10.1029/2020WR029207
DO - 10.1029/2020WR029207
M3 - Article
AN - SCOPUS:85113402046
SN - 0043-1397
VL - 57
JO - Water Resources Research
JF - Water Resources Research
IS - 8
M1 - e2020WR029207
ER -