Concentration-discharge (CQ) relationships have been used to infer catchment-scale processes controlling dynamics of dissolved and particulate organic matter. Various competing factors have been proposed to influence the CQ relationships of dissolved organic carbon (DOC), including the dependence of carbon decomposition rates on soil moisture, soil organic carbon (SOC) distribution, and hydrological flow pathways. However, due to the integrative nature of streams with both hydrological and biogeochemical signals, it is still not clear which factors dominate under different watershed characteristics and environmental conditions. We hypothesize that hydrological pathways predominantly control the CQ relationship of DOC. To test this hypothesis, we utilized data from the Susquehanna Shale Hills Critical Zone Observatory (CZO) in Pennsylvania and applied a recently developed hydrological land surface and biogeochemical reactive transport code, BioRT-Flux-PIHM, to better understand the process-based interactions between DOC production and transport. Simulation results show that DOC flushing behavior (increasing concentrations with increasing discharge) persists regardless of different DOC production rates (including linear, non-linear, and threshold rate dependence on soil moisture), as long as the proportions of shallow soil lateral flow and deep groundwater contribution to the stream flow remain similar. Further comparisons of heterogeneous versus homogeneous SOC distribution indicate that hydrologic connectivity between the stream and flow-converging area (i.e., swales with deep and SOC-rich soils) drives observed dilution behavior (decreasing concentration with increasing discharge). Stream water derived from swales under low flow conditions is enriched in DOC whereas under high flow conditions, it is diluted by DOC-poor water from planar hillslopes. This study underscores the intertwining characteristics of DOC production and transport, and the significant role of hydrological flow pathways resulting from critical zone subsurface structures that drive the export of DOC and possibly other reactive solutes into aquatic systems.
Hang Wen*, Li Li, Julia N Perdrial, Benjamin Abbott, Thomas Adler, Susana Bernal, Remi Dupas, Sarah Godsey, Rebecca L Hale, Adrian Harpold, Donna M Rizzo, Gary Sterle, Kristen Underwood (2018): Hydrologic Control of Catchment-Scale Dissolved Organic Carbon (DOC) Dynamics. Abstract H13J-1880 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec .