Identifying Catchment Scale DOC Export Dynamics Using Specific Conductivity Traced End‐Member Mixing Analysis Method

Abstract

Quantifying event‐scale dissolved organic carbon (DOC) sources and flow paths is crucial for understanding stream DOC dynamics and hydrological controls on DOC export. However, few continuous DOC estimations from different pathways have limited our understanding of DOC export mechanisms in data‐scarce catchments. Here, a method coupling end‐member mixing analysis (EMMA) and DOC concentration‐discharge relationships (DOC‐ Q ) was developed, to quantify the contributions of event water, mobilized shallow subsurface water, and deep baseflow to DOC fluxes and yields at a mountainous forested catchment. Results demonstrated that the coupling approach effectively captured streamflow DOC dynamics with median Nash‐Sutcliffe efficiency of 0.69. The end‐member DOC‐ Q fitting revealed distinct DOC concentrations in event water (10.2 ± 1.9 mg L ⁻¹ ), mobilized shallow subsurface water (7.2 ± 1.2 mg L ⁻¹ ), and deep baseflow (4.7 ± 0.2 mg L ⁻¹ ). Across five studied storms, flow pathways consistently played a disproportionate role in catchment DOC export. Event water exhibited an outsized effect on DOC yield relative to water yield, whereas mobilized shallow subsurface water and deep baseflow contributed undersized effects to DOC yield. Whether DOC predominantly originated from event water or mobilized shallow subsurface water depends on antecedent hydrological conditions and rainfall‐runoff processes. Streamflow DOC‐ Q relationship showed steeper under dry antecedent conditions; while it could flatten under specific wet antecedent conditions due to the DOC supply limitation and the flow dilute effect. This study provides a cost‐effective, high‐frequency method for identifying DOC dynamics and advances our understanding of hydrological controls on DOC export. The findings provide valuable insights for process‐based modeling, particularly in data‐limited regions.