A multi-tracer hydrological model reveals dissolved organic carbon dynamics in a cold mountainous basin

Dissolved organic carbon (DOC) dynamics in cold mountainous basins, significantly influenced by cryospheric processes, remain poorly understood due to model limitations and scarce observations. This study explores DOC dynamics in the Source Region of the Yangtze River (SRYR) on the Tibetan Plateau using a novel multi-tracer-aided hydrological model, THREW-IC. The model integrates modules for water isotopes and DOC into the distributed THREW framework, explicitly representing key cryospheric processes (snow, glacier, frozen soil). It was calibrated and validated against daily streamflow, streamwater δ¹⁸O, and streamwater/groundwater DOC concentrations from 2010-2018. Results demonstrate satisfactory model performance across all objectives, confirming its capability to simulate coupled hydrological-biogeochemical processes. Sensitivity analysis revealed that parameters governing runoff generation  were most influential for streamflow simulation, whereas DOC-specific parameters were crucial for capturing DOC dynamics but less critical for water flux or isotope simulations, highlighting the value of multi-tracer constraints. Spatially, DOC production was predominantly linked to water storage in unsaturated soils. The analysis further elucidated the modulating role of soil freeze-thaw cycles on DOC production and transport. This study underscores the efficacy of a multi-tracer approach in reducing model uncertainty and advancing the mechanistic understanding of DOC generation and export in complex, cold mountainous environments.