Exploring dissolved organic carbon dynamics in a cold mountainous basin using a multi-tracer-aided hydrological model

Cold-region headwaters on the Tibetan Plateau are highly sensitive components of regional water and carbon cycles, yet the catchment-scale controls on dissolved organic carbon (DOC) export remain poorly resolved because runoff generation and flow pathways cannot be reliably inferred from streamflow alone. Here we developed THREW-IC, a distributed multi-tracer-aided hydrological model that couples cryospheric processes with stable isotope and DOC dynamics, and applied it to the Source Region of the Yangtze River, a permafrost-affected alpine basin. The model was jointly calibrated against discharge, stream-water δ¹⁸O, and DOC concentrations in stream water and groundwater. The results show that: (1) THREW-IC reasonably reproduces the main temporal dynamics of discharge, stream-water δ¹⁸O, and DOC concentrations in stream water and groundwater. (2) The combined tracer constraints identify internal runoff partitioning more effectively than discharge alone, with isotopes providing stronger constraints on surface–subsurface runoff partitioning and DOC observations more effectively constraining the groundwater-outflow parameter KKA. (3) Spatial correlation analysis showed that SCL was significantly associated with total DOC export and surface DOC export, but not with subsurface DOC export, highlighting the pathway-dependent coupling between terrestrial carbon availability and hydrological transport in controlling basin-scale DOC export. (4) In the 2015 sensitivity experiment, representing frozen-soil effects increased simulated annual total DOC export by approximately 18.6%, primarily through a 35.6% increase in subsurface DOC export while exerting negligible influence on surface DOC export. This effect was strongly seasonal, suppressing DOC export during the frozen low-flow period but enhancing it during the thaw and wet period through the seasonal redistribution of subsurface runoff. These results show that integrating conservative and reactive tracers within a distributed framework can reduce process uncertainty and improve mechanistic understanding of water–carbon coupling in cold alpine headwaters.