Dissolved organic carbon export mechanism at a typical flash flood catchment: insights from high-frequency measurements and modellings

Understanding the patterns and mechanisms of DOC export from catchments is key to interpreting and quantifying land-to-ocean lateral carbon transport process. The DOC export is highly dynamic and storm-driven, especially in mountainous headwater catchments. Yet traditional low-frequency monitoring and modelling often misses rapid DOC changes and fail to identify potential shifts on DOC export pattern and process, limiting a deeper understanding of short-term export mechanisms. We have conducted high-frequency measurements combined with process-based modeling in a mountainous flash flood headwater catchment in China, to reveal how hydrological processes control DOC mobilization and export under varying storm conditions. Based on high-frequency measurements and hysteresis analysis, we have found a three-phase concentration-discharge (C-Q) relationship of DOC and a shift on DOC export pattern from transport-limited to source-limited across extreme storms. The studied catchment streamflow and DOC dynamics were successful reproduced by the process-based INCA-C model at hourly steps, further supporting the quantification of different flow pathways’ contributions on DOC output. The results showed that more DOC were exported by subsurface flows from shallow organic soil with greater peaks and shorter time-to-peaks at higher storm intensities. DOC is primarily sourced from subsurface runoff from the mineral layer (73 %–77 %) during moderate events, whereas it is primarily sourced from subsurface runoff from the organic layer (61 %–79 %) during extreme events. The two contrasting contributions suggest that hydrological pathway controls and DOC dynamic patterns can shift owing to runoff generation influenced by storm intensity. Our research revealed mechanisms on shifted DOC export regimes at a typical flash flood catchment with increasing storm intensity and changing flow-path contributions. The findings highlight high-frequency measurements and modellings for insights into hydrological controls on DOC export process.