Hydrological controls on temporal contributions of three nested forested subcatchments to the export of dissolved organic carbon

Abstract. Assessing the export of dissolved organic carbon (DOC) from terrestrial systems into inland waters reliably is of vital importance to understand the global carbon cycle in detail. In this context, hydrological connectivity within catchments is the prerequisite for transport processes from terrestrial systems into inland waters. Using high-frequency measurements of DOC concentrations via ultraviolet–visible (UV–Vis) spectrometry, we quantified the DOC export at the outlets of three nested forested subcatchments within a 3.5 km2 headwater catchment in the Bavarian Forest National Park (BFNP), Germany, during a 12-month period. The subcatchments differ with respect to topography, elevation, vegetation, and soil types. We observed a high flow-weighted DOC export from the entire headwater catchment during spring and autumn. In contrast, during snowmelt, summer, and winter, DOC export was low due to either low runoff generated and/or relatively lower DOC available for mobilization. Flow-weighted DOC export also varied between the three subcatchments. Flow-weighted DOC export was always higher in the flat lower subcatchment than in the steep upper subcatchments, indicating a large DOC store that can be activated whenever hydrological connectivity is established. This was particularly evident during autumn, when large precipitation events mobilized DOC which had accumulated during the dry summer period and was also delivered from fresh leaf litter of deciduous trees. Our data show the strong hydrological control on seasonal DOC export. However, the runoff-based contribution of subcatchments over time is modulated by the interplay of soils, vegetation, topography, and microclimate, which can be seen as secondary controls. As hydrological connectivity varies with topography, the relative contribution of topographically different subcatchments varies seasonally. Since climate change is predicted to influence precipitation patterns, spatial and temporal DOC export patterns are likely to change depending on topography.