Hydrological Controls on Temporal Contributions of Three Nested Forested Subcatchments to DOC Export

Abstract. Assessing DOC export from terrestrial systems into inland waters reliably is of paramount importance to understand all processes of the global carbon cycle. Using high-frequency measurements of DOC concentrations via 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, Germany, during a 12 month period. The subcatchments differ with respect to topography, elevation, vegetation and soils. 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 low DOC availability and a limited hydrological connectivity, which is the prerequisite for transport processes from terrestrial systems into inland waters. Flow-weighted DOC export also varied between the three subcatchments. Flow-weighted DOC export was always higher in the lower, flat subcatchment than in the upper steep 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 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.