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. Understanding the controls on event-driven dissolved organic carbon (DOC) export is crucial as DOC is an important link between the terrestrial and the aquatic carbon cycles. We hypothesized that topography is a key driver of DOC export in headwater catchments because it influences hydrological connectivity, which can inhibit or facilitate DOC mobilization. To test this hypothesis, we studied the mechanisms controlling DOC mobilization and export in the Große Ohe catchment, a forested headwater in a mid-elevation mountainous region in southeastern Germany. Discharge and stream DOC concentrations were measured at an interval of 15 min using in situ UV-Vis (ultraviolet–visible) spectrometry from June 2018 until October 2020 at two topographically contrasting subcatchments of the same stream. At the upper location (888 m above sea level, a.s.l.), the stream drains steep hillslopes, whereas, at the lower location (771 m a.s.l.), it drains a larger area, including a flat and wide riparian zone. We focus on four events with contrasting antecedent wetness conditions and event size. During the events, in-stream DOC concentrations increased up to 19 mg L−1 in comparison to 2–3 mg L−1 during baseflow. The concentration–discharge relationships exhibited pronounced but almost exclusively counterclockwise hysteresis loops which were generally wider in the lower catchment than in the upper catchment due to a delayed DOC mobilization in the flat riparian zone. The riparian zone released considerable amounts of DOC, which led to a DOC load up to 7.4 kg h−1. The DOC load increased with the total catchmentwetness. We found a disproportionally high contribution to the total DOCexport of the upper catchment during events following a long dry period. Weattribute this to the low hydrological connectivity in the lower catchmentduring drought, which inhibited DOC mobilization, especially at thebeginning of the events. Our data show that not only event size but alsoantecedent wetness conditions strongly influence the hydrologicalconnectivity during events, leading to a varying contribution to DOC exportof subcatchments, depending on topography. As the frequency of prolongeddrought periods is predicted to increase, the relative contribution ofdifferent subcatchments to DOC export may change in the future whenhydrological connectivity will be reduced more often.

Low hydrological connectivity after summer drought inhibits DOC export in a forested headwater catchment