Missing connectivity during summer drought controls DOCmobilization and export in a small, forested catchment

Abstract. Understanding the controls on event-driven DOC export is crucial, as DOC is an important link between the terrestrial and the aquatic carbon cycles. We hypothesize that topography is a key driver of DOC export because it influences hydrologic connectivity, which can inhibit or facilitate DOC mobilization. To test this we studied the mechanisms controlling DOC mobilization and export in the Grose Ohe catchment, a forested headwater in a mid-elevation mountainous region in Southeastern Germany. Discharge and stream DOC concentrations were continuously measured using in-situ UV-Vis spectrometry from June 2018 until October 2020 at two topographically contrasting sub-catchments of the same stream: at a steep hillslope (888 m.a.s.l.) and in a flat and wide riparian zone (771 m.a.s.l). We focus on four events with contrasting antecedent hydrological conditions and event size. During 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 anti-clockwise 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 total DOC load up to 522 kg per event. The total DOC load increased with the total catchment wetness. We found a disproportionally high contribution to the total DOC export of the upper catchment during events following a long dry period. We attribute this to the lack of hydrological connectivity in the lower catchment during drought, which inhibited DOC mobilization, especially at the beginning of the events. Our data show that not only event size but also antecedent hydrological conditions strongly influence the hydrological connectivity during events, leading to a varying contribution to DOC export of different catchment parts depending on topography. As the frequency of prolonged drought periods is predicted to increase, the relative contribution of different catchment parts to DOC export may change in the future, when hydrological connectivity will occur less often.