High-resolution DOC measurements indicate differences in DOC mobilization processes depending on topography

Since the 1980s, an increase in dissolved organic carbon (DOC) concentrations in waters of the Northern hemisphere has been observed. However, no general explanation has been found so far. Our study focuses on investigating the mechanisms influencing DOC mobilization and export in the streams of a forested headwater catchment in the Bavarian Forest National Park, Germany, as controlled by topography and hydrological conditions. Our goal is to identify differences in DOC mobilization processes between steep hillslopes and riparian zones and between different precipitation events. We hypothesize that different hydrological conditions and the topographical position (steep hillslopes vs. riparian zones) influence sources of DOC and mobilization processes. 

Three continuous sampling sites were established in different topographical positions within the catchment of the Große Ohe in the Bavarian Forest National Park along one of the streams: at a steep hillslope (880 m.a.s.l.), in a transition zone where the steep hillsides level off (805 m.a.s.l.) and in a flat and wide riparian zone (770 m.a.s.l). At these three locations, DOC concentrations in stream water have been measured continuously using UV-Vis spectrometry since early summer 2018, in combination with continuous discharge measurements. In addition, we regularly conducted a longitudinal sampling in order to analyze stream water chemistry parameters at 16 sampling points along the investigated stretch of about 3 km.

We analyzed discharge and DOC dynamics and DOC-Q hysteresis patterns, derived from the high-resolution data, to investigate if DOC mobilization differed between the topographical positions. We focus on two large events with different antecedent hydrological conditions in October 2018 (P_tot: 30 mm, API₁₄: 1.9 mm) and May 2019 (P_tot: 28.9 mm, API₁₄: 46.8 mm). At all topographical positions, maximal DOC concentrations were higher during the event in October 2018 (up to 15 mg/l) than during the event in May 2019 (up to 10 mg/l). These maximal concentrations also persisted much longer on the falling limb of the hydrograph during the October event, following dry conditions, than during the May event, following wet conditions.  This behavior results in wider hysteresis loops at all topographical positions during the event in October 2018 than in May 2019. However, peak concentrations dropped more quickly at the site of the steep hillslope than at the site of the transition zone and the riparian zone, resulting in more compressed hysteresis loops during both events. We use these differences in the DOC-Q hysteresis patterns to identify key processes for DOC mobilization and to create a perceptual model for DOC export from small, forested catchments.