BayCEER Workshop 2020

Topographic and hydrologic controls of event-driven DOC mobilization and export in a small, forested catchment

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. We hypothesize that hydrological conditions, such as antecedent wetness and event size, and the topographical position (steep hillslopes vs. flat riparian zones (RZ)) influence sources of DOC and mobilization processes. 

We have been measuring discharge and stream DOC (using UV-Vis spectrometry) continuously since early summer 2018 at two topographically different positions within the Große Ohe catchment in the Bavarian Forest National Park, Germany: at a steep hillslope (888 m.a.s.l.) and in a flat and wide RZ (771 m.a.s.l). Here, we focus on three similarly sized events with different antecedent hydrological conditions in June 2018 (total precipitation P_tot: 29 mm, antecedent precipitation index API₁₄: 110 mm), October 2018 (P_tot: 33 mm, API₁₄: 2 mm) and May 2019 (P_tot: 24 mm, API₁₄: 45 mm). We observed a rather slow DOC mobilization in the flat RZ, but once DOC mobilization has started, the RZ releases considerable amounts of DOC. We attribute this to a delayed establishment of connectivity between DOC source areas and the stream. However, area-normalized DOC export shows a varying relative contribution of the upper steep catchment part to total DOC export. Especially during the event in October 2018, which followed a very dry summer, the upper part of the catchment contributed with 49 % to the total DOC export (vs. 32 % expected in terms of area).  We attribute this to the missing hydrological connectivity at the lower part of the catchment, which inhibits DOC mobilization. Our data show that topography in the Große Ohe catchment influences important hydrological processes that facilitate or inhibit DOC transport to the stream.