In order to understand mechanisms influencing DOC sources and export we investigate DOC quality parameters. Our goal is to identify differences in DOC and its mobilization processes between steep hillslopes and riparian zones and between different precipitation events. We hypothesize that different antecedent hydrological conditions and the topographical position influence sources of DOC and mobilization processes.
Material and Methods
We continuously measured absorption from 200 to 750 nm via UV-Vis spectrometry at two sampling sites at a small stream (riparian zone and steep hillslope). We used the absorption values to investigate the ratio of A254 and A365 and the specific DOC concentration normalized UV absorbance at 254 nm (SUVA254) during three large precipitation events with different antecedent hydrological conditions.
The A254/A365 ratio values usually fluctuated between 2 and 5. However, the variation of the ratio values was more pronounced during the event following dry hydrological conditions than the one following wet hydrological conditions and less pronounced at the hillslopes than at the riparian zone. SUVA254 values at the steep hillslope sampling site were close to 3 L/(mg m) and did not vary greatly during rain events. This stands in contrast to the riparian zone sampling site where SUVA254 values reached 12 L/(mg m) and showed a clear variation during precipitation events. This variation was also more pronounced during the precipitation event following dry hydrological conditions than the one following wet hydrological conditions.
The change of DOC quality parameters during events indicates a change of DOC sources or flowpaths. Our results suggest that mechanisms influencing the mobilization of DOC differ between the riparian zone and the hillslopes. Furthermore, hydrological preconditions seem to influence the composition of DOC reaching the stream.