Abstract 2011: Dissolved organic carbon (DOC) is transported in considerable amounts from terrestrial to aquatic systems and plays an important role in the global carbon cycle. The objective of this study was to analyze the long term trend and the short term dynamics of DOC in a forested catchment, characterized by a large proportion of Histosols.
The long term data set on discharge and runoff chemistry from the Lehstenbach catchment, Germany, from 1987 to 2009 is based on biweekly sampling and daily average discharge. In addition, DOC in runoff was measured during several discharge events in 2010 and 2011 in high temporal resolution (15 min) by a spectrometric device. Simultaneously, the groundwater table was monitored by pressure sensors in a fen site close to the watershed outlet.
The long term data revealed that DOC concentrations in runoff increased from about 5 mg DOC L-1 in 1987 to 9 mg DOC L-1 in 2009. DOC concentrations were highly variable at biweekly time scales. Runoff did not change significantly in the given period of time, while sulfate concentrations decreased significantly.
Following rain events the discharge increases within a few hours. Measurements with high temporal resolution showed that DOC increased with discharge and maximum concentrations were observed shortly after maximum discharge. The relation of discharge and DOC is characterized by a hysteresis with higher concentrations at falling discharge as compared to rising discharge. Furthermore, the relation is temperature dependent with largest response during the summer period. High concentrations of DOC in runoff coincide with near surface groundwater table, indicating that the peaty riparian zones are the major source of DOC in runoff.
In the Lehstenbach catchment, the long term trend in DOC seems to be due to decreased sulphate deposition. The short term temporal variation of DOC in runoff is hydrologically driven and overlapped by temperature. Changes in the climatic regime, e.g. by changing precipitation pattern and changing temperature will influence DOC export from the watershed.
Keywords: DOC, runoff, forested watershed, hydrologic control, temperature
project description in detail from proceedings of 2011 TERRECO Science Conference GAP
Abstract 2013: Dissolved organic carbon (DOC) is an important constituent in both terrestrial and aquatic ecosystems. DOC influences nutrient cycling, pollutant transport, soil formation, gross carbon cycling, light penetration, and acidification/eutrophication. While DOC in runoff has been investigated for a time, little is known about the temporal dynamics of DOC in headwater stream catchments in response to precipitation events. Here, we review the results of two studies analyzing the dynamics of DOC in forested headwater stream catchments. The DOC concentrations in both studies were estimated using a novel technique based on UV/UVvis spectroscopy which allows high-frequency measurements (< 30 minutes). One catchment is located in the north of South Korea (the Haean Catchment) characterized by steep slopes and a predominantly broad-leaved forest type. The second catchment is located in the Fichtel Mountains, northern Bavaria, Germany, characterized by moderate slopes, 30% of wetland area, and Norway spruce in monoculture. The DOC concentration range in runoff was about 1-5 and 5-35 mg liter-1, in the Haean and in the Fichtelgebirge catchment, respectively. In both headwater catchments a pronounced hysteresis was found, with a general clockwise concentration-discharge response in the Haean catchment, whereas the DOC concentration-discharge response was counter-clockwise for the Bavarian catchment. The differences in the direction of the concentration-discharge response are due to distinguished runoff generation processes and mechanisms of DOC mobilization. While the DOC in the Haean catchment is mobilized on the rising limb of the hydrograph, and therefore transported without temporal delay to the stream, the DOC in the Fichtelgebirge catchment is mobilized on the falling limb of the hydrograph due to bypass mechanisms caused by ditches and the influence of pre-conditional water table levels in the wetland areas of the catchment. The results of both studies demonstrate the potential of in-situ optical measurements for the analysis of fine-resolution dynamics of DOC in catchments, especially during high precipitation events.
Key words: dissolved organic carbon, hysteresis, concentration-discharge response, headwater stream
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