Concentrations and fluxes of dissolved organic carbon in runoff from a forested catchment: insights from high frequency measurements

Concentrations of dissolved organic carbon (DOC) in runoff from catchments are often subject to substantial short term variations. The aim of this study was to identify the spatial sources of DOC and the causes for short term variations in runoff from a forested catchment. Furthermore, we investigated the implication of short term variations for the calculation of annual runoff fluxes. High frequency measurements (30 min intervals) of DOC in runoff, of discharge and groundwater table were conducted for one year in the 4.2 km² forested Lehstenbach catchment, Germany. Riparian wetland soils represent about 30% of the catchment area. The quality of DOC was investigated by three dimensional fluorescence excitation-emission matrices in samples taken from runoff, deep groundwater and shallow groundwater from the riparian wetland soils. The concentrations of DOC in runoff were highly variable at an hourly to daily time scale, ranging from 2.6 mg l⁻¹ to 34 mg l⁻¹ with an annual average of 9.2 mg l⁻¹. The concentrations were positively related to discharge, with a pronounced, counter clockwise hysteresis. Relations of DOC to discharge were steeper in the summer/fall than in the winter/spring season. Dynamics of groundwater table, discharge, DOC concentrations and DOC quality parameters indicated that DOC in runoff originated mainly from the riparian wetland soils, both under low and high flow conditions. The annual export of DOC from the catchment was 84 kg C ha⁻¹ yr⁻¹ when calculated from the high frequency measurements. If the annual export was calculated by simulated random fortnightly samplings, the range was 47 to 124 kg C ha⁻¹ yr⁻¹. Calculations of DOC export fluxes might result in significant errors when based on infrequent (e.g. fortnightly) sampling intervals. Future changes in the precipitation and discharge patterns will influence the DOC dynamics in this catchment, with largest effects in the summer season.