Concentrations and fluxes of dissolved organic carbon in runoff from a forested catchment: insights from high frequency measurements
Abstract. 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 compartmental sources of DOC in a forested catchment and the causes for short-term variations in runoff. 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 counter clockwise hysteresis. Relations of DOC to discharge were steeper and the degree of hysteresis larger in the summer/fall than in the winter/spring period. 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 samplings of >2 days intervals substantial deviations resulted.
Predicted changes in precipitation and discharge patterns as well as generally increasing temperatures likely will cause raising DOC exports from this catchment.