Sources of dissolved organic carbon during stormflow in a headwater agricultural catchment

Abstract : Increasing dissolved organic carbon (DOC) concentrations have been reported during the last 15 years in streams from the United Kingdom, Northern Europe and North America. Identifying the sources of DOC and the controls of the delivery to the stream is important to understand the significance of these trends. This relies on the availability of observations of DOC dynamics during storm events, since much of the DOC export from soils to streams occurs during high flows. This study analyses DOC data for eight storm events during winter 2005-2006 in a small agricultural experimental catchment - the Kervidy-Naizin experimental catchment - located in Western France. A four end-member mixing approach was applied to the eight monitored storm events to identify DOC sources and quantify their respective contribution to DOC stream fluxes, using DOC, nitrate, sulphate and chloride as tracers. The results show that DOC concentrations in the stream at the outlet of this catchment increase markedly during storm events. The slope of the linear regression between DOC concentration and discharge was not constant for the eight events and depended on pre-event hydrological conditions. Between 64 and 86% of the DOC that enter the stream during storms originated from the upper layers of the riparian wetland soils. The variation of the delivery of DOC seems to be controlled by hydrological processes only, the wetland soils acting as a non-limiting store.