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

Abstract

International audienceIncreasing 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

High‐frequency monitoring of catchment nutrient exports reveals highly variable storm event responses and dynamic source zone activation

“…At the seasonal scale, streamflow decreased through summer in response to relatively warm, yet not unusually dry, conditions with high evapotranspiration potential, and then [Jaffé et al, 2008]. Such DOC concentrations are akin to those reported for a similar sized agricultural catchment in northern France [Morel et al, 2009] but relatively high when compared to values reported from many mixed-use catchments in both Europe and North America [Raymond and Oh, 2007;Monteith et al, 2007]. Base flow NO 3 -N concentrations of around 6.5 mg/L were similar to those reported previously for agricultural catchments [Lloyd et al, 2016b;Thomas et al, 2016] and were likely driven by current fertilizer application and legacy fertilizer At diurnal scales, streamflow exhibited minor fluctuations during base flow conditions, most likely driven by contrasts in evapotranspiration demand between day and night but also potentially due to temperaturedriven changes in water viscosity and associated changes in bed sediment hydraulic conductivity [Schwab et al, 2016;Constantz et al, 1994].…”

New molecular evidence for surface and sub-surface soil erosion controls on the composition of stream DOM during storm events

“…Previous work performed in the Kervidy-Naizin catchment (Morel et al, 2009) and in other headwater catchments (Inamdar and Mitchell, 2006;Van Gaelen et al, 2014) have shown that during base flow periods, the stream water was mostly sustained by deep and shallow groundwater. However, during storm events, the increase in discharge was mostly due to the increase in the soil solution contribution (Lambert et al, 2011;Morel et al, 2009). As the same modifications were recorded in the soil solution and stream DOM composition during storm events, the soil solution may be a possible origin of stream water DOM modification during storm events.…”

“…The use of chemical components such as these is common in order to trace the contribution of sources to the stream Lambert et al, 2014;Morel et al, 2009). The soil solution analysis revealed variable concentrations during the event (Fig.…”

“…The intensity of DOM export varies over seasons and hydrological conditions depending on the sources and water flow paths. When considering annual river DOM fluxes, more than 50 % is exported during storm events (Buffam et al, 2001;Morel et al, 2009;Raymond and Saiers, 2010). This largest export is attributed to the rise of the water table in organic-rich soil horizons which become hydrologically connected to the river (Boyer et al, 1996), and to 5040 M. Denis et al: New molecular evidence for surface and sub-surface soil erosion controls the fact that river discharge is mostly sustained by soil water fluxes during storm events (Hagedorn et al, 2000;Lambert et al, 2011).…”

Motifs temporels de la qualité de l'eau. Analyse d'une décennie de données haute-fréquence mesurées dans un observatoire agro-hydrologique

“…Ces travaux, servant à constituer les hypothèses de départ, portaient sur un élément (ou groupe d'éléments iso-chimiques) comme le nitrate [Molénat et al, 2005 ;Molénat et al, 2008], le COD [Lambert et al, 2011 ;Morel et al, 2009] ou encore sur les terres rares [Dia et al, 2000] (la liste des publications est loin d'être exhaustive).…”