Contrasting suspended sediment export in two small agricultural catchments: Cross‐influence of hydrological behaviour and landscape degradation or stream bank management

Abstract

AbstractThe study aimed to identify hydrological and landscape factors that influence sediment transport in 2 agricultural catchments in north‐western France—Moulinet and Kervidy‐Naizin—with the same climate, size, and bedrock but with contrasting suspended sediment (SS) fluxes. Discharge and SS concentrations were continuously monitored at the catchment outlets for 9 hydrological years. Data were examined at annual, seasonal, and individual storm‐flow event scales. Storm events were classified into 3 types—single small, successive, and major events—whose effect on sediment transport was studied. At the annual scale, SS yields were higher in Moulinet (16–63 × 103 kg km−2) than in Kervidy‐Naizin (3–22 × 103 kg km−2) despite similar water fluxes. However, Kervidy‐Naizin had higher potential for hillslope erosion because of more frequent occurrence of saturation excess runoff, more frequent bare soil in cultivated area, and fewer hedgerows. Major storm events were also stronger in this catchment. Although high SS concentrations and fluxes occurred only during a few major events in Kervidy‐Naizin, SS export was always substantial throughout the year in Moulinet. We hypothesised that bank degradation due to cattle trampling generated the higher SS export in the Moulinet catchment. This hypothesis was confirmed by the large decrease in SS fluxes following construction of watering troughs in the middle of the study period. In the Kervidy‐Naizin catchment, natural woody vegetation or grass buffer strips along the stream effectively protect the stream from bank erosion or a possible influx of hillslope erosion particles, except during some major events in winter.