Estimation of sediment yield and soil loss using suspended sediment load and 137Cs measurements on agricultural land, Brie Plateau, France

Abstract

Recent land use changes have resulted in environmental disturbances on agricultural land in NW Europe. The development of underground drainage networks in regions characterized by temporary hydromorphic soils has altered the delivery of water discharge and sediment loads in rivers. To implement soil and water conservation strategies, space-time movements of fine sediment particles must be investigated at watershed scale. A cultivated watershed in the Brie Plateau (upstream of Paris) was chosen for this study. In the short-term, sediment fluxes were measured at three monitoring stations within nested watersheds (drainage pipe under a cultivated field of 6.4 ha, Vannetin River 4.6 km2 and 30.6 km2 areas). Suspended sediment loads, estimated over a four year period (1991–1995), revealed wide seasonal and interannual variations reflecting the seasonal rainfall distribution. Sediment yields at the outlet of the drainage system were large (0.24 t ha−1 yr −1) during a year with high rainfall. This incurs a risk of impoverishment of fine particles in soil over the medium-term. Sediment yields in the river could reach 0.30–0.47 t ha T−1 yr−1 for a rainy year. Large sediment deliveries only accompanied a widespread overland flows on slopes. In the medium-term, the time-integrated 137Cs technique is ideal for the Brie Plateau, where overland flow does not produce significant rill erosion features in the landscape. The 137Cs sampling grid was relatively dense because soil erosion and soil redistribution varied greatly on the gentle slopes (4–5% mean slope angle). The 137Cs tracer was used to integrate soil redistribution for a medium term-period (33 years). The study of a 7 ha field (downstream part of a 24 ha crop catchment) revealed a complex soil redistribution pattern and enabled the construction of a sediment budget. Five representative topographic units were investigated. Two of them exhibited net soil loss (sediment delivery ratio 74% on the hillslope and 83% at the outlet unit of the drainage basin). In the other spatial units, deposition exceeded erosion because of upslope and/or lateral sediment transfers. The latter units represent the main buffer areas of the slope.