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

Recent land use changes have resulted in environmental disturbances on agri cultural land in NW Europe. The development of underground drainage network s 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 p articles must be investigated at watershed scale. A cultivated watershed in the Brie Plateau (upstream of Paris) was chosen for this study. In the sho rt-term, sediment fluxes were measured at three monitoring stations within nested watersheds (drainage pipe under a cultivated field of 6.4 ha, Vannet in River 4.6 km(2) and 30.6 km(2) areas). Suspended sediment loads, estimat ed over a four year period (1991-1995), revealed wide seasonal and interann ual variations reflecting the seasonal rainfall distribution. Sediment yiel ds at the outlet of the drainage system were large (0.24 t ha(-1) yr(-1)) d uring a year with high rainfall. This incurs a risk of impoverishment of fi ne particles in soil over the medium-term. Sediment yields in the river cou ld reach 0.30-0.47 t ha(-1) yr(-1) for a rainy year. Large sediment deliver ies only accompanied a widespread overland flows on slopes. In the medium-t erm, the time-integrated Cs-137 technique is ideal for the Brie Plateau, wh ere overland flow does not produce significant rill erosion features in the landscape. The Cs-137 sampling grid was relatively dense because soil eros ion and soil redistribution varied greatly on the gentle slopes (4-5% mean slope angle). The Cs-137 tracer was used to integrate soil redistribution f or a medium term-period (33 years). The study of a 7 ha field (downstream p art of a 24 ha crop catchment) revealed a complex soil redistribution patte rn and enabled the construction of a sediment budget. Five representative t opographic units were investigated. Two of them exhibited net soil loss (se diment delivery ratio 74% on the hillslope and 83% at the outlet unit of th e drainage basin). In the other spatial units, deposition exceeded erosion because of upslope and/or lateral sediment transfers. The latter units repr esent the main buffer areas of the slope.