Long-term soil redistribution in a small French watershed as estimated from cesium-137 data

Changes initiated in the 1970s in the agricultural landscape of the north European plains have resulted in increased water erosion. The spatial redistribution of Cs-137 was used to assess the magnitude of long-term soil movements in a 180 ha watershed located in northern France. The Cs-137 base level was estimated at 2000 +/- 200 Bq m(-2) The point data suggest annual soil movements ranging from a loss of 18 Mg ha(-1) to a deposition of 19 Mg ha(-1). After spatializing these data, it is concluded that 41% of the watershed area ran be considered as stable. Some 45% of the watershed area suffered a net soil loss, with an average rate of 6 Mg ha(-1) yr(-1) Net deposition occurred over 14% of the area, with an average annual rate of 7 Mg ha(-1). From these values, it was estimated that a net output of 1.9 Mg ha(-1) yr(-1) of eroded material would have left the watershed. This represents a sediment delivery ratio of 59%. These results are consistent with those of other European researchers under comparable environments. They also stress the point that redeposition is an important part of the global erosion process. Finally, these results highlight some of the advantages of the Cs-137 technique, which makes possible the development of long-term sediment budgets at the watershed scale.