Assessment of soil erosion in a small agricultural basin of the St. Lawrence River watershed

In the last decades, the conservation of soil and water resources has becom e a major environmental concern worldwide. The authors used an anthropogeni c radiotracer (Cs-137) to assess the magnitude and the spatial distribution of soil movement in a 80 ha agricultural watershed located in Southeastern Quebec (Canada). This project is part of a larger one aimed at increasing our understanding of erosive processes, in order to minimize their impacts on soil resources and downstream hydrosystems. Using this technique, it was possible to establish a budget and a map of soil redistribution. The resul ts could help farm operators to implement a resource conservation strategy. It was also possible to assess the environmental impact of this soil movem ent, by estimating the magnitude of the net output of sediment from the stu died watershed. This was estimated at an average of 237 tons per year, over a period of some 33 years. The output represents 93% of the total soil dis placement within the watershed boundaries. Although slightly higher, these results are consistent with those obtained, over a time period limited to 2 years, by water quality monitoring at the watershed outlet and by model si mulation. This intensive study, involving more than 500 sampling points for Cs-137, has yielded valuable information on the assessment and the spatial representation of erosive processes. Such information is needed to improve the management of watersheds and can help identify the areas most at risk and establish priorities for corrective actions. In this way, nonpoint poll ution of downstream hydrosystems may be controlled more efficiently.