Fight against non point pollution in rural areas - risk diagnosis approach in the Cétrais watershed

Abstract

In the western part of France, the protection of surface waters has become of major concern for farmers and agricultural organisations. But most of the programs implemented for the restoration of water quality act at the farm level. In fact, a greater efficiency would be reached if the restoration programs could be reasoned both at field and watershed levels. To help managers to implement such programs, a research has been developped since 1997. The monitoring of farmers' practices and water quality on a small watershed (Cétrais watershed, 34 square kilometers) has allowed the building of a non point source pollution risk diagnosis, which will soon supply managers with references and methods for Don watershed, which is much larger (600 square kilometers). In this paper, we discuss the method of non point source pollution risk diagnosis which has been build on the smaller watershed. the method has been applied both on nitrogen and pesticides. The comparisons between these two diagnosis axis has shown great differences in the main pathways and risk eriods for the two molecules. For nitrogen pollution, the diagnosis has allowed the building of pollution risk indexes, more simple to use than models. Extrapolation to larger watersheds has been explored. For pesticides, particular indexes are proposed, but more work is still necessary to describe the importance of flows and retention in ditches.