Assessing the risks of nonpoint nitrogen pollution from agricultural sources on the scale of catchment areas: advantages of a modelling approach with SWAT®.

Abstract

Decisions on water quality issues are usually made on large territories ranging hundreds to thousands of km². The challenge for decision makers is to better understand the interactions between agriculture, the physical environment and water quality. The SWAT® model is a tool for combining those relationships. We will present here its implementation on two test watersheds in the region of "Pays de la Loire": the "Moine" watershed (385 km²) and the "Rochereau" watershed (206 km²). We outline difficulties linked to data availability. The model can predict relatively well flows and nitrogen fluxes at the catchment scale. Mapping nitrogen pollution risks due to agricultural activities becomes possible. As a result, one can differentiate among the different factors participating in the overall Nitrogen fluxes (agricultural practices or physical characteristics). This modelling experience brings interesting opportunities to simulate the effects of changes of the practices and use of the land on water quality.