The effectiveness of changes in agricultural practices on water quality as simulated by the SWAT model
Reducing the negative impact of agriculture on water quality requires changes in agricultural practices. The actions proposed by local stakeholders have variable effects on water quality depending on the kind of environment and existing agricultural activities. In this context, agro-hydrological modelling constitutes an approach for assessing the impacts of agricultural practices on water quality at the watershed level, offering valuable decision-making support regarding a range of policy alternatives. More specifically, modelling provides a means for evaluating policy choices in terms of environmental efficiency. In this article, we present the implementation and results of a simulation involving new agricultural practices on a 385 km2 watershed in western France. This project involved a partnership with local planners in order to evaluate possible changes of water quality over the mid-term. The data used are a daily series of precipitation and temperature measurements, a Digital Elevation Model, a soil map, a crop rotation map for two successive years resulting from satellite imagery, and information concerning agricultural practices based on field surveys. For the 2000-2001 calibration period, the model provided good hydrological results with a 0.81 Nash index between simulated and observed flows. The dynamics of nitrogen and phosphorus fluxes in the river were also well represented by the model, although the insufficient frequency of validation data should be kept in mind when interpreting these results. After the retrospective calibration of the model, the efficiency of two changes in agricultural practices was evaluated: i) the introduction of a winter crop cover (mustard); and ii) no tillage. The no-tillage method appeared to be the most effective in reducing phosphorus fluxes, which represent the major source of pollution in the study area.