Land use, agronomic management and water quality in a small Northern Italian watershed

European union (EU) agricultural policy has induced significant changes in crop rotations, especially in marginal areas. The evaluation of the impact on water quality induced by this new agronomic framework is presented in this paper. The discharge, the sediment content and the concentrations of herbicides and nitrates in the Centonara creek, draining a hilly watershed near Bologna, were measured from October 1994 to September 1996. A geographic information system (GIS) and the crop simulation model CropSyst were used to characterize the relationships between cropping systems, land use, pedological and morphological properties of the watershed as well as nitrate losses. Hydrological results showed that the Centonara creek discharge was characterized by low base flows and by fast increments during flooding. Herbicide concentrations were above the EU 0.1 microgram l(-1) limit on several occasions, whereas nitrate concentrations were always below the 50 mg l(-1) limit established by EU for drinking waters. It was estimated that more than 30% of the nitrogen input in the watershed is due to atmospheric depositions. The purpose of GIS was to subdivide the watershed in 86 "agronomically homogeneous areas", which were then utilized as the basis for the application of CropSyst. Simulations obtained by the model showed that the greatest leaching losses of nitrates were higher than 10 kg ha(-1) yr(-1) exclusively in the agronomically homogeneous areas characterized by coarser textured soils. Overall nitrate and herbicide losses were low, mainly due to the differentiation of the cropping systems in the watershed. The combined use of GIS and CropSyst enabled the characterization of the environmental vulnerability in relation to the land use in the watershed by means of pedologic cartography, land use maps and meteorological data. In particular, erosion and herbicide losses were higher in sloping areas planted with spring-summer crops. The increase in row crops cultivations, determined by EU agricultural policy, represents the main impact on water quality of the investigated area.