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 th is paper. The discharge, the sediment content and the concentrations of her bicides and nitrates in the Centonara creek, draining a hilly watershed nea r Bologna, were measured from October 1994 to September 1996. A geographic information system (GIS) and the crop simulation model CropSyst were used t o characterize the relationships between cropping systems, land use, pedolo gical and morphological properties of the watershed as well as nitrate loss es. Hydrological results showed that the Centonara creek discharge was char acterized by low base flows and by fast increments during flooding. Herbici de concentrations were above the EU 0.1 mug l(-1) limit on several occasion s, whereas nitrate concentrations were always below the 50 mg l(-1) limit e stablished by EU for drinking waters. It was estimated that more than 30% o f the nitrogen input in the watershed is due to atmospheric depositions. The purpose of GIS was to subdivide the watershed in 86 "agronomically homo geneous areas", which were then utilized as the basis for the application o f CropSyst. Simulations obtained by the model showed that the greatest leac hing losses of nitrates were higher than 10 kg ha(-1) yr(-1) exclusively in the agronomically homogeneous areas characterized by coarser textured soil s. Overall nitrate and herbicide losses were low, mainly due to the differe ntiation of the cropping systems in the watershed. The combined use of GIS and CropSyst enabled the characterization of the environmental vulnerabilit y in relation to the land use in the watershed by means of pedologic cartog raphy, land use maps and meteorological data. In particular, erosion and he rbicide losses were higher in sloping areas planted with spring-summer crop s. The increase in row crops cultivations, determined by EU agricultural po licy, represents the main impact on water quality of the investigated area. (C) 2001 Elsevier Science B.V. All rights reserved.