Spatial dynamics of agricultural practices on a basin territory: a retrospective study to implement models simulating nitrate flow. The case of the Seine basin

Within an interdisciplinary research programme which has aimed at modelling the nitrate flow evolution in the whole Seine basin since the seventies, we propose a methodological process in order to reconstitute and spatialise cropping systems' dynamics, whose characteristics are used to implement the STICS agronomic model. This process makes use of both expert opinions and departmental or national agricultural statistics that are compared, step by step, in order to build the most reliable database in relation to the time and space scales considered. Data mining and statistical cartography methods are, respectively used to model the crop sequences' temporal evolution and to spatialise them over the spatial pattern of the agricultural districts. The results show an important spatial and temporal differentiation of the cropping systems, both in the cropping sequences developed and in the cultivation techniques employed. Crossing several information sources allows the identification of historical trends, but with a decreasing precision as we go back in time.