Phosphorus is an important pollutant leading to lake eutrophication. Diffuse pollution accounts for a sub- stancial share of the total phosphorus load. To improve our knowledge of this pollution, we conducted a watershed study in a small, rural, 302-ha watershed (Lake Leman area). To study the spatial distribution and temporal dynamics of phosphorus in the watershed and the corresponding variation in phosphorus speciation, 40 stormflow events over a 4- year period were analysed. We began by defining the 'state' of the watershed using a) periodic field surveys of signs of erosion, b) the length of the flowing ditches and streams (active hydrological network) and the waterpathways on the fields, c) the overall water budget, and d) grab samples of stream water from a network of subwatersheds. Four states that reflected contrasting watershed conditions were identified. Analysis of the data on phosphorus flux at the outlet of the watershed, in conjunction with the observations on the state of the watershed, allowed us to develop a conceptual model of phosphorus export from the watershed and several hypotheses about phosphorus transfer. (© Inra/Elsevier, Paris.)