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International audience; Excessive nutrient delivery into freshwater bodies results in increased eutrophication risk worldwide. Because high-frequency monitoring cannot be generalised to all rivers, methods are needed to assess eutrophication risk in contexts with scarce data. To this end, we present an assessment framework which includes: (i) a mass-balance model to estimate diffuse N and P transfer/retention in unmonitored catchments and (ii) a set of indicators based on N:P:Si molar ratios to assess the risk of eutrophication in freshwaters. The model, called Nutting, integrates variables that describe both agricultural pressures and physical attributes of catchments (climate, topography, soil). Nutting refines previous mass-balance models by describing nutrient pressures with soil N surplus and soil P content instead of N and P inputs, and by considering physical attributes not only as lumped variables over the entire area but also within river corridors. The model was calibrated on a set of 160 independent catchments across France and applied to all headwater catchments. We found that apparent N and P retention represented 53 +/- 24% and 95 +/- 29% of soil N and P surplus, respectively, and was mainly controlled by the climate and a hydrology-related connectivity index. The spatial organisation of the landscape was of secondary importance compared to the refined description of agricultural pressures. Estimated eutrophication risk was highly sensitive to assumptions about P bioavailability, hence the potential range of headwaters at risk of eutrophication spanned 26-63% of the catchments, depending on assumptions. This framework provides a generic method to assess the relative contribution of agriculture to nutrient loads and the subsequent risk of eutrophication. (V) 2014 Elsevier Ltd. All rights reserved.

International audience; Watershed losses of phosphorus (P) have been a topic of concern for water resource managers over recent decades. To evaluate environmental policies or before implementing mitigation options at the watershed (catchment area) scale, stakeholders often need to analyze the patterns of point and diffuse sources of phosphorus. This information is often not easy to obtain in field conditions. Several statistical modeling approaches have been developed in recent years to respond to this basic operational demand. Point and diffuse sources are often evaluated from power functions established between phosphorus concentration and water discharge. Such models do not explicitly account for in-stream processes which control P concentrations in the hydrographic network and differentiate the P export dynamics of the various forms and inputs of P. To identify the phosphorus sources and evaluate their change in response to environmental policies, we developed a simple and loaded-oriented model (POPEYE - PhOsPhorus, Evaluation of the efficiencY of Environmental policy measures) that computes retention, settling and re-suspension rates of fine and coarse P fractions and their relation to P concentration of bed sediments. The model is calibrated to a long-term database (25 years), and describes the weekly water and chemical fluxes of a tributary of Lac Leman (Lake Geneva, Venoge river watershed, 240 km(2)). It adequately predicts observed values of fine and particulate phosphorus and reflects the gradual decrease of point and diffuse inputs over the studied period due to agricultural and sewage treatment policies implemented for the control of lake pollution. (c) 2012 Elsevier B.V. All rights reserved.

Resume Une crue artificielle est realisee sur un troncon de ruisseau du bassin versant du Mercube (Haute-Savoie, France) afin de rendre compte du role du reseau hydrographique dans la transmission et l’emission de particules et de phosphore. Seul le phosphore d’origine diffuse est concerne. Cette experimentation se deroule en injectant de l’eau naturellement marquee par une teneur elevee en sulfates et chargee de suspensions. Elle met en evidence (1) les zones de stockage et de relargage des matieres en suspension dans le reseau hydrographique, (2) l’importance de l’effet piston sur le renouvellement des eaux en crue et (3) la capture du phosphore soluble au cours du transfert. L’augmentation du debit liee a l’onde de pression met en mouvement les sediments deposes au fond du lit. De ce fait, le phosphore est emis a l’exutoire d’abord lieaux particules remises en suspension, puis associea l’eau injectee. La contribution du reseau hydrographique a l’emission de phosphore n’est pas negligeable. Le meme phenomene se retrouve dans une crue reelle d’intensitecomparable.

Resume Ce papier presente les resultats d'une etude sur les transferts des nutriments (P, N) et la qualite de l'eau, menee sur un petit bassin versant (14 ha) en milieu rural, situe en bordure du Lac Leman. L'etude du bilan annuel et de la variabilite des concentrations en N et P permet d'evaluer les risques de degradation des eaux lies aux pratiques agricoles. L'analyse d'une vingtaine de crues nous a permis de degager 3 types d'evenements: (1) les crues liees surtout a une mise en charge de la nappe et caracterisees par des exportations limitees de N et de P; (2) les crues a forte composante de ruissellement qui exportent surtout de grandes quantites de P sous forme particulaire; (3) les crues a ecoulement hypodermique important qui exportent beaucoup de N mais aussi de P sous forme soluble. Les flux sortant du bassin experimental s'ecoulent vers un marecage et a l'exutoire de ce marais, on constate une nette amelioration de la qualite de l'eau par denitrification et par insolubilisation et decantation de P. La gestion et la lutte contre la pollution diffuse agricole passe ainsi par un amenagement a la fois de la zone agricole elle-meme et des differentes zones tampons le long du parcours des eaux. Le type d'amenagement n'est pas forcement le meme pour N et pour P. Dans certains cas, il peut meme apparaitre des effets contraires.