Identification et modélisation des processus à l'origine des transferts de phosphore dissous dans un bassin versant agricole

Phosphorus (P) is a controlling factor of eutrophication. Its presence in water bodies is partly due to agricultural diffuse emissions. The objective of this thesis was to identify and quantify the processes controlling diffuse P transfer, with an approach combining analysis of multi-scale observation data and modelling.Analysis of a water chemistry time series acquired at the outlet of a small agricultural catchment revealed that particulate and dissolved P forms had different spatial origin within the landscape, and were transferred through distinct mechanisms. Particulate P originates from stream bed sediments, bank erosion and occasionally from erosion on hillslopes. Dissolved P originates from riparian soils; it is transferred via subsurface flow when the water table fluctuations create a hydrological connection with the stream.Multi-site monitoring of P concentration in the soil pore water of the riparian zone evidenced that groundwater fluctuations controlled not only P transfer, but also its solubilisation. Two critical moments were identified: the summer dry period is favourable for the build-up of a pool of mobile P forms in soils, which is transferred in the autumn; when groundwater stagnates in soil in anoxic conditions, reductive dissolution of iron oxides causes a second P release. A coupled hydrological-biogeochemical model confirmed the hypotheses regarding the role of groundwater fluctuation, the soil P content and variability in soil temperature and moisture. An assessment of the information content in the data and the propagation of uncertainty enabLe phosphore (P) est un facteur de contrôle de l’eutrophisation. Sa présence dans les masses d’eau est liée en partie à des émissions diffuses agricoles. L'objectif de cette thèse a été d'identifier et de quantifier les mécanismes à l'origine des transferts de P diffus, par une démarche intégrant analyse de données d'observations multi-échelle et modélisation.L'analyse d'une chronique de chimie de l'eau à l'exutoire d'un petit bassin versant agricole a révélé que les formes particulaires et dissoutes du P avaient des origines différentes dans le paysage, et étaient transférées de manière indépendante. Le P particulaire provient des sédiments du cours d'eau, de l'érosion des berges et occasionnellement d'épisodes érosifs dans les versants. Le P dissous provient des sols de la zone riparienne; il est transféré par écoulement de subsurface lorsque les fluctuations de nappe créent une connexion hydrologique avec la rivière.Un suivi multi-site de la concentration en P dans l'eau des sols ripariens a permis de mettre en évidence que la nappe agissait non seulement sur le transfert, mais aussi sur la solubilisation du P. Deux moments importants ont été identifiés : la période de sécheresse estivale est favorable à la constitution d'un pool de P mobile dans les sols, exporté à l'automne; lorsque la nappe stagne dans les sols, la dissolution réductrice des oxydes de Fer provoque un second relargage de P. Un modèle couplé hydrologie-biogéochimie a permis de préciser le rôle de la nappe, de la teneur en P des sols et des variations d'humidité et de température. Une analyse de l

Distribution of Landscape Units Within Catchments Influences Nutrient Export Dynamics

Excess nutrients from agriculture have caused persistent eutrophication in aquatic ecosystems worldwide. Here, we present a conceptual framework for landscape management to achieve one or several water quality targets along the river continuum from headwaters to estuaries. Based on monitoring of representative headwaters and downstream reaches, we divide catchments into elementary landscape units defined by ecosystem properties and anthropogenic land use. We use a theoretical simulation to evaluate our hypothesis that the water-quality responses of redistributing these elementary units within the catchment will vary depending on the water quality targets (e.g., reduction in concentration or load). This landscape unit distribution (LUD) framework can efficiently assess the current ecohydrological functioning of a catchment and provide simple but robust predictions of its response to landscape management changes. Using simulated data, we show that different scenarios of landscape redistribution can allow attainment of one or several, but often not all desired water quality targets. Therefore, we recommend that water quality targets must be clearly defined and prioritized prior to designing landscape management strategies

Nonlinear empirical modeling to estimate phosphorus exports using continuous records of turbidity and discharge

Special section: Continuous nutrient sensing in research and management: applications and lessons learned across aquatic environments and watershedsInternational audienceWe tested an empirical modeling approach using relatively low‐cost continuous records of turbidity and discharge as proxies to estimate phosphorus (P) concentrations at a subhourly time step for estimating loads. The method takes into account nonlinearity and hysteresis effects during storm events, and hydrological conditions variability. High‐frequency records of total P and reactive P originating from four contrasting European agricultural catchments in terms of P loads were used to test the method. The models were calibrated on weekly grab sampling data combined with 10 storms surveyed subhourly per year (weekly+ survey) and then used to reconstruct P concentrations during all storm events for computing annual loads. For total P, results showed that this modeling approach allowed the estimation of annual loads with limited uncertainties (≈ −10% ± 15%), more reliable than estimations based on simple linear regressions using turbidity, based on interpolated weekly+ data without storm event reconstruction, or on discharge weighted calculations from weekly series or monthly series. For reactive P, load uncertainties based on the nonlinear model were similar to uncertainties based on storm event reconstruction using simple linear regression (≈ 20% ± 30%), and remained lower than uncertainties obtained without storm reconstruction on weekly or monthly series, but larger than uncertainties based on interpolated weekly+ data (≈ −15% ± 20%). These empirical models showed we could estimate reliable P exports from noncontinuous P time series when using continuous proxies, and this could potentially be very useful for completing time‐series data sets in high‐frequency surveys, even over extended periods

Identification and modelling of processes controlling dissolved phosphorus transfer in an agricultural catchment

Le phosphore (P) est un facteur de contrôle de l’eutrophisation. Sa présence dans les masses d’eau est liée en partie à des émissions diffuses agricoles. L'objectif de cette thèse a été d'identifier et de quantifier les mécanismes à l'origine des transferts de P diffus, par une démarche intégrant analyse de données d'observations multi-échelle et modélisation.L'analyse d'une chronique de chimie de l'eau à l'exutoire d'un petit bassin versant agricole a révélé que les formes particulaires et dissoutes du P avaient des origines différentes dans le paysage, et étaient transférées de manière indépendante. Le P particulaire provient des sédiments du cours d'eau, de l'érosion des berges et occasionnellement d'épisodes érosifs dans les versants. Le P dissous provient des sols de la zone riparienne; il est transféré par écoulement de subsurface lorsque les fluctuations de nappe créent une connexion hydrologique avec la rivière.Un suivi multi-site de la concentration en P dans l'eau des sols ripariens a permis de mettre en évidence que la nappe agissait non seulement sur le transfert, mais aussi sur la solubilisation du P. Deux moments importants ont été identifiés : la période de sécheresse estivale est favorable à la constitution d'un pool de P mobile dans les sols, exporté à l'automne; lorsque la nappe stagne dans les sols, la dissolution réductrice des oxydes de Fer provoque un second relargage de P. Un modèle couplé hydrologie-biogéochimie a permis de préciser le rôle de la nappe, de la teneur en P des sols et des variations d'humidité et de température. Une analyse de l'Phosphorus (P) is a controlling factor of eutrophication. Its presence in water bodies is partly due to agricultural diffuse emissions. The objective of this thesis was to identify and quantify the processes controlling diffuse P transfer, with an approach combining analysis of multi-scale observation data and modelling.Analysis of a water chemistry time series acquired at the outlet of a small agricultural catchment revealed that particulate and dissolved P forms had different spatial origin within the landscape, and were transferred through distinct mechanisms. Particulate P originates from stream bed sediments, bank erosion and occasionally from erosion on hillslopes. Dissolved P originates from riparian soils; it is transferred via subsurface flow when the water table fluctuations create a hydrological connection with the stream.Multi-site monitoring of P concentration in the soil pore water of the riparian zone evidenced that groundwater fluctuations controlled not only P transfer, but also its solubilisation. Two critical moments were identified: the summer dry period is favourable for the build-up of a pool of mobile P forms in soils, which is transferred in the autumn; when groundwater stagnates in soil in anoxic conditions, reductive dissolution of iron oxides causes a second P release. A coupled hydrological-biogeochemical model confirmed the hypotheses regarding the role of groundwater fluctuation, the soil P content and variability in soil temperature and moisture. An assessment of the information content in the data and the propagation of uncertainty ena

Outil d'aide à la maintenance préventive basé sur l’apprentissage automatique

International audienceLa maintenance est une fonction économique primordiale dont l’objectif est d’assurer la sûreté de fonctionnement des équipements et optimiser les coûts. Dans ce contexte l'operateur de maintenance est confronté à deux problèmes. Le premier concerne le manque de connaissances utiles pour effectuer une opération de maintenance. L'exploitation effective de ces connaissances constitue le deuxième problème. L'utilisation des systèmes experts ou systèmes à bases de connaissances (SBC) peut contribuer avantageusement à résoudre le premier problème et permet donc de rendre disponible sur les sites de production les connaissances de maintenance. En revanche, malgré l'intérêt indéniable de ces systèmes pour la maintenance, leur développement souffre toujours du manque de méthodes et techniques formelles pour l'élaboration de leur base de connaissances. L'apprentissage symbolique automatique (ASA) en se basant sur des techniques de regroupement conceptuel, de classification, d'induction et de généralisation, permet de pallier cette lacune. Ainsi notre contribution consiste à partir des deux outils précédemment cités (SBC et ASA) à réaliser un système expert d'aide à la maintenance préventive afin d'apporter une solution au problème qui demeure posé : l'exploitation des connaissances de maintenance. Ce système est destiné à évaluer la stratégie de maintenance appliquée et à orienter son évolution à partir de l'analyse des historiques d'un matériel donné

Hydroclimatic Controls on the Seasonal and Inter-Annual Variability of Dissolved Phosphorus Concentration in a Lowland Agricultural Catchment

International audienc

The influence of landscape organized heterogeneity on riverine nitrate dynamics

International audienceLandscape organized (or structured) heterogeneity is often assumed to influence hydrological and biogeochemical patterns across space and time. In this study, we quantified landscape organized heterogeneity with two indices describing the spatial configuration of nitrogen sources or sinks regarding 1) their hydrological distance to the nearest stream (i.e. upslope/downslope heterogeneity: in the lateral dimension) and 2) their hydrological distance to the outlet in the river network (i.e. upstream/downstream heterogeneity: in the longitudinal dimension). The nitrogen sources considered are agricultural fields, defined from interpretation of satellite images, and the sinks are riparian wetland, defined from a topoclimatic index. Using public nitrate concentration and discharge data from 180 catchments in western France (5-150km²), we tested whether landscape organized heterogeneity influenced riverine nitrate concentration and dynamics. The metrics computed to characterize nitrate concentration and dynamics were the flow-weighted concentration (FWNO3), the slope of the log(C)-log(Q) relationship (slope b) and the ratio of the coefficients of variation of concentration and discharge (CVratio). Results showed a high positive correlation between slope b and the CVratio, but no correlation between the later and FWNO3. 43% of the catchment exhibited a positive b slope, indicating maximum nitrate during the winter high flow period and 17% exhibited a negative b slope, indicating maximum nitrate during the summer/fall low flow period; the remaining 40% exhibited a near-zero slope. Landscape organized heterogeneity was larger in the lateral dimension for both nitrogen source and sinks than in the longitudinal dimension. In the lateral dimension, nitrogen sources were primarily located upslope and nitrate sinks downslope. In the longitudinal dimension, no general trend was observed for nitrogen sources and nitrate sinks were rather located upstream. Heterogeneity in the lateral dimension was highly variable among catchments for the smaller catchments and less variable for the larger ones. Heterogeneity in the longitudinal dimension did not exhibit a visible relationship with catchment size. No relationship was found between indices of landscape heterogeneity and FWNO3, arguably because other primary factors (such as the nitrogen surplus or runoff) control most of the regional variability in FWNO3. We found non-linear relationships between our indices of nitrogen sink organization and the b-slope or the CVratio, both in the lateral and longitudinal dimensions. The catchments with a negative b-slope (maximum nitrate during low-flow season) had their wetlands located more upstream and/or more upslope than the average. The relationship with nitrogen sources were opposite by construction (agricultural fields are often located outside wetland areas) but less clear. Further work is ongoing to explore the influence of landscape spatial organization on phosphorus concentration and dynamics

Evidence of colloids as important phosphorus carriers in natural soil/stream waters in an agricultural catchment

International audienceColloids (1‐1000 nm) are important phosphorus (P) carriers in agricultural soils. However, most studies are based on colloids from soil waters extracted in the laboratory, thus limiting the understanding of the natural transfer of colloidal P along the soil‐to‐stream continuum. Here, we conducted a field study on the colloidal P in both natural soil waters and their adjacent stream waters in an agricultural catchment (Kervidy‐Naizin, western France). Soil waters (10‐15 cm, Albeluvisol) of two riparian wetlands and the adjacent stream waters were sampled monthly during wet seasons of the 2015–2016 hydrological year (7 dates in total). Ultrafiltration at three pore sizes (5 kDa, 30 kDa and 0.45 μm) was combined with ICP‐MS to investigate variability in colloidal P concentration and its concomitant elemental composition. Results showed that colloidal P represented on average 45% and 30% of the total P (< 0.45 μm) in the soil waters and stream waters, respectively. We found that colloidal P was preferentially associated with i) organic carbon in the fine nanoparticle fraction (5‐30 kDa) and ii) Fe‐oxyhydroxides and organic carbon in the coarse colloidal fraction (30 kDa ‐ 0.45 μm). The results confirmed that colloidal P is an important component of total P in both soil waters and stream waters under field conditions, suggesting that riparian wetlands are hotspot zones for the production of colloidal P at the catchment scale which have the potential to be transported to adjacent streams. We emphasize that these findings were based on limited sampling times and further longer‐term monitoring and application of tracer or isotope methods would be necessary to better assess colloidal P variations and its transfer from soils to streams

Hydrologic controls on the export dynamics of dissolved and particulate phosphorus in a lowland, headwater agricultural catchment

International audiencePhosphorus (P) availability controls eutrophication in freshwater ecosystems, since P is generally the limiting nutrient to algal development. The contribution of diffuse P emission to surface waters is significant in intensively livestock farmed catchments as a result of high application rates of P-rich animal waste and subsequent enrichment of soils. This study investigates the transport dynamics of particulate phosphorus (PP), suspended sediments (SS), and dissolved phosphorus (DP) with the aim of elucidating the relationship between PP and DP transport mechanisms and water dynamics in lowland, headwater catchments. The selected catchment (Kervidy-Naizin catchment, France) is particularly suitable for this purpose as it benefits of a 5 years, high-frequency monitoring of PP and DP concentrations at its outlet, including data recovered both during base flow and storm periods, with the monitoring of more than 50 storm flow events. The data analysis includes interpretation of concentration-discharge relationships at the annual time scale and on an event basis, seasonal analysis of flood characteristics and empirical modeling. Annual DP and PP concentration-discharge relationships of interflood samples display a hysteretic pattern, with higher concentrations during the autumn and spring periods, and progressive decrease during winter. No hysteretic pattern is visible for interflood SS concentration, which follows a classical C=a*Q^b relationship. During floods, the dynamic of PP export is similar to that of SS during most of the events: the concentration peak occurs during the rising limb of the hydrogram (clockwise hysteresis), suggesting a source close to or within the stream. The amplitude and the hysteresis' loop size for SS and PP are a function of maximum discharge and rate of change in discharge. On the contrary, there is a strong decoupling between DP and SS (and thus PP) during most of the floods (no significant correlation), with DP concentration peaks occurring several hours after discharge (anticlockwise hysteresis). The dynamic of DP export appears in phase with the water table fluctuations measured at the bottom of the slope domains of the catchment. However, maximum DP concentrations during flood tend to be low during extended periods of soil water saturation, even though these periods corresponded to periods of high flow in the streams. These results show that the hydraulic energy of the stream controls SS and PP dynamics during floods, whilst DP dynamic is influenced by water table fluctuation. Empirical SS/PP/DP models were built considering these findings. Further investigation is currently being made to test how water table fluctuation and redox conditions could affect P availability in soils

Groundwater dynamics in wetland soils control the production andtransfer mechanisms of dissolved reactive phosphorus in an agriculturallandscape

International audienceBecause of its high sorption affinity on soils solid phase, mitigation options to reduce diffuse P transfer usuallyfocus on trapping particulate P forms delivered via surface flowpaths. Therefore, vegetated buffer zones placedbetween croplands and watercourses have been promoted worldwide, sometimes in wetland areas. To investigatethe risk of such P trapping riparian wetlands (RWs) releasing dissolved P to rivers, we monitored molybdatereactive P (MRP) in the free soil solution of two RWs in an intensively farmed catchment. Two main mechanismscausing MRP release were identified in light of the geochemical and hydrological conditions in the RWs, controlledby groundwater dynamics. First, soil rewetting after the dry summer was associated with the presence of a poolof mobile P, limited in size. Its mobilization started under conditions of water saturation caused by groundwateruprise in RWorgano-mineral soil horizons. Second, the establishment of anoxic conditions in the end of the wintercaused reductive solubilization of Fe oxide-hydroxide, along with release of P. Comparison between sites revealedthat the first MRP release occurred only in a RW with P enriched soils, whereas the second was recorded even ina RW with a low soil P status. Seasonal variations in MRP concentrations in the stream were synchronized withthose in RW soils. Hence, enriched and/or periodically anoxic RWs can act as a key component of the P transfercontinuum in agricultural landscapes by converting particulate P from croplands into MRP released to rivers