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International audience; Le Laboratoire d’Analyses en Soutien aux Exploitants (LASE) au CEA Saclay est chargé de la caractérisation d’effluents et déchets nucléaires des producteurs de déchets du CEA de Saclay, notamment des analyses chimiques et radiologiques. Dans ce cadre, les paramètres physico-chimiques et radiologiques sont analysés avec divers appareils analytiques du laboratoire sur des matrices variées composant les déchets nucléaires telles que des effluents, terres, résines, métaux, huiles et bétons.Du fait de la diversité des échantillons et des différents paramètres à analyser, les circuits d’intercomparaisons permettent de valider nos méthodes, qualifier nos appareils, habiliter le personnel et surtout déterminer l’exactitude des mesures en intégrant les différentes étapes du processus analytique, de la préparation des échantillons jusqu’à l’analyse. Les organisateurs d’intercomparaisons remettent des rapports permettant de situer les performances du LASE par rapport aux valeurs de référence ou de consensus et à celles des autres laboratoires participants. Les résultats du LASE sont analysés en termes de z-scores et d’écarts relatifs par rapport aux valeurs de référence de manière à vérifier la justesse des méthodes et les incertitudes associées.Depuis plus de 10 ans, le LASE participe régulièrement à des circuits d’intercomparaisons organisés par des organismes agréés pour différentes matrices dont l’ensemble balaie la majeure partie des techniques analytiques du laboratoire (environ 20 par an). Ces circuits sont principalement réalisés par des organismes français (AGLAE, BIPEA, CETAMA, LNHB) mais également internationaux (NPL, LGC), ou encore lors de projets européens (INSIDER, NKS).Le retour d’expériences important des circuits d’intercomparaisons auxquels le laboratoire participe, nous permet d’améliorer nos méthodes analytiques mais également de prendre des mesures spécifiques en cas d’éventuels problèmes détectés.

Forests provide a number of ecological and hydrological services, for instance, contributing to decreased water and sediment yields through increased infiltration and reduced soil erosion. However, forest fires can turn positive forest services into drawbacks, enhancing surface runoff and soil erosion and damaging both hillslopes and downstream aquatic life in rivers. Therefore, appropriate mitigation strategies should be developed to limit these negative effects. Using a runoff and erosion model (the WaterSed model), we proposed forest fire and firebreak scenarios to analyse their respective effects on sediment loads. The model reproduced the measured discharge and sediment loads over an entire hydrological year, including 21 flood events occurring from November 2010 to May 2011 in a 72-km2 Mediterranean catchment (Celone catchment, Puglia, Italy). Eight different forest fire scenarios were then proposed. While the mean burnt areas remained below 2% of the total catchment area, forest fires significantly affected the sediment yield. Indeed, the sediment yield increased over the different forest fire scenarios, from 1.97 to 2.70 t ha−1.yr−1, corresponding to a 37% increase. At the flood-event scale, the sediment load after fire represented up to 324% of the unburnt catchment sediment load in the worst-case scenario. By using realistic firebreaks, the sediment load could be dramatically reduced, from 324% to 165%, in the worst-case scenario. Because rural catchments, such as the Celone catchment, are currently experiencing land abandonment, forested areas are expected to replace crops and expand in the future. This change will likely increase forest ecological services, which may, however, be punctually balanced by negative fire effects. More studies addressing the global impacts of forest growth, fires and firebreaks on sediment transfers are therefore needed in similar environments.

International audience; Runoff events are usually very concentrated in both space and time, and are therefore challenging to monitor. They result from the interactions of rainfall characteristics with both soil surface and intrinsic characteristics and catchment morphology. To improve our understanding of this complex process, five observatories were installed to monitor the runoff response of eleven catchments to natural rainfall events. Measurements were performed with a high frequency (≤ 15 min-time step) and over long time periods (1.5 to 11 years). 1640 rainfall-runoff events were extracted from rainfall and discharge time series. Statistical analyses were performed in order to determine significant triggering factors. The rainfall-runoff response of each catchment is highly variable, but some common features were identified across contrasted catchments. Under highly variable soil types, the catchment runoff exhibited contrasted patterns. Soil surface state, and land use were demonstrated to highly affect runoff. The scattering in the rainfall-runoff relationship demonstrated the key role of the initial soil moisture and rainfall characteristics. Lessons learnt from previous studies in these various environments, as well as their consequences on runoff understanding and modelling are discussed.; Le ruissellement est un phénomène épisodique et localisé, donc difficile à mesurer. Il résulte de l'interaction de nombreux facteurs, dont les caractéristiques intrinsèques et de surface du sol, la morphologie du bassin versant, mais également les caractéristiques pluviométriques. Afin d'améliorer la compréhension de la genèse et de la propagation du ruissellement au sein de bassins versants de tête, cinq observatoires du ruissellement sont présentés, incluant 11 bassins versants. Pour chacun d'entre eux, la pluie et le ruissellement sont mesurés sur le terrain à haute fréquence (≤ 15 min) pendant des durées longues (1,5 à 11 ans). 1640 évènements pluvio-ruisselants sont extraits de ces chroniques en continu et statistiquement analysés, afin de cerner les facteurs influant significativement le ruissellement. La réponse ruisselante de chaque bassin versant est extrêmement complexe mais permet d'identifier des comportements communs. L'importance des caractéristiques des sols est montrée, ainsi que celle de leur occupation, de leurs états de surface et état hydrique. La dispersion de la relation pluie-débit observée au sein de chacun des observatoires permet par ailleurs de montrer et de quantifier l'impact des caractéristiques pluviométriques sur la lame d'eau ruisselée. Les leçons tirées de chacun des observatoires et leurs conséquences sur la compréhension et la modélisation du ruissellement dans différents contextes sont discutées.

International audience

International audience; Soil erosion is recognized as one of the main processes triggering the physical and chemical degradation of water bodies. Although erosive dynamics have been widely described in morphogenic areas, much less is known about this process in lowland agricultural catchments.To address this issue, a multidisciplinary study was carried out at the scale of a small agricultural lowland catchment (Louroux catchment, France, 25 km (2)) to reconstruct the evolution of the soil erosion rates and sources, from the hillslopes to the deposition areas in the water bodies. This study's goal is specifically to understand the impact of changes in land use and agricultural practices after 1950 (field re-design, ditch creation, drainage) on sediment dynamics For this purpose, sediment cores were collected in a medieval pond (870AD) located at the outlet of the catchment. These cores have been dated and used to reconstruct the temporal variations in sediment fluxes over time (from 1954 to 2013). In addition, sediment sources were identified using fallout radionuclide measurements (cesium-137 and beryllium7 / excess lead-210 ratios). These results will contribute to the implementation of effective management measures to reduce the transfer of sediments from the hillslopes to the hydrosystems.; 14 15 L'érosion des sols est reconnue comme étant l'un des principaux processus à l'origine de la dégradation physico-16 chimique des masses d'eau. Les phénomènes érosifs, largement décrits en contexte morphogène, restent pourtant 17 peu étudiés en contexte de plaine. 18 Pour combler ce manque, une étude pluridisciplinaire a été menée à l'échelle d'un petit bassin versant (25 km²) 19 de plaine agricole drainée pour reconstruire l'évolution des taux et des sources d'érosion, depuis les zones 20 d'accumulation de matière jusqu'aux zones sources. Cette étude vise ainsi à comprendre l'impact des 21 aménagements intervenus dans le bassin après 1950 (remembrements, création de fossés, drainage) sur la 22 dynamique sédimentaire actuelle et passée. 23 À cette fin, des carottages ont été réalisés dans un étang médiéval (870AD) localisé à l'exutoire du bassin versant 24 du Louroux (France). Les carottes ont été datées et ont permis de reconstruire les variations temporelles du flux 25 sédimentaire (de 1954 à 2013), mais aussi de les associer avec les changements d'usage des sols. Par ailleurs, les 26 sources de sédiments ont été identifiées à partir de mesures de radionucléides (césium 137 et rapport béryllium 27 7/plomb 210 en excès). Ces résultats vont permettre de guider la mise en place d'aménagements pour lutter contre 28 le départ de matière sur les versants. 29 MOTS CLEFS : érosion, sédiment, colmatage, archive sédimentaire, traçage 30 31 Increase of erosion source contributions to rivers and lakes (1950-2010): 32 the case of the Louroux Pond (Central France) 33 Soil erosion is recognized as one of the main processes triggering the physical and chemical degradation of water 34 bodies. Although erosive dynamics have been widely described in morphogenic areas, much less is known about 35 this process in lowland agricultural catchments. 36 To address this issue, a multidisciplinary study was carried out at the scale of a small agricultural lowland 37 catchment (Louroux catchment, France, 25 km²) to reconstruct the evolution of the soil erosion rates and sources, 38 from the hillslopes to the deposition areas in the water bodies. This study's goal is specifically to understand the 39 impact of changes in land use and agricultural practices after 1950 (field redesign , ditch creation, drainage) on 40 sediment dynamics 41 For this purpose, sediment cores were collected in a medieval pond (870AD) located at the outlet of the 42 catchment. These cores have been dated and used to reconstruct the temporal variations in sediment fluxes over 43 time (from 1954 to 2013). In addition, sediment sources were identified using fallout radionuclide measurements 44 (cesium-137 and beryllium7 / excess lead-210 ratios). These results will contribute to the implementation of 45 effective management measures to reduce the transfer of sediments from the hillslopes to the hydrosystems. 46 KEY WORDS: erosion, sediment, siltation, lake deposits, sediment fingerprinting 47 48 49

International audience; In this paper, we analyse how the performance and calibration of a distributed event-based soil erosion model at the hillslopescale is affected by different simplifications on the parameterizations used to compute the production of suspended sediment byrainfall and runoff. Six modelling scenarios of different complexity are used to evaluate the temporal variability of thesedimentograph at the outlet of a 60 m long cultivated hillslope. The six scenarios are calibrated within the generalized likelihooduncertainty estimation framework in order to account for parameter uncertainty, and their performance is evaluated againstexperimental data registered during five storm events. The Nash–Sutcliffe efficiency, percent bias and coverage performanceratios show that the sedimentary response of the hillslope in terms of mass flux of eroded soil can be efficiently captured by amodel structure including only two soil erodibility parameters, which control the rainfall and runoff production of suspendedsediment. Increasing the number of parameters makes the calibration process more complex without increasing in a noticeablemanner the predictive capability of the model.

The suspended particles of catchment networks are dependent on both river and hillslope erosion processes. During this thesis, the particle size dynamics was studied along this continuum in order to improve the understanding of particle delivery from hillslopes to the outlets of headwater catchments. Field measurements were conducted at the headwater catchment scale (~20 km²). The discharge displayed a positive correlation with the particle size. An original measurement protocol has been set up and it demonstrated that particles were mostly aggregated. The inputs from hillslopes were possibly involved in some of the variations of the measured particle size. Laboratory experiments carried out using an annular flume demonstrated that a part of these variations could be explained by disaggregation or flocculation within the flow. Important variations due to the soil type were observed. However, they were less pronounced in the falling limbs of the schematic flood events, suggesting that flow conditions progressively became more important than the soil signature. The latter encouraged the analysis of hillslope processes, among which a special attention was given to the rainfall effects. Rainfall simulation experiments (~1 m²) demonstrated for two soils that an increase in the rainfall kinetic energy resulted in smallest aggregates detached from the soil matrix. The importance of this mecanism at the hillslope scale (~ 100 m²) with regard to runoff selectivity was demonstrated developing a size-dependent detachment parametrisation included in two physically based numerical models. Finally, the effects of the rainfall kinetic energy on the particle size were observed during field measurements made at the plot scale as well, underlining the need to adequatly describe the rainfall forcing field at this scale., Les particules en suspension transportées dans les réseaux hydrographiques résultent des processus de rivière et des apports depuis les versants. Nous avons étudié dans cette thèse la dynamique des tailles de particules le long du continuum versant-rivière afin d'apporter des éléments de réponse à la réflexion aujourd'hui menée sur les distances de transport et sur le concept de connectivité sédimentaire. Des observations de terrain sont menées à l'exutoire d'un bassin versant de tête (~20 km²). Elles mettent en évidence une corrélation positive entre débit liquide et taille des particules. L'établissement et la mise en oeuvre d'un protocole de mesure original montre que les particules sont agrégées. A cette échelle, les apports des versants semblent importants pour expliquer les variations de taille des particules. Des expériences de laboratoire utilisant un canal annulaire sont menées et indiquent qu'une partie de ces variations peut être attribuée à la désagrégation ou à la floculation des particules dans l'écoulement. Des variations de taille notables sont dues au type de sol. Elles sont moindres à la fin des évènements schématiques simulés en canal, suggérant que l'écoulement prend une part prépondérante pour expliquer les variations de taille des particules. Cet effet du type de sol a motivé l'étude des processus de versant, et en particulier ceux de la pluie. Des expériences de simulations de pluie menées en laboratoire (~1 m²) sur deux sols révèlent que l'augmentation de l'énergie cinétique de la pluie a tendance à générer des agrégats plus fins. Une paramétrisation du détachement par la pluie par fraction granulométrique est développée sur la base de ces expériences et implémentée au sein de deux modèles numériques d'érosion hydrique à base physique. Les simulations numériques confirment que cette tendance a des impacts sur les exports à l'échelle du versant. Enfin, des variations de granulométrie en lien avec l'énergie cinétique de la pluie sont perceptibles lors d'observations de terrain à l'échelle du versant (~ 100 m²), confirmant l'importance d'une description correcte du forçage pluviométrique.

Connectivity has been embraced by the geosciences community as a useful concept to understand and describe hydrological functioning and sediment movement through catchments. Mathematical modelling has been used for decades to quantify and predict erosion and transport of sediments, e.g. in scenarios of land use change or conservation measures. Being intrigued by both models and the connectivity concept, as a group of modellers we aimed at investigating what different models could tell us about connectivity. Therefore, we evaluated the response of contrasted spatially-distributed models to landscape connectivity features and explained the differences based on different model structures. A total of 53 scenarios were built with varying field sizes and orientations, as well as the implementation of soil conservation measures. These scenarios were simulated, for two rainfall intensities, with five event- and process-based water and soil erosion models – EROSION3D, FullSWOF_2D, LandSoil, OpenLISEM and Watersed. Results showed that rainfall amount plays the most important role in determining relative export and connected area of runoff and sediment in all models, indicating that functional aspects of connectivity were more important than structural connectivity. As for the role of structural landscape elements, there was no overall agreement between models regarding the effects of field sizes, crop allocation pattern, and conservation practices; agreement was also low on the spatial patterns of connectivity. This overall disagreement between models was unexpected. The results of this exercise suggest that the correct parameterization of runoff and sediment production and of routing patterns may be an important issue. Thus, incorporating connectivity functions based on routing would help modelling forward. Our results also suggest that structural connectivity indices may not suffice to represent connectivity in this type of catchment (relatively simple and monotonous land cover), and functional connectivity indices should be applied., Geomorphology, 367, ISSN:0169-555x, ISSN:1872-695X