Variabilité spatiale de la teneur en eau de surface des sols nus par mesures in situ et imagerie radar

National audienceOn présente l'analyse géostatistique de la teneur en eau de surface (0-6 cm de profondeur) collectée les 12 et 13 Mars 2009, sur une quinzaine de parcelles de sol nu d'un petit bassin péri-urbain proche de Lyon. Les mesures in situ, ont été collectées à deux échelles : une échelle locale sur des croix de longueur 20m et un pas d'espace de 1m et une échelle parcellaire sur 3 transects avec un pas de 20m environ. Les résultats montrent une corrélation de quelques m à échelle fine et de 20 à 50m à l'échelle de la parcelle. Après correction du bruit, calibration radiométrique et correction des effets géométriques et de pente, la comparaison des moyennes par parcelles issues de l'image radar TerraSAR-X et des mesures in situ est satisfaisante (R2=0.43) mais l'analyse intra-parcellaire reste à affiner. / This paper presents the geostatistical analysis of surface soil water content (0-6 cm depth), collected on March 12-13 2009, in about 15 bare soil fields located in a small suburban catchment close to Lyon. In situ data were sampled at two scales : a local scale on 20m-long crosses with a space step of about 1m; a field scale, with 3 transects and a space scale of about 20m. The results show a correlation of a few meters at the local scale and of about 20-50m at the field scale. After correction of the noise, radiometric calibration, geometric and slope effect correction, the comparison of the field averages derived from the TerraSAR-X image and of in situ data is satisfactory (R2=0.43), but the intra-field variability should be studied in more details

Three very high resolution optical images for land use mapping of a suburban catchment: input to distributed hydrological models

International audienceUrbanization and other modifications of land use affect the hydrological cycle of suburban catchments. In order to quantify these impacts, the AVuPUR project (Assessing the Vulnerability of Peri-Urban Rivers) is currently developing a distributed hydrological model that includes anthropogenic features. The case study is the Yzeron catchment (150 km2), located close to Lyon city, France. This catchment experiences a growing of urbanization and a modification of traditional land use since the middle of the 20th century, resulting in an increase of flooding, water pollution and river banks erosion. This contribution discusses the potentials of automated data processing techniques on three different VHR images, in order to produce appropriate and detailed land cover data for the models. Of particular interest is the identification of impermeable surfaces (buildings, roads, and parking places) and permeable surfaces (forest areas, agricultural fields, gardens, trees. . . ) within the catchment, because their infiltration capacity and their impact on runoff generation are different. Three aerial and spatial images were acquired: (1) BD Ortho IGN aerial images, 0.50 m resolution, visible bands, may 5th 2008; (2) QuickBird satellite image, 2.44 m resolution, visible and near-infrared bands, august 29th 2008; (3) Spot satellite image, 2.50 m resolution, visible and near-infrared bands, September 22nd 2008. From these images, we developed three image processing methods: (1) a pixel-based method associated to a segmentation using Matlab®, (2) a pixel-based method using ENVI®, (3) an object-based classification using Definiens®. We extracted six land cover types from the BD Ortho IGN (visible bands) and height classes from the satellite images (visible and near infrared bands). The three classified images are resampled in the same low resolution of 2.5 m and compared in order to evaluate the accuracy of different image processing methods and to determine for each cover type, the more appropriate image and/or method. This comparison provides hydrologists with a synthetic land cover map. Four parameters affect the accuracy of land cover mapping: firstly the addition of the NIR band improves vegetation classification such as the distinction between coniferous forest and broad-leaved forest. Moreover the intensity of chlorophyllian activity allows us to characterize the use of agricultural fields. Secondly, the images were taken at three dates in the agricultural calendar. This multi-date data allows the discrimination between permanently vegetalized pastures, and temporarily bare crops, a useful information for hydrologists who study surfaces hydraulic properties. Thirdly, the high resolution of the BD Ortho IGN image emphasizes the heterogeneity inside the spatial entities. Thus, in urbanised areas, high-resolution imagery allows the precise identification of objects > 5 m2 and consequently the quantification of impervious and pervious surfaces. However, the continuity of forest areas is not maintained because of the presence of small entities with sparser tree cover that were classified as herbaceous areas. Finally, image characteristics are more crucial than classification methods for the accuracy of land cover mapping. However, object based approach improves the classification of mixed pixels on the edge between different objects. It's particularly true for buildings and roads

The AVuPUR project (Assessing the Vulnerabiliy of Peri-Urbans Rivers): experimental set up, modelling strategy and first results

International audienceLe projet AVuPUR a pour objectif de progresser sur la compréhension et la modélisation des flux d'eau dans les bassins versants péri-urbains. Il s'agit plus particulièrement de fournir des outils permettant de quantifier l'impact d'objets anthropiques tels que zones urbaines, routes, fossés sur les régimes hydrologiques des cours d'eau dans ces bassins. Cet article présente la stratégie expérimentale et de collecte de données mise en ½uvre dans le projet et les pistes proposées pour l'amélioration des outils de modélisation existants et le développement d'outils novateurs. Enfin, nous présentons comment ces outils seront utilisés pour simuler et quantifier l'impact des modifications d'occupation des sols et/ou du climat sur les régimes hydrologiques des bassins étudiés. / The aim of the AVuPUR project is to enhance our understanding and modelling capacity of water fluxes within suburban watersheds. In particular, the objective is to deliver tools allowing to quantify the impact of anthropogenic elements such as urban areas, roads, ditches on the hydrological regime of suburban rivers. This paper presents the observation and data collection strategy set up by the project, and the directions for improving existing modelling tools or proposing innovative ones. Finally, we present how these tools will be used to simulate and quantify the impact of land use and climate changes on the hydrological regimes of the studied catchments

Compréhension et modélisation des processus hydrologiques dans les petits bassins versants péri-urbains à l'aide d'une approche distribuée orientée objet et modulaire. Application aux sous-bassins de la Chaudanne et du Mercier (bassin versant de l'Yzeron, France)

Urban expansion mainly affects peri-urban areas. These areas are subject to rapid modifications such as an increase of impervious areas or concentration of runoff in sewer systems. These changes have an impact on local hydrology and can induce floods, pollution or decrease of groundwater resource. Modelling tools allowing a quantification of the sensitivity of peri-urban catchments to urbanization are therefore useful in this context. The hypothesis underlying this PhD is that a continuous distributed hydrological model, taking explicitly into account the spatial organization of the landscape (urban, agricultural, forest areas, hedges,..) and the water pathways, as determined by topography but also roads and sewer networks, can help to understand and hierarchize the role of various landscape elements on the hydrological response of small hydrosystems. We therefore designed the Peri-Urban Model for landscape MAnagement (PUMMA) simulating the rainfall-runoff processes both in urban and in rural areas. For this, the urban model URBS was integrated into the LIQUID modelling framework already containing modules describing hydrological processes in rural areas. Additionally, three process modules were developed describing sewer overflow devices, overland flow as well as retention basins and lakes. PUMMA follows an object-oriented approach. The landscape is discretized into cadastral parcels in urban areas and irregular hydrological response units in rural areas. In order to apply PUMMA to the catchment scale, automatic methods were developed for the pre-processing of the geographical data. Furthermore, a method for the delineation of suburban catchments including the separation into dry and wet weather contributing areas was developed. The model was then applied to the Chaudanne catchment, a sub-basin of the Yzeron, located in the peri-urban area of Lyon, France. The model was run continuously for two contrasting years (dry and humid) using parameters values taken from observations and the literature. Although summer peak discharge is often overestimated, the results show that, the model is able to simulate realistically the observed discharges and in particular different responses under dry and wet conditions, controlled by the soil saturation. Sensitivity tests to various processes/parameters showed the importance of the urban influenced processes on the hydrological response, in particular surface runoff generation on impervious and natural urban surfaces, infiltration into the sewer system and the connexion of urban areas to the natural hydrographic network. Soil depth and lateral saturated hydraulic conductivity were also found influential on the base flow dynamics. We finally showed the model potential for the evaluation of various rain water management scenarios.La densification actuelle de l’urbanisation conduit à un changement d’occupation du sol et du réseau de drainage en zone péri-urbaine. L’écoulement est concentré dans des fossés ou des réseaux d’assainissement et ainsi accéléré. Ces phénomènes peuvent avoir des conséquences importantes pour les nappes et les cours d’eau, comme par exemple l’aggravation des crues et des sécheresses et l’altération de la qualité chimique et/ou biologique du milieu. Sous la pression réglementaire (Directive Cadre Européenne sur l’Eau) et la demande sociale, les gestionnaires sont donc confrontés à des choix complexes en terme d’aménagement. Ainsi, il est nécessaire de mettre au point des méthodes et des modèles capables de quantifier l’impact de l’augmentation de l’urbanisation sur la vulnérabilité des hydro-systèmes péri-urbains. Dans ce travail, nous formulons l’hypothèse qu’une modélisation hydrologique spatialisée continue, prenant explicitement en compte les objets des paysages périurbains (parcelles urbaines, agricoles, forestières, haies,..) et les éléments déterminant les chemins de l’eau (topographie, mais aussi réseaux d’assainissement ou de routes) peut aider à comprendre et hiérarchiser le rôle des différents objets du paysage sur la réponse hydrologique. Pour ce faire, le modèle distribué PUMMA (Peri-Urban Model for landscape Management) adapté aux bassins versants péri-urbains, a été développé dans le cadre de cette thèse. Il consiste en une intégration du modèle existant URBS, décrivant des processus hydrologiques urbains à l’échelle d’une parcelle cadastrale, dans la plate-forme de modélisation LIQUID, qui contient déjà des modules représentants des processus hydrologiques en zone rurale. PUMMA a également été complété par de nouveaux modules simulant les déversoirs d’orage, les bassins de rétention et le transfert du ruissellement de surface. Le modèle suit une approche orientée objet dans laquelle le paysage est divisé en mailles irrégualières, correspondant aux parcelles cadastrales en zone urbaine et aux unités de réponse hydrologiques (HRUs) en zone rurale. Afin de pouvoir appliquer le modèle à l’échelle d’un bassin versant, des méthodes automatiques pour la préparation des données géographiques ont été mises au point. De plus, une méthode a été développée pour déterminer les contours de bassins versants péri-urbains, en distinguant les surfaces contributives de temps sec et humide. Le modèle a été appliqué au bassin versant de la Chaudanne, un sous-bassin de l’Yzeron, situé en zone péri-urbaine lyonnaise pour deux années en conditions contrastées (sèche et humide). Les paramètres du modèle ont été spécifiés à partir des observations disponibles et des données de la littérature. Les résultats montrent un comportement du modèle réaliste et une aptitude à représenter les comportements différents en période sèche et humide, en lien avec des degrés de saturation des sols différents, même si les pics de débits d’été sont en général surestimés. Différents tests de sensibilité sur certains processus/paramètres montrent l’importance des processus urbains sur la réponse hydrologique du bassin, comme en particulier la génération de ruissellement de surface par les surfaces imperméables et naturelles urbaines, le drainage de l’eau du sol par les réseaux d’assainissement et les connexions entre les îlots urbains et le réseau hydrographique naturel et artificiel. L’épaisseur des sols et la conductivité hydraulique à saturation latérale jouent aussi un rôle important sur la dynamique du débit de base. Nous montrons aussi le potentiel du modèle pour tester différents scenarii d’aménagement ou de gestion des eaux pluviales

Land cover mapping using aerial and VHR satellite images for distributed hydrological modelling of periurban catchments: Application to the Yzeron catchment (Lyon, France)

International audienceThe rapid progression of urbanization in periurban areas affects the hydrological cycle of periurban rivers. To quantify these changes, distributed hydrological modelling tools able to simulate the hydrology of peri-urban catchments are being developed. Land cover information is one of the data sources used to define the model mesh and parameters. The land cover in periurban catchments is characterized by a very large het-erogeneity, where the vegetated and the artificial surfaces are finely overlapping. The study is conducted in the Yzeron catchment (150 km 2), close to the city of Lyon, France. We explore the potential of very high-resolution (VHR) optical images (0.50–2.50 m) for retrieving information useful for those distributed hydrological models at two scales. For detailed object-oriented models, applicable to catchments of a few km 2 , where hydrological units are based on the cadastral units, manual digitizing based on the 0.5 m resolution image, was found to be the most accurate to provide the required information. For larger catchments of about 100 km 2 , three semi-automated mapping procedures (pixel based and object-oriented classifications), applied to aerial images (BD-Ortho Ò IGN), and two satellite images (Quickbird and Spot 5) were compared. We showed that each image/processing provided some interesting and accurate information about some of the land cover classes. We proposed to combine them into a synthesis map, taking profit of the strength of each image/processing in identifying the land cover classes and their physical properties. This synthesis map was shown to be more accurate than each map separately. We illustrate the interest of the derived maps in terms of distributed hydrological modelling. The maps were used to propose a classification of the Yzeron sub-catchments in terms of dominant vegetation cover and imperviousness. We showed that according to the image processing and images characteristics, the calculated imperviousness rates were different. This can lead to significant differences in the hydrological response

Avancées dans la modélisation hydrologique intégrée avec la plate-forme LIQUID

International audienceEnvironmental modelling frameworks are valuable and increasingly used tools for building customized models, in a context where the complexity of management issues and the availability of data require much flexibility. The LIQUID framework has been developed since 2005. It is mostly dedicated to hydrological modelling. It aims at easily integrating hydrological processes while preserving their characteristic temporal and spatial scales. LIQUID allows the user to build and run integrated models on the basis of reusable and exchangeable modules. It provides templates for easy development of new modules, connections to databases and GIS for data input and output, and module coupling mechanisms, that synchronize different time steps and handle irregular geometries. LIQUID suits a wide range of applications, involving various spatial scales and process conceptualisations. The framework is also able to simulate complex interactions between modules, in particular including feedback. The paper will present the recent advances of LIQUID , in terms of concepts as well as in terms of technical specifications, and a brief overview of the ongoing main applications. Those deal with the assessment of landscape management impact on hydrology in agricultural and suburban areas, and with the analysis of hydrological responses in the context of flash floods

Artificial Wetland and Forest Buffer Zone: Hydraulic and Tracer Characterization

As part of a European LIFE ArtWET project, two on-site buffer zones, an artificial wetland and a forest plot, are being evaluated for their capacity to mitigate pesticide pollution. As treatment efficiency is highly dependent on the systems' hydrology, the present work focuses on the watershed and both systems' hydrological functioning. The design strategy involved limited inlet flow rates to 70 L s-1: 99% of watershed outlet flow rates were lower than this limit. Approximately half of the flows of greatest concern passed through the artificial wetland, whereas the forest only received 2% of these flows. A tracer experiment was conducted under a low steady flow rate while little vegetation was present in the artificial wetland. A water dye tracer (sulforhodamine B, SB) and two molecules of contrasting properties, uranine (Ur, photodegrading) and isoproturon (mobile and only slightly sorptive, IPU) were injected. Dilution, sorption, and photodegradation were observed. The forest plot, which presented a high organic matter content, showed more sorption (IPU, SB) but lower photodecay (Ur) than did the artificial wetland. Total IPU losses in the forest buffer were high (79%). In the artificial wetland, 30% IPU losses were found, whereas a 66.5-h mean retention time was determined and good hydraulic efficiency (0.55) was calculated. Few dead zones and short-circuits were found, suggesting good hydrological functioning. Implementing buffer zones in subsurface pipe-drained watersheds actively participates in the reduction of pesticide transfer to natural water bodies

Utilisation de logiciels SIG open-source pour le pré-traitement de modèles hydrologiques spatialisés

International audienceIn distributed hydrological models, elementary modelling units are often derived from raster and vector geographical maps such as digital elevation models (DEM), land use and soil properties maps. Model pre-processing can be defined as the operations transforming raw GIS data into the adequate modelling units. This paper describes our experience with open-source GIS for these pre-processing operations through a set of examples. The softwares tested were PostGIS, OpenJump, OrbisGIS, GRASS, SAGA, MapWindow and QuantumGIS. The pre-processing operations dealt with DEM processing, topology corrections, and advanced editing of vector layers. None of the tested software provided a ready to use solution. However GRASS GIS appeared as the most promising candidate for performing all the preprocessing operations and automating them. Although sometimes tricky to handle by GIS non-expert users, open-source GIS allow easy tests and comparisons and therefore are well suited to research purposes

Cartographie de l'occupation du sol pour la modélisation hydrologique spatialisée du cycle de l'eau en zone péri-urbaine

[Departement_IRSTEA]Eaux [TR1_IRSTEA]ARCEAUNational audiencePeri-urban basins, face a quick land-use change, increasing pollution and flood risks. Taking into account the surface heterogeneity (mixture of rural and urbanised areas), and also the natural and artificial water pathways in a continuous and long term modelling of the hydrological cycle in peri-urban areas, is one of the focus of the AVuPUR (Assessing the Vulnerability of Peri-Urban Rivers) project. This requires, among others, a land cover mapping at various scales. The latter is performed by using aerial photography and very high resolution satellite imagery. Manual interpretation and semi automated methods are developed on both images sources and compared. Constraints due to mapping data integration in distributed hydrological models are analyzed.Les bassins-versants péri-urbains soumis à un changement rapide de l'occupation du sol, sont exposés à l'accroissement des risques de pollution et d'inondations. La prise en compte de l'hétérogénéité de la surface (coexistence de zones rurales et urbanisées) et des chemins de l'eau naturels et artificiels, dans une simulation en continu sur quelques dizaines d'années de l'hydrologie de ces zones, est l'une des questions-clé du projet AVuPUR (Assessing the Vulnerability of Peri-Urban Rivers). Ceci requiert entre autres, une cartographie de l'occupation du sol qui est menée à différentes échelles à l'aide d'images aériennes et spatiales à très haute résolution. Des méthodes manuelles et semi-automatiques d'extraction sont développées sur ces différents types d'images. Les contraintes liées à l'introduction de ces données dans des modèles hydrologiques spatialisés sont analysées

Computer-assisted mesh generation based on hydrological response units for distributed hydrological modeling

Artículo de publicación ISIDistributed hydrological models rely on a spatial discretization composed of homogeneous units representing different areas within the catchment. Hydrological Response Units (HRUs) typically form the basis of such a discretization. HRUs are generally obtained by intersecting raster or vector layers of land uses, soil types, geology and sub-catchments. Polylines maps representing ditches and river drainage networks can also be used. However this overlapping may result in a mesh with numerical and topological problems not highly representative of the terrain. Thus, a pre-processing is needed to improve the mesh in order to avoid negative effects on the performance of the hydrological model. This paper proposes computer-assisted mesh generation tools to obtain a more regular and physically meaningful mesh of HRUs suitable for hydrologic modeling. We combined existing tools with newly developed scripts implemented in GRASS GIS. The developed scripts address the following problems: (1) high heterogeneity in Digital Elevation Model derived properties within the HRUs, (2) correction of concave polygons or polygons with holes inside, (3) segmentation of very large polygons, and (4) bad estimations of units’ perimeter and distances among them. The improvement process was applied and tested using two small catchments in France. The improvement of the spatial discretization was further assessed by comparing the representation and arrangement of overland flow paths in the original and improved meshes. Overall, a more realistic physical representation was obtained with the improved meshes, which should enhance the computation of surface and sub-surface flows in a hydrologic model