Structures spatiales en hydrographie continentale

La compréhension de l'effet des hétérogénéités spatio-temporelles de surface sur la réponse hydrologique et l'adaptation par l'homme de ces hétérogénéités représentent un des enjeux majeurs de l'hydrologie tout en dépassant le cadre de la seule recherche académique. Les recherches en géographie quantitative que j'ai réalisées au cours de ces dix dernières années au sein de l'ENGREF puis d'AgroParisTech s'inscrivent dans cette problématique générale et se sont focalisées sur les hétérogénéités spatiales des supports de transfert des flux hydriques de surface concentrés : les réseaux hydrographiques au sens large (cours-d'eau, plans d'eau, ravines, etc). Les approches développées relèvent de modèles de représentation d'hétérogénéités spatiales sur réseau qui combinent des approches métrologiques (télédétection) et de modélisation spatiale stochastique (géostatistique), pour combler les données manquantes et estimer les incertitudes de représentation. Ainsi, comme je tenterai de l'illustrer, ces modèles de représentation spatiale stochastiques contribuent soit à mesurer l'effet de la précision de la représentation des réseaux sur la réponse hydrologique (finalité méthodologique) soit à explorer numériquement l'effet des hétérogénéités spatiales de réseaux sur la réponse hydrologique ainsi que leurs lois d'échelle (finalités cognitives et prédictives). Parce que les capacités d’adaptation des hétérogénéités spatio-temporelles de ces réseaux dans les paysages cultivés sont un enjeu fort agro-environnemental, j'envisage pour mon projet d'étendre ces modèles à la dynamique des propriétés des réseaux hydrographiques des agrosystèmes

Cheminement des eaux superficielles et télédétection pour la modélisation hydrologique distribuée

Communication invitée lors du 13. International Symposium on Spatial Data Handling: 2008; Montpellier, France: 2008/07/23-25Les cheminements superficiels des eaux sont de mieux en mieux décrits à partir des informations spatiales. En particulier les MNT à très haute résolution spatiale apportent des données nouvelles qui autorisent une description fine des réseaux d'écoulement superficiel et donc l'intégration de ces structures dans les modélisations hydrologiques distribuées. Mais si les techniques numériques d'observation ont fait de grands progrès dans les années passées, par contre il n'en est pas de même pour les concepts en modélisation et les expérimentations de terrain, ce qui provoque un point de blocage pour une réelle utilisation des données de télédétection finement résolues dans les modèles. Le développement de nouvelles techniques de mesure sur le terrain, de type low cost et réseaux communicants, est porteur de renouveau dans ce domaine. / Water paths are better and better described through spatial data. High resolution DTM are a good solution for mapping fine water networks and their integration into hydrological modelling. But if, in past years, big progresses concerned spatial data techniques, on the other hand they did concern concepts in modelling and ground experiments. And this is a difficulty for a real use of remote sensing data into finely resolved models. New possibilities are carrying revival in this field, through development of low cost ground techniques and communicating wireless networks

Extraction of thalweg networks from DTMs: application to badlands

Contact: [email protected] study gully spatial patterns in the badlands using a continuous thalweg vector network, this paper presents methods to extract the badlands' thalweg network from a regular grid digital terrain model (DTM) by combining terrain morphology indices with a drainage algorithm. This method will delineate a thalweg only where the DTM denotes a significant curvature with respect to DTM accuracy and relies on three major steps. First, discontinuous concave areas were detected from the DTM using morphological criteria, either the plan curvature or the convergence index. Second, the concave areas were connected using a drainage algorithm, which provides a continuous, thick, tree-structured scheme. We assumed that these areas were physically significant and corresponded to a gully floor. Finally, the thick path was reduced to its main course and vectorised to obtain a thalweg network. The methods were applied to both virtual and actual DTM cases. The actual case was a LiDAR DTM of the Draix badlands in the French Alps. The obtained networks were quantitatively compared, both with a network obtained using the usual drainage area criteria and with a reference network mapped in the field. The CI-based network showed the great potential for thalweg network extraction

Robust badlands thalwegs network extraction from DTM for topological characterisation

International audienceThis paper presents methods to extract badlands thalwegs network from regular grid DTM by combining a terrain morphology indices to a drainage algorithm. The computation of a continuous vector network will permit the study of the badlands spatial patterns. Thess methods aim at delineating a thalweg only where the DTM denotes a significant curvature with respect to DTM accuracy. It relies on three major steps. Firstly, discontinuous concave areas are detected from the DTM using morphological criteria; the plan curvature and the convergence index . Secondly, the concave convergence areas are connected using a drainage algorithm which provides continuous and tree-structured thick scheme. We assume that these areas present physical significance and corresponds to a gully floor area. Finally, the thick path is reduced to its main curve and vectorised to obtain a thalwegs network. The methods are applied on both virtual and actual cases DTM. The actual case is a LiDAR DTM of Draix Badlands (French Alps). The obtained networks are quantitatively compared both to the one obtained with usual drainage area criteria and to a reference network. The networks comparison shows the great potential of the converge index based method for thalweg network extraction

Etude de la précision du satellite lidar GLAS-ICESat pour l'altimétrie des eaux continentales

For the coming century, the control of water resources will be certainly the key of all the stakes for billions of human beings. Unfortunately a reduction in the number of stations is observed with a decline of measurements quality. Remote sensing, which saw the development of numerous satellite radar altimeters and more recently the launch of the satellite lidar ICESat, could be an interesting alternative for the study of the hydrological networks. The objective of this study is to estimate the potential of ICESat for monitoring continental water missions through the cases of the Lake Geneva (Switzerland and France) and rivers of Metropolitan France. Our first axis of study concerned the satellite-based assessment of ICESat on the Lake Geneva by comparing laser data to hydrological gauge water levels. Two hydrological stations (Chillon and Saint Prex) were used to evaluate the accuracy of ICESat elevations. First it was necessary that all data was in the same datum to conduct a consistent comparison. ICESat elevations, which are referenced in the Topex ellipsoid, were converted into orthometric elevations by a translation between Topex ellipsoid and WGS84 and then into the vertical reference IGN69 (RGF93) with the grid RAF98. The shots of water alone were then extracted track by track and the mean elevation calculated for each track was used for the comparison with reference elevations (hydrological gauges). The error RMS is 33 cm (-0.20 cm ± 0.21 cm) without any saturation correction. When the saturation correction is supplied and different from -999.000, the quality of water elevation data is improved : the error RMS is 14 cm (0.01 cm ± 0.10 cm). However GLAS temporal profiles show a slow progressive adaptation of GLAS sensor before proposing correct elevations. On the passage of ICESat from the land to water, the first spots elevations are higher than reference elevation and the following spots from 30 cm to 50 cm. The progressive return to the normal can last 0.2 s. It corresponds to 8 measurements and an adaptation distance of 1.360 km. When the transition footprints are excluded, the accuracy for the ICESat elevation measurements is 5 cm. Besides hydrological objects with a small size (small lakes, small rivers), which can not apply a margin of 1.5 km to remove transition footprints, could not be monitoring using ICESat with a good accuracy. Next the accuracy of ICESat was investigated on French rivers with a width larger than the size of ICESat footprint (about 55 m for the laser 3). The error RMS is 1.15 m (0.03 m ± 1.17 m) due to the time of ICESat adaptation on the passage from land to water. ICESat is not adapted for the monitoring of the continental water resource

A spatial stochastic algorithm to reconstruct artificial drainage networks from incomplete network delineations

Contact: [email protected], [email protected], [email protected] spatial stochastic algorithm that aims to reconstruct an entire artificial drainage network of a cultivated landscape from disconnected reaches of the network is proposed here. This algorithm uses random network initialisation and a simulated annealing algorithm, both of which are based on random pruning or branching processes, to converge the multi-objective properties of the networks; the reconstructed networks are directed tree graphs, conform to a given cumulative length and maximise the proportion of reconnected reaches. This algorithm runs within a directed plot boundaries lattice, with the direction governed by elevation. The proposed algorithm was applied to a 2.6-km2 catchment of a Languedocian vineyard in the south of France. The 24-km-long reconstructed networks maximised the reconnection of the reaches obtained either from a hydrographic database or remote sensing data processing. The distribution of the reconstructed networks compared to the actual networks was determined using specific topographical and topological metrics on the networks. The results show that adding data on disconnected reaches to constrain reconstruction, while increasing the accuracy of the reconstructed network topology, also adds biases to the geometry and topography of the reconstructed network. This network reconstruction method allows the mapping of uncertainties in the representation while integrating most of the available knowledge about the networks, including local data and global characteristics. It also permits the assessment of the benefits of the remote sensing partial detection process in drainage network mapping

A hierarchical graph matching method to assess accuracy of network extraction from DTM

International audienceMore and more elevation data and methods are available to automatically map hydrographic or thalweg networks. However, there are few methods to assess the network quality. The most used method to compare an extracted network to a reference network gives global quality information on only geographic criterion. The method proposed in this paper allows a network assessment compared to a reference network whose results can be interpreted more easily and more related to networks morphologies. This method is based on a hierarchical node matching within a graph. Nodes are classified by hierarchical level according to their importance in the tree-structured network. Then, a matching process seeks for nodes pairs between the two networks based on the geographic distance. The hierarchy introduces a priority order in the matching. The relative location of nodes pairs is checked in order to ensure a topological consistency. Finally, similarity statistics based on nodes matching counts are computed. While the usual method only takes into account a geographic criterion, the presented method integrates geographic, geometric and topologic criteria. It is an interactive and object-by-object matching. Moreover, the hierarchical approach helps comparing networks represented at different scales. It provides global statistics but also step-by-step maps that helps characterizing the spatial distribution of network delineation errors

How far spatial accuracy governs land-use changes monitoring frequency: the urban sprawl monitoring example

International audienceIn this paper, we illustrate how far spatial accuracy of a land-use map governs land-use changes monitoring frequency on a urban sprawl monitoring case study. From a specific Monte Carlo approach propagating uncertainties, conf4idence curves for minimal monitoring frequency to detect significant changes in urban sprawl indicators were built. Results showed that frequency decreased when uspscaling indicators but it also showed very low monitoring frequency for indicators at the lower level. INTRODUCTION When setting up land-use monitoring systems, spatial uncertainties and their impact on the detection of indicator changes are usually ignored. To capture a significant change in land-use indicators is thus strongly related to its spatial resolution, the velocities of the process it represents and the accuracy of the used indicators. As a consequence, the required monitoring land-use change frequency, corresponding to the mimimum time step to ensure a significant change in indicators, also depends on these three factors: indicator spatial resolution, change process velocity and spatial indicator accuracy

Analyzing urban sprawl indicators under uncertainties

International audienceUrban sprawl causes the sealing of lands closest to the urban centers by transforming productive agricultural fields into impervious areas, with numerous economic, social and environmental impacts. Integrated monitoring is important to help urban policies and urban sprawl modelling, proposing urban sprawl indicators with combining spatial and social aspects of the phenomenon by spatial operators in GIS. For that, geographical information of sealing patches and territorial social data are used, but spatial uncertainties and their impact on the detection of indicator changes are usually ignored. The paper proposes (1) to evaluate uncertainties of indicator spatial and (2) to analyze the effect of upscaling on these uncertainties. The method used proposes to create impervious polygons according to their measured geometric and thematic uncertainties using a Monte Carlo simulation approach and to simulate social data according to census uncertainties. Impervious polygons are used in a closing operation, with different radius values, required to map morphological urban areas. The case study focused on three indicators (area, dispersion coefficient and population density) of the morphological urban areas for four administrative levels of administrative territorial units of Languedoc-Roussillon region, France. Results show that indicator uncertainties are generally higher for less densely populated areas than for the others at the finest territorial level, that the closing radius had a slightly influence on indicator uncertainties, and that uncertainties decreases with the upper territorial entities

Vegetation patch effects on flow resistance at channel scale

International audienceThanks to a specific experimental design in a controlled channel, this paper aimed at quantifying how patches of four different ditches plant species affect integrated flow resistance parameters, the Manning coefficient. These plants, frequently encountered in the farmland ditches and irrigation channels of the south of France, were selected according to a large range of hydrophilic requirements, flexibility and branching complexity related to the plant blockage factor. Eight different spatial patches (regular, random, lateral or central patches) of each plant with crescent or similar plant densities were implanted at the bottom of a controlled channel where the water levels and water velocities were measured for three different discharges in steady and unsteady flow conditions. Resistance parameters (Manning parameters) were then estimated from the total head-loss, or from flow propagation velocity in the channel thanks to inversion of an hydrodynamic model. These experiments allow us to test the significance effect of channel vegetation patches and densities on flow resistance parameters at the reach scale