Which spatial discretization for distributed hydrological models? Proposition of a methodology and illustration for medium to large-scale catchments

Distributed hydrological models are valuable tools to derive distributed estimation of water balance components or to study the impact of land-use or climate change on water resources and water quality. In these models, the choice of an appropriate spatial discretization is a crucial issue. It is obviously linked to the available data, their spatial resolution and the dominant hydrological processes. For a given catchment and a given data set, the "optimal" spatial discretization should be adapted to the modelling objectives, as the latter determine the dominant hydrological processes considered in the modelling. For small catchments, landscape heterogeneity can be represented explicitly, whereas for large catchments such fine representation is not feasible and simplification is needed. The question is thus: is it possible to design a flexible methodology to represent landscape heterogeneity efficiently, according to the problem to be solved? This methodology should allow a controlled and objective trade-off between available data, the scale of the dominant water cycle components and the modelling objectives. <br><br> In this paper, we propose a general methodology for such catchment discretization. It is based on the use of nested discretizations. The first level of discretization is composed of the sub-catchments, organised by the river network topology. The sub-catchment variability can be described using a second level of discretizations, which is called hydro-landscape units. This level of discretization is only performed if it is consistent with the modelling objectives, the active hydrological processes and data availability. The hydro-landscapes take into account different geophysical factors such as topography, land-use, pedology, but also suitable hydrological discontinuities such as ditches, hedges, dams, etc. For numerical reasons these hydro-landscapes can be further subdivided into smaller elements that will constitute the modelling units (third level of discretization). <br><br> The first part of the paper presents a review about catchment discretization in hydrological models from which we derived the principles of our general methodology. The second part of the paper focuses on the derivation of hydro-landscape units for medium to large scale catchments. For this sub-catchment discretization, we propose the use of principles borrowed from landscape classification. These principles are independent of the catchment size. They allow retaining suitable features required in the catchment description in order to fulfil a specific modelling objective. The method leads to unstructured and homogeneous areas within the sub-catchments, which can be used to derive modelling meshes. It avoids map smoothing by suppressing the smallest units, the role of which can be very important in hydrology, and provides a confidence map (the distance map) for the classification. The confidence map can be used for further uncertainty analysis of modelling results. The final discretization remains consistent with the resolution of input data and that of the source maps. The last part of the paper illustrates the method using available data for the upper Saône catchment in France. The interest of the method for an efficient representation of landscape heterogeneity is illustrated by a comparison with more traditional mapping approaches. Examples of possible models, which can be built on this spatial discretization, are finally given as perspectives for the work

Which spatial discretization for which distributed hydrological model?

International audienceDistributed hydrological models are valuable tools to derive distributed estimation of water balance components or to study the impact of land-use or climate change on water resources and water quality. In these models, the choice of an appropriate spatial scale for the modelling units is a crucial issue. It is obviously linked to the available data and their scale, but not only. For a given catchment and a given data set, the "optimal" spatial discretization should be different according to the problem to be solved and the objectives of the modelling. Thus a flexible methodology is needed, especially for large catchments, to derive modelling units by performing suitable trade-off between available data, the dominant hydrological processes, their representation scale and the modelling objectives. In order to represent catchment heterogeneity efficiently according to the modelling goals, and the availability of the input data, we propose to use nested discretization, starting from a hierarchy of sub-catchments, linked by the river network topology. If consistent with the modelling objectives, the active hydrological processes and data availability, sub-catchment variability can be described using a finer nested discretization. The latter takes into account different geophysical factors such as topography, land-use, pedology, but also suitable hydrological discontinuities such as ditches, hedges, dams, etc. For small catchments, the landscape features such as agricultural fields, buildings, hedges, river reaches can be represented explicitly, as well as the water pathways between them. For larger catchments, such a representation is not feasible and simplification is necessary. For the sub-catchments discretization in these large catchments, we propose a flexible methodology based on the principles of landscape classification, using reference zones. These principles are independent from the catchment size. They allow to keep suitable features which are required in the catchment description in order to fulfil a specific modelling objective. The method leads to unstructured and homogeneous areas within the sub-catchments, which can be used as modelling units. It avoids map smoothing by suppressing the smallest units, the role of which can be very important in hydrology, and provides a confidence map (the distance map) for the classification. The confidence map can be used for further uncertainty analysis of modelling results. The final discretization remains consistent with the scale of input data and that of the source maps. We present an illustration of the method using available data from the upper Saône catchment (11 700 km2) in France. We compare the results with more traditional mapping approach, according to the landscape representation and input data scale

Impact of land use on the hydraulic properties of the topsoil in a small French catchment

The hydraulic properties of the topsoil control the partition of rainfall into infiltration and runoff at the soil surface. They must be characterized for distributed hydrological modelling. This study presents the results of a field campaign documenting topsoil hydraulic properties in a small French suburban catchment (7 km2) located near Lyon, France. Two types of infiltration tests were performed: single ring infiltration tests under positive head and tension disk infiltration using a mini-disk. Both categories were processed using the BEST Beerkan Estimation of Soil Transfer parameters- method to derive parameters describing the retention and hydraulic conductivity curves. Dry bulk density and particle size data were also sampled. Almost all the topsoils were found to belong to the sandy loam soil class. No significant differences in hydraulic properties were found in terms of pedologic units, but the results showed a high impact of land use on these properties. The lowest dry bulk density values were obtained in forested soils with the highest organic matter content. Permanent pasture soils showed intermediate values, whereas the highest values were encountered in cultivated lands. For saturated hydraulic conductivity, the highest values were found in broad leaved forests and small woods. The complementary use of tension disk and positive head infiltration tests highlighted a sharp increase of hydraulic conductivity between near saturation and saturated conditions, attributed to macroporosity effect. The ratio of median saturated hydraulic conductivity to median hydraulic conductivity at a pressure of -20 mm of water, was about 50. The study suggests that soil texture, such as used in most pedo-transfer functions, might not be sufficient to properly map the variability of soil hydraulic properties. Land use information should be considered in the parameterizations of topsoil within hydrological models to better represent in situ conditions, as illustrated in the paper

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

Projet IRIP : Rapport bibliographique sur la cartographie de l'aléa inondation par ruissellement

[Departement_IRSTEA]Eaux [TR1_IRSTEA]ARCEAUIl s'agit de faire un point sur les travaux contribuant à définir et évaluer un risque d'inondation par ruissellement. A partir des travaux existants sur le ruissellement, et de réflexions complémentaires alimentées par des cas observés ou étudiés par le Cemagref en territoire rural, cette note précise les outils disponibles et les perspectives de recherches à mener pour la prévision des zones présentant de tels risques. D'autres contextes, comme la prévention des inondations par débordement de rivière ou encore la conception d'aménagements pour la maîtrise des écoulements, ne sont pas abordés dans ce document. La note se divise en 4 parties : (1) des définitions, essentielles pour préciser la terminologie des expressions techniques utilisées (2) un état des connaissances disponibles autour du ruissellement en tant que phénomène hydrologique, (3) des réflexions sur l'identification des facteurs d'influence et des zones sensibles aux inondations par ruissellement, et (4) une synthèse des barrières qu'il nous semble nécessaire de lever pour améliorer la cartographie de l'aléa inondation par ruissellement, et éventuellement de mettre au point d'un système de prévision dédié aux risques d'inondation par ruissellement

Rapport technique du projet IRIP : Cartographie de l'aléa ruissellement

[Departement_IRSTEA]Eaux [TR1_IRSTEA]ARCEAULe projet IRIP a permis de développer une méthodologie spécifique à la cartographie d'un indicateur de l'aléa spatial inondation par ruissellement. Elle se base sur une première étape de caractérisation de l'aléa pluviométrique, suivi d'une étape de caractérisation de la prédisposition d'un territoire à produire, transférer et accumuler du ruissellement (ANSETR). La méthodologie proposée a été testée et évaluer sur 2 bassins versant à deux échelles différentes. Les éléments d'évaluation intègre à grande échelle l'utilisation de la base de données des arrêtés de catastrophes naturelles. A échelle fine, un dispositif d'observation spatialisée (DEVERSE) a été mis en place et exploité afin de confronter la réalité du ruissellement aux résultats cartographiques. Une analyse de sensibilité a permis de tester la robustesse/limites de la méthodologie proposée lorsque les données utilisées sont grossières. Enfin, plusieurs cas d'application ont été réalisé sur différents bassins de tailles différents : Loire amont (7000 Km2), les bassins de l'Yzeron (140 km2) , du Garon (200 Km2), et les petits bassins d'apports de quelques kilomètres carré sur les communes de Vienne , de Givors et de Genay

Actions exploratoires : Evaluation du risque de ruissellement

[Departement_IRSTEA]Eaux [TR1_IRSTEA]ARCEAU [Relecteur_IRSTEA]Chastan, B.Il s'agit de faire un point sur les travaux contribuant à définir et évaluer un risque d'inondation par ruissellement. A partir des travaux existants sur le ruissellement, et de réflexions complémentaires alimentées par des cas observés ou étudiés par le Cemagref en territoire rural, cette note précise les outils disponibles et les perspectives de recherches à mener pour la prévision des événements présentant de tels risques. D'autres contextes, comme la prévention des inondations par débordement de rivière ou encore la conception d'aménagements pour la maîtrise des écoulements, ne sont pas abordés dans ce document. La note se divise en 4 parties : (1) des définitions, essentielles pour préciser la terminologie des expressions techniques utilisées (2) un état des connaissances disponibles autour du ruissellement en tant que phénomène hydrologique, (3) des réflexions sur l'identification des facteurs d'influence et des zones sensibles aux inondations par ruissellement, et (4) une synthèse des barrières qu'il nous semble nécessaire de lever pour la mise au point d'un système de prévision dédié aux risques d'inondation par ruissellement

Modeling of hydrological processes using unstructured and irregular grids: 2D groundwater application.

18 Pags.To better handle landscape heterogeneities in distributed hydrological modeling, an earlier work proposed a discretization based on nested levels, which leads to fully unstructured modeling meshes. Upon such a discretization, traditional numerical solutions must be adapted, especially to describe lateral flow between the unstructured mesh elements. In this paper, we illustrated the feasibility of the numeric solution of the diffusion equation, representing groundwater flow, using unstructured meshes. Thus, a two-dimensional (2D) groundwater model (BOUSS2D), adapted to convex unstructured and irregular meshes was developed. It is based on the approximation of the 2D Boussinesq equation using numeric techniques suitable for nonorthogonal grids. The handling of vertical and horizontal aquifer heterogeneities is also addressed. The fluxes through the interfaces among joined mesh elements are estimated by the finite volume method and the gradient approximation method. Comparisons between the BOUSS2D predictions and analytical solutions or predictions from existing codes suggest the acceptable performance of the BOUSS2D model. These results therefore encourage the further development of hydrological models using unstructured meshes that are capable of better representing the landscape heterogeneities.Peer reviewe