Simulating the effects of spatial configurations of agricultural ditch drainage networks on surface runoff from agricultural catchments

The study of runoff is a crucial issue because it is closely related to flooding, water quality and erosion. In cultivated catchments, agricultural ditch drainage networks are known to influence runoff. As anthropogenic elements, agricultural ditch drainage networks can therefore be altered to better manage surface runoff in cultivated catchments. However, the relationship between the spatial configuration, i.e., the density and the topology, of agricultural ditch drainage networks and surface runoff in cultivated catchments is not understood. We studied this relationship by using a random network simulator that was coupled to a distributed hydrological model. The simulations explored a large variety of spatial configurations corresponding to a thousand stochastic agricultural ditch drainage networks on a 6.4 km2 Mediterranean cultivated catchment. Next, several distributed hydrological functions were used to compute water flow-paths and runoff for each simulation. The results showed that (i) denser networks increased the drained volume and the peak discharge and decreased hillslopes runoff, (ii) greater network density did not affect the surface runoff any further above a given network density, (iii) the correlation between network density and runoff was weaker for small subcatchments (< 2 km2) where the variability in the drained area that resulted from changes in agricultural ditch drainage networks increased the variability of runoff and (iv) the actual agricultural ditch drainage network appeared to be well optimized for managing runoff as compared with the simulated networks. Finally, our results highlighted the role of agricultural ditch drainage networks in intercepting and decreasing overland flow on hillslopes and increasing runoff in drainage networks

Framework to guide the choice of the hydrologic model for applications on gauged, poorly gouged and ungauged small catchments : case of natural vc farmed catchments

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Analyses piscicoles de la Durance dans les secteurs de Cheval Blanc et de Pont de Cadenet. Campagne 2004

Cette étude a été effectuée en collaboration avec le Conseil Supérieur de la Pêche dans le cadre du suivi des peuplements piscicoles de la Basse-Durance et d'une étude d'impact des opérations de délestage/délimonage de l'ouvrage EDF de Mallemort. Elle a été réalisée sur les stations du Réseau Hydrobiologique et Piscicole de "Pont de Cadenet" le 23 juin 2004 et de "Cheval Blanc" le 24 juin 2004. Les débits étaient de 9 m3/s au Pont de Cadenet et de 11 m3/s à Cheval Blanc. L'échantillonnage piscicole a été effectué par pêche électrique de 14 ambiances à Pont de Cadenet et 16 à Cheval Blanc, supposées représentatives des faciès existants dans les deux stations. Elles ont fait l'objet de relevés topographiques, de mesures de vitesses et de profondeurs, d'une description qualitative des habitats. Les hydrosignatures des ambiances ont été calculées. Du point de vue piscicole, cette station présente des similitudes avec la station de Bonpas. Les effectifs des captures sont globalement faibles en regard d'une chronique disponible sur les deux stations du RHP. Une utilisation du modèle habitats/poissons 5M7 a permis de calculer les taux de saturation de 14 taxons (espèces par classes de taille) pour leurs habitats disponibles (barbeau fluviatile, chevaine, loche franche, spirlin et vairon). En 2004, les taux de saturation sont sensiblement plus élevés au Pont de Cadenet qu'à Cheval Blanc pour la loche franche, le spirlin et le vairon

Geo-MHYDAS: A landscape discretization tool for distributed hydrological modeling of cultivated areas

Corresponding author: Fax:+334 6763 2614. E-mail address: [email protected] audienceThe representation of landscape variabilities by means of an adequate landscape discretization is of major importance in distributed hydrological modeling. In this paper, we present Geo-MHYDAS, a landscape discretization tool that allows explicit representation of landscape features, particularly man-made ones, that are known to have a great impact on water and mass flows across the landscape. The landscape discretizations that are produced include user-controlled delineated irregular, linear or areal units connected to each other along a tree-like structure. Geo-MHYDAS includes three steps: (i) processing (i.e., the importation or the creation), followed by the modifications of the geographical objects, the limits of which are considered in the modeling as hydrological discontinuities, (ii) creation of the areal and linear units for hydrological modeling by a “selective cleaning” procedure after overlay that preserves, as much as possible, the object limits defined in the previous step, while having sizes and shapes that remain compatible with the application of the water flow functions of the hydrological model and (iii) building an oriented topology between irregularly shaped areal and linear units that allows the routing of the simulated water flows across the landscape. Geo-MHYDAS was developed using the open source free Geographic Information Systems (GIS) software GRASS. The use of Geo-MHYDAS was illustrated by running the hydrological model MHYDAS for different scenarios of man-made features, their presence and spatial organization within a small vineyard catchment located in the south of France (the Roujan catchment). Comparisons with hydrological modeling performed with usual landscape discretizations showed significant differences in the simulated hydrograms. This comparison illustrates well the strong impact of landscape discretizations on hydrological modeling, specifically on the man-made landscape features represented in Geo-MHYDA

Modélisation des incertitudes de ruissellement de bassins cultivés par algorithme de drainage stochastique

International audienceAnthropogenic ditch drainage networks have a strong impact on the runoff of small cultivated catchments and are more and more considered in hydrological modelling. However, maps of ditch drainage networks are not usually available which results in uncertainties of water flow-paths. In this context, this study aimed to assess runoff uncertainties entailed by uncertainties of ditch drainage network. We used a coupling approach to propagate uncertainties generated by a random network generation method in a hydrological model. First, we used a stochastic vector drainage algorithm running within the lattice of the field boundaries valued by elevation. It simulated equi-probable networks on a small cultivated catchment, with respect to morphology and uncertainties of elevation data. A thousand simulations represented uncertainty of the spatial organization and density of the network. Next, we propagated uncertainties of the water flow-paths through the hydrological model MHYDAS. Uncertainty of network runoff was high: the coefficient of variation of total volume was equal to 21\% at a subcatchment scale and equal to 18\% at the catchment scale. This uncertainty can be partly related to uncertainty of the network density. In addition to uncertainties of network runoff, uncertainty occurred about diffuse flow-paths too, due to the change of the topology of the fields. This uncertainty of overland flow was higher than for the network (coefficient of variation of overland flow indicator equal to 123\%) and closely related to ditch drainage density. Finally, this study (i) proposed a way to map runoff uncertainty at different scales in the case of an unknown actual network, (ii) allowed to evaluate the relative importance of the ditch drainage network in runoff simulations

OpenFLUID : a software platform for spatial modelling in landscapes

OpenFLUID is a fully featured environment for spatial modelling of landscapes dynamics.The modelling and simulation cycle can be implemented with OpenFLUID, from model development to executions and analyses of simulations.OpenFLUID (Software environment for modelling Fluxes In Landscapes) est une plate-forme logicielle de simulation de flux dans le paysage. Elle permet la construction de modèles et l'exécution de simulations. Elle est constitué de trois composants logiciels : le moteur de simulation OpenFLUID-engine, l'interface graphique OpenFLUID-builder, les ressources en ligne OpenFLUID-web.OpenFLUID-engine est le composant principal de la plate-forme OpenFLUID. Il assure l'exécution de codes de calcul branchés dynamiquement (plug-ins) à un noyau logiciel qui supervise la simulation spatio-temporelle des processus

Uncertainties of cultivated landscape drainage network mapping and its consequences on hydrological fluxes estimations

International audienceIn this study, we use a network generation method to simulate equi-probable artificial drainage networks on a small Mediterranean cultivated catchment. The method consists in a stochastic algorithm assimilating sampled observations on the network that generates a ditch network based on the map of field boundaries. A set of one hundred simulations is used to represent the uncertainty on the ditch network. With regards to geometrical (lengths indices), and topographical metrics (slope indices), simulations similarities compared to the real network are computed. Secondly, uncertainty on the network is propagated through the hydrological model MHYDAS. The induced hydrological responses in hydrographs present a variability that can be linked to previous networks metrics. At this stage of the network generation process, this uncertainty propagation study allows to drive the choice of criteria for future network generation process improvement

Incertitudes des chemins de l'eau des paysages cultivés numériques : conséquences sur les ruissellements de surface et débits

International audienceArtificial drainage networks are of great importance in hydrology of little cultivated catchments. However, artificial drainage networks mapping is very time-consuming. Thus, these networks are usually unknown. In this context, this study aims to assess hydrological consequences caused by uncertainties in the artificial drainage network mapping. In order to carry it out, a network generation method is used to simulate equi-probable artificial drainage networks on a 6.4 km2 Mediterranean cultivated catchment. This method consists in a stochastic drainage algorithm within the map of field boundaries directed by a DTM and assimilating sampled observations on the network. A few hundreds simulations are used to represent the uncertainty of the ditch network spatial organization and of its density. Network simulations obtained in this way present some geometrical, topological and topographical differences which can be computed. This uncertainty of drainage networks also modifies sub-catchments boundaries and areas for each simulation. Finally, in addition to changes in channelized flow-pathes, uncertainties in drainage network mapping lead to changes in diffusive flow-pathes too, due to the modification of the fields topology. So, both overland flow and channel run-off will vary. Next, uncertainties on the water flow-pathes are propagated through the hydrological model MHYDAS. The induced hydrological responses are studied at different scales : a method is defined to compute field overland flow at different aggregation level while channelized flow is analysed from sources to the outlet. The observed hydrological uncertainty depends of scale and can be linked to networks metrics. In the end, this study (i) proposes a way to map hydrological uncertainty at different scales when the actual network is unknown, (ii) allows to evaluate the relative importance of the artificial drainage network representation in hydrological simulations and (iii) assesses on a catchment the impact of varying the drainage density and the network spatial organization

Distributed hydrological modelling of farmed catchments (MHYDAS) : assessing the impact of man-made structures on hydrological processes

National audienceManagement of water resources of agricultural catchments has emerged as an environmental priority due to the effects of anthropogenic discontinuities or activities such as the field limits, embankments, drains, ditches, and agricultural practices on runoff, erosion and pollutant transport. MHYDAS, a physically based distributed hydrological model, was especially developed to model water, pollutant and erosion transport, taking into account these discontinuities and practices. A modular approach was undertaken using the platform OpenFLUID, which enables the user to build his own version of MHYDAS by combining various processes (interception, runoff, channel routing, infiltration, transfer in the soil, pollutant fate and transfer, erosion transfer) as a function of the objective of the study and the availability and the accuracy of the data. Application cases are shown and compared in various agro-hydro-climatic conditions in order to study the impact on water and pollutant transfer of tillage practices in the vineyard, stem-flow in tropical volcanic zone and drains in drained catchments. Finally, the model was applied to simulate the impact of various land-use scenarios