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

Are crop sequence evolutions influenced by farm territory dynamics?

Crop sequence can be defined as the temporal arrangement of crops and is influenced by four major types of constraint: the timing of agricultural operations, the minimum area of each crop, the minimum return period between the same crop, and the benefits or risks associated to preceding-following crop pairs (Castellazzi et al., 2008). Crop sequence is a key factor for assessing the environmental impact of farming systems (Leteinturier et al., 2006). Thus, analyzing the evolution of crop sequences and its drivers (e.g. European regulations or changes in farm size) is essential to understand farming system dynamics. The aim of this work was to analyze the link between farm territory dynamics and crop sequence evolutions. We specifically analyzed the evolutions of crops preceding winter wheat on cereal farms with contrasted dynamics of their territory (growth or stability of their Used Agricultural Area (UAA))

Characterization of crop rotations variability by combining modelling and local farm interviews

The characterization of actual crop rotations on a large territory is a major challenge for local stakeholders in order to understand agricultural impacts on natural resources, e.g., nitrate leaching (Beaudouin et al., 2005). Although various mathematical descriptions and simulations of crop rotations have been proposed (e.g., Castellazi et al., 2010), few models propose to reconstruct actual crop sequences and to simplify their diversity in a reasonable number of crop rotations. The aim of this work was thus to combine modelling and farm interviews to characterize crop rotations which were representative of different soil characteristics and farming systems over a whole agricultural area of 70,000 ha

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

Landscape structure and hydrological functioning : application to ditch networks in Mediterranean vineyards

L'influence des fossés dans les phénomènes de crue, d'érosion hydrique des sols ou de transferts de polluants agricoles est bien connue à l'échelle locale. Cependant, on ne dispose que de peu de connaissances sur la variabilité spatio-temporelle des réseaux de fossés et de l'effet de cette variabilité sur les processus hydrologiques. L'objectif de ce travail de thèse vise donc à quantifier la relation entre l'organisation spatiale et temporelle des réseaux de fossés, éléments de la structure du paysage, et le fonctionnement hydrologique des paysages viticoles méditerranéens.Dans un premier temps, on propose d'analyser comment la densité des réseaux de fossés varie dans les paysages et à quel point elle est conditionnée par le milieu physique. Ensuite, un algorithme de simulation de réseaux est chaîné à un modèle hydrologique afin de quantifier le rôle de la densité des réseaux dans la régulation des écoulements de surface. En lien avec son rôle d'interception du ruissellement, l'effet anti-érosif des réseaux de fossés est alors analysé grâce à l'utilisation d'indicateurs géomorphologiques.Dans un second temps, on s'intéresse à la dynamique spatio-temporelle de la végétation des fossés et à son impact sur les transferts de pesticides. Pour cela, on caractérise puis simule les pratiques d'entretien des fossés et leurs impacts sur la végétation. On montre alors en mobilisant différents indicateurs que les pratiques actuelles ne sont pas optimales d'un point de vue de la rétention des pesticides.Cette thèse, qui s'appuie sur des méthodes de simulation du paysage, montre l'intérêt du chaînage entre modèles de structure et de fonctionnement du paysage. Ce chaînage a permis de quantifier le rôle des réseaux de fossés dans la modulation des processus hydrologiques.The impact of ditch networks on runoff, soil erosion and pollutant transfer is well known at local scale. However, the spatio-temporal variability of ditch networks and the impact of this variability on hydrological processes has not been quantified. The aim of this thesis is thus to quantify the relation between the spatial configuration of ditch networks and the hydrological functioning of Mediterranean vineyards.First, the variability of ditch network density and the extent to which this density depends on the landscape attributes are analyzed. Then, a network simulator is used with a hydrological model to quantify the impact of ditch network density on runoff. In relation to the role of ditch networks in the interception of runoff on hillslopes, the protection against soil erosion procured by ditch networks is studied thanks to geomorphological indicators.Secondly, the spatio-temporal evolution of vegetation covers in ditch networks and its impact on the transfer of pesticides is analyzed. The ditch management regimes and their impact on ditch vegetation are simulated. Thanks to several indicators, it is shown that current management regimes are not optimal in view of pesticide retention.This thesis shows the interest in using a landscape structure model and a landscape functioning model. This allowed to quantify the impact of ditch networks on hydrological processes

Structure du paysage et fonctionnement hydrologique : application aux réseaux de fossés en zone viticole méditerranéenne.

The impact of ditch networks on runoff, soil erosion and pollutant transfer is well known at local scale. However, the spatio-temporal variability of ditch networks and the impact of this variability on hydrological processes has not been quantified. The aim of this thesis is thus to quantify the relation between the spatial configuration of ditch networks and the hydrological functioning of Mediterranean vineyards. First, the variability of ditch network density and the extent to which this density depends on the landscape attributes are analyzed. Then, a network simulator is used with a hydrological model to quantify the impact of ditch network density on runoff. In relation to the role of ditch networks in the interception of runoff on hillslopes, the protection against soil erosion procured by ditch networks is studied thanks to geomorphological indicators. Secondly, the spatio-temporal evolution of vegetation covers in ditch networks and its impact on the transfer of pesticides is analyzed. The ditch management regimes and their impact on ditch vegetation are simulated. Thanks to several indicators, it is shown that current management regimes are not optimal in view of pesticide retention. This thesis shows the interest in using a landscape structure model and a landscape functioning model. This allowed to quantify the impact of ditch networks on hydrological processes.L'influence des fossés dans les phénomènes de crue, d'érosion hydrique des sols ou de transferts de polluants agricoles est bien connue à l'échelle locale. Cependant, on ne dispose que de peu de connaissances sur la variabilité spatio-temporelle des réseaux de fossés et de l'effet de cette variabilité sur les processus hydrologiques. L'objectif de ce travail de thèse vise donc à quantifier la relation entre l'organisation spatiale et temporelle des réseaux de fossés, éléments de la structure du paysage, et le fonctionnement hydrologique des paysages viticoles méditerranéens. Dans un premier temps, on propose d'analyser comment la densité des réseaux de fossés varie dans les paysages et à quel point elle est conditionnée par le milieu physique. Ensuite, un algorithme de simulation de réseaux est chaîné à un modèle hydrologique afin de quantifier le rôle de la densité des réseaux dans la régulation des écoulements de surface. En lien avec son rôle d'interception du ruissellement, l'effet anti-érosif des réseaux de fossés est alors analysé grâce à l'utilisation d'indicateurs géomorphologiques. Dans un second temps, on s'intéresse à la dynamique spatio-temporelle de la végétation des fossés et à son impact sur les transferts de pesticides. Pour cela, on caractérise puis simule les pratiques d'entretien des fossés et leurs impacts sur la végétation. On montre alors en mobilisant différents indicateurs que les pratiques actuelles ne sont pas optimales d'un point de vue de la rétention des pesticides. Cette thèse, qui s'appuie sur des méthodes de simulation du paysage, montre l'intérêt du chaînage entre modèles de structure et de fonctionnement du paysage. Ce chaînage a permis de quantifier le rôle des réseaux de fossés dans la modulation des processus hydrologiques

Construction de scénarios d'évolutions des territoires agricoles intégrant les dynamiques locales

Afin de lutter contre les pollutions diffuses, les collectivités locales mettent en place des démarches de protection des aires d’alimentation de captage, qui s’articulent en trois étapes successives : la délimitation de l’aire d’alimentation de captage (AAC), c’est-à-dire la surface sur laquelle l'eau qui s'infiltre ou ruisselle alimente le captage ; l'identification des pressions polluantes qui s’exercent sur l’AAC ; la construction d'un plan d'action visant à réduire les pressions polluantes sur l’AAC. Une partie importante de ces AAC concerne des territoires agricoles. L’identification des pressions comme la construction d’un plan d’actions doivent alors intégrer les problématiques et enjeux propres à ces territoires agricoles. Les collectivités qui ont la maîtrise d’ouvrage pour ces opérations sont parfois démunies pour établir un diagnostic correct de l’impact des pratiques agricoles actuelles, ainsi que pour proposer des solutions adaptées au contexte local. L’objectif de ce document est de présenter les principaux apports de la démarche mis en oeuvre sur l’AAC du Vivier pour répondre aux objectifs indiqués ci-dessus

Elaboration d'outils et méthodes d'analyse de la dynamique des espaces cultivés

Elaboration d'outils et méthodes d'analyse de la dynamique des espaces cultivés. Groupe de Travail Captage ONEM

Potential of linear features detection in a Mediterranean landscape from 3D VHR optical date: application to terrace walls

International audienceMediterranean cultivated landscapes are prone to floods, erosion and water pollution. These landscapes are also considered as hot spots for biodiversity. As hydraulic and linear artificial settlements, terrace fronts or terrace walls consisting in historic manmade levees on agricultural plot margins, can alter hydrological fluxes, limit erosion, favour soil water storage and biodiversity (habitat and corridor effects) [1]. More and more spatially explicit models and indicators need the map of these landscape linear features to diagnose the hydrological risk or biodiversity at catchment or region scale [2]. However, terraces walls are almost never available in any map agency databases. Therefore, the potential of very high spatial resolution remote sensing data, in 2D or 3D scenes, to map these linear elements, need to be addressed. Some recent studies showed that terraces from 2D VHR scenes, i.e images, can be poorly detected, due to both vegetation cover altering objects visibility having poor spectral signature or due to anisotropic shadowing effects. The use of 3D scenes, i.e. digital terrain models (DTM) and digital surface models (DSM), appears therefore as more reliable. A lot of literature exist in landscapes linear elements detection from remote sensing 3D scenes, e.g. for hedges corresponding to surface convexities or hydrographic networks corresponding in terrain concavities [3, 4]. However, very few remote sensing or geomorphometry literature exists for terrace walls linear detection, corresponding to terrain stairs or discontinuities. This paper aims to propose an innovative DTM processing methodology chaining existing algorithms to map terrace walls from optical VHR satellite data