Apport de la géostatistique dans l'analyse et la modélisation spatialisée de l'organisation et du fonctionnement des sols

A travers des exemples de travaux publiés, nous illustrons l’apport de la géostatistique dans l’analyse et la modélisation spatialisée de l’organisation et du fonctionnement des sols. La majeure partie de ces travaux de recherche concerne des applications de la géostatistique dans des voies très diversifiées du point de vue des méthodes. Ainsi, l’analyse structurale, l’estimation linéaire et les modèles non stationnaires ont été mobilisés, pour analyser et modéliser l’organisation spatiale des sols en fonction des facteurs de la pédogenèse. L’interpolation géostatistique multivariée a permis de compléter quelques mesures coûteuses de l'attribut d'intérêt par des données plus abondantes sur des attributs qui lui sont corrélés. Les méthodes multivariées, telles que l'analyse factorielle krigeante, ont été utilisées pour étudier comment la corrélation entre les variables change en fonction de l'échelle spatiale, et pour améliorer notre compréhension des échelles de dépendance des processus physiques. La déconvolution d’un signal caractérisant un sol grâce au krigeage des composantes a permis d’extraire l’information pertinente dans des données exhaustives à référence spatiale. Il a ainsi été démontré que le krigeage des composantes permet de détecter puis de modéliser et de cartographier séparément les différentes échelles de variabilité d’un signal. Grâce à cette méthode, nous avons pu focaliser sur une échelle particulière du signal en l’incorporant dans le système d’équations du krigeage pour améliorer l’estimation d’une variable cible. La conjugaison de deux méthodes (l'analyse factorielle krigeante et le krigeage des composantes) a permis de définir les zones de fonctionnement homogène d’une couverture pédologique dans le cadre de projets sur l’Agriculture de Précision. Enfin, la question de la quantification de l’incertitude inhérente à la prédiction spatiale a été traitée par des techniques de simulations conditionnelles

A snapshot of soil water composition as an indicator of contrasted redox environments in a hedged farmland plot

International audienceWhile soil water composition has long been recognised as being related to soil type (characteristics of the horizons), the influence of structures resulting from agricultural activities (hedges, ditches, wheel ruts, etc) is still under discussion. This work was undertaken to show that a snapshot of spatial variability of the geochemical characteristics of soil water at the scale of a plot can improve our understanding of soil geochemistry in a farmland setting. We selected a 3 hectare hedged plot located on a hillside, limited by a stream and used as pasture where soils have developed in weathered gneiss. The water depth, electrical conductivity, major ions, temperature, pH, dissolved organic carbon (DOC) content, dissolved oxygen content, fluorescence, alkalinity, Fe2+, Mn2+, NO2−, Fe(III) and F− contents were measured in 62 auger holes randomly drilled on the site. Four sectors were identified in order to describe the distribution of the main geochemical parameters. Electrical conductivity and some major ions, especially sulphate, had larger concentrations near hedges where oxic conditions prevailed. These features were attributed to the impact of the linear anthropogenic network on the circulation of subsurface soil waters and evapo-transpiration and represent sector I. Dissolved Mn was an indicator of well channelled runoff subsurfaces facilitating the circulation of more highly reducing water (sector III), while DOC probably marked areas drained less well, with a prolonged contact time between soil solutions and organic topsoil horizons (sector II). The presence of dissolved Mn and Fe(II) indicates bottomland anoxic conditions (sector IV). It is concluded that a survey of the chemical composition of soil water may be a direct approach to show the influence of permanent structures on current soil properties and dynamics

Classification and mapping of anthropogenic landforms on cultivated hillslopes using DEMs and soil thickness data - Example from the SW Parisian Basin, France

International audienceThis study focuses on linear anthropogenic landforms of decametric width on cultivated hillslopes and their relations to soil thickness variability. The 16 ha study area shows a rolling topography supported by Cretaceous chalk of the SW Parisian Basin, France. Two types of landforms were identified: lynchets, similar to those described as soil terraces occurring on downslope field parts in other contexts, and undulations, linear, convex landforms that cut across fields. Accurate DEM construction and a detailed soil thickness survey were performed all over the study area. Soil samples were classified considering their location on specific types of anthropogenic landforms. The classification tree (CT) method was applied to assess whether lynchets and undulations can be discriminated through morphometric attributes (slope, curvature, profile curvature and planform curvature) and soil thickness (CTsoil) or through morphometric attributes only (CTtopo). The CT application establishes predictive classification models to map the spatial distribution of lynchets and undulations over the whole study area. The validation results of the CTsoil and CTtopo applications show model efficiencies of 83% and 67%, respectively. Both models performed well for lynchets. Errors arise mainly from difficulties in unequivocally discriminating gently convex undulations and undifferentiated surfaces, especially when soil thickness is not accounted for. Mean values of soil thickness are 1.08, 0.62 and 0.45 m in lynchets, undulations and undifferentiated areas, respectively. The general shape of the thickened soil is characteristic to each type of anthropogenic landform. Multi-temporal mapping of field border networks shows that undulations are linked to borders that were removed during the latest land consolidation. Lynchets are associated with current field borders. Lynchets and undulations, which cover 39% of the study area, define topographic indicators of human-induced soil accumulations. The method involves perspectives for efficiently mapping and quantifying the anthropogenically modified spatial variability of soil thickness on agricultural hillsides

Distribution spatiale des objets archéologiques et mouvements de matière à l’échelle des versants cultivés : le cas de Mougon (Indre-et-Loire, France)

Cette contribution pose la question de l’utilisation de la géostatistique pour l’interprétation de la répartition des artefacts recueillis en prospection archéologique. L’expérience a été réalisée à partir des données issues des prospections réalisées à Mougon (commune de Crouzilles, Indre-et-Loire, France) dans le cadre d’un projet retenu par l’Action Concertée Incitative « Terrain, Technique, Théorie » du ministère de la recherche. L’objectif initial visait à établir une distinction entre les concentrations d’artefacts susceptibles de signaler la présence de structures archéologiques sous-jacentes et les zones de répartition spatiale aléatoire des artefacts, liée au fumage des terres cultivées et aux processus post-dépositionnels. Après le tri, le comptage et l’enregistrement informatique du mobilier par catégories chronologiques et fonctionnelles, les artefacts collectés en surface ont fait l’objet d’un traitement géostatistique destiné à faciliter la lecture et l’interprétation de leur répartition : la distribution des artefacts a été étudiée au moyen des méthodes géostatistiques de krigeage ordinaire et de la simulation conditionnelle. L’exploitation des données de surface par le biais de la géostatistique a permis, d’une part, de circonscrire précisément l’emplacement des structures archéologiques potentielles et d’autre part, de révéler l’importance des processus post-dépositionnels dans la redistribution des artefacts en surface. Cette expérience est également à l’origine d’une réflexion sur la pertinence des unités de collecte et de représentation du mobilier de prospection.This contribution aims at discussing the use of geostatistics for the interpretation of the spatial surface distribution of archaeological artefacts. The experience was carried out with the artefacts collected in Mougon (Crouzilles, Indre-et-Loire, France) for the ACI TTT project of the French Ministry of Research. The initial purpose of the study was to distinguish the concentrations of artefacts indicating the presence of buried archaeological remains from a random spatial distribution of artefacts due to manuring and post-depositional processes. After all the artefacts were sorted out, counted and recorded, a geostatistical analysis has been performed to interpret the spatial distribution of the surface artefacts : ordinary kriging and conditional simulation were used. The use of geostatistics allowed a precise delimitation of archaeological features. It also revealed the influence of post-depositional processes on the spatial distribution of artefacts. Furthermore, this experiment led us to re-estimate the relevance of the units we used to collect and represent the survey data

The STICS model to predict nitrate leaching following agricultural pratices.

The aim of this paper was to develop an upscaling approach for the soil-crop model STICS in order to predict the impact of agricultural practices on nitrate leaching on both plot and regional scales. A case study was carried out on a "Nitrate Vulnerable Zone" located in central France. The performance of the spatial approach was evaluated by accounting for all the spatial and temporal variability existing within the studied area. The results indicate that N leaching and nitrate concentration in drainage water were slightly underestimated; by 3 kg N·ha-1 (16%) and 8 mg NO3-·L-1 (11%), respectively. The STICS scaling approach was used to assess the effectiveness of "Good Agricultural Practice" established within the area over a seven-year period. The simulation results provided evidence that such a practice had reduced the nitrate concentration by about 30% (36 mg NO3-·L-1). However, the rate of nitrate leaching remains too large and further improvements to agricultural practices are required

3D representation of soil distribution: An approach for understanding pedogenesis Représentation 3D des sols, apports à la compréhension de la pédogenèse

International audienceSoils are characterised by a spatial variability in the three dimensions (3D) of space. However, 3D studies remain scarce due to the qualitative nature of many soil horizon characteristics, notably the horizon designation. Indeed, existing 3D tools are mainly developed for quantitative data. To solve this difficulty, we propose a new approach based on the interpolation of the horizon thickness to derive digital elevation models for both the upper and the lower limits of each horizon. This approach was applied to Planosols previously extensively studied with 2D approaches. The pseudo 3D obtained representation evidences soil processes that were missed in 2D approaches. As an example, we evidence the impact of differential weathering, resulting from the mineralogical heterogeneity of the parent material, on the subsequent pedogenesis. Les sols sont des continuums tridimensionnels (3D), caractérisés par une variabilité spatiale dans l'ensemble des directions de l'espace, mais les études 3D demeurent rares. Une des difficultés majeures des approches 3D est due à la nature qualitative de nombreuses caractéristiques des horizons, dans la mesure où la plupart des outils 3D existants sont principalement développés pour l'interpolation 3D de données quantitatives. En substitution à l'interpolation directe de la variable « nom de l'horizon », nous proposons une approche basée sur l'interpolation de l'épaisseur des horizons pour calculer les modèles numériques d'altitude correspondant aux limites de chaque horizon, permettant ainsi une visualisation pseudo 3D. Cette étude, appliquée à des Planosols bien caractérisés en 2D par ailleurs, montre que la représentation 3D des sols permet de souligner des processus omis lors des approches précédentes, tels que l'impact de l'altération différentielle, résultant de l'hétérogénéité initiale du matériau parental, sur une pédogenèse ultérieure

Three-dimensional structure of a highly heterogeneous soil horizon derived by Electrical Resistivity Tomography

Introduction The soil consists in a natural filter of water transfer to the groundwater. Its structure influences water storage and transfer properties or gas diffusivity, which induce major consequences on both environment and agronomy. Knowing the soil structure is thus essential to predict the soil hydraulic functioning and the 3D characterization of the structure at the horizon scale is necessary to describe the hydraulic functioning of the soil. In heterogeneous horizons, the structure (..

Landscape design for soil conservation under land use and climate change

International audienceSoils and landscapes evolve simultaneously. Soil evolution is controlled by redistribution and transformation processes influenced by topographic and climatic parameters, with also a major contribution of management strategies. The perennial landscape features have a strong influence on soil spatial distribution (geometry) and soil genesis. Building landscapes which enhance soil resilience to degradation processes and increase soil services appears as a promising way to adapt to forthcoming climatic and land use evolutions. The presentation aims to synthetize major results from a research program nicknamed Landsoil which focused on the evolution of agricultural soils over medium time scales (decades to centuries) in relation to changing conditions of land use and climate. Precise study of the soil 3D organization in three contrasted landscapes (Brittany, Touraine, Languedoc-Roussillon) enabled to link soil redistribution in space to landscape components (field geometry, hedges or ditches network) and their past evolution. A dynamic and high resolution spatial modeling approach was developed coupling erosion processes and soil organic matter evolution and was calibrated over past evolution using dating techniques (Cs137, C14, OSL). The resulting Landsoil model was afterwards applied in a prospective manner under different scenarios of land use and climate change over the 21th century. Indicators of soil vulnerability and soil resilience were defined and tested by the comparison of several prospective scenarios applied on a same landscape and by comparison of the contrasted landscapes

Etude des lois de distribution spatiale des sols de Petite Beauce. Application à la cartographie d'un horizon par couplage de méthodes morphométriques et géostatistiques

* INRA, Centre de Recherche d'Orléans, Unité de Science du Sol, Ardon 45160 Olivet (FRA) Diffusion du document : INRA, Centre de Recherche d'Orléans, Unité de Science du Sol, Ardon 45160 Olivet (FRA) Diplôme : Dr. d'Universit