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Application et développement de méthodes de cartographie numérique des propriétés des sols à l'échelle régionale : cas du Languedoc-Roussillon

Abstract : Depicting and mapping the soil variability is an important issue since the ecosystem services provided by soils play an important role in solving the current agro-environmental challenges. At the French national scale, the pedological data are currently provided by regional soil databases (« Référentiel Régionaux Pédologiques », RRP) at 1:250,000. However they provide soil information at a spatial resolution that is too coarse for addressing these challenges. This thesis undertakes a Digital Soil Mapping approach at the regional scale in a region (Languedoc-Roussillon) characterized by a great extent (27 236 km ²) and a low density of soil observations (1 observation/13.5 km2). The goal is to produce a new regional infrastructure of pedological data that could satisfy the specifications enacted in the international project GlobalSoilMap and that meets the needs of the local end-users. In a first step, several known approaches of digital soil mapping using the various pedological data available in the RRP were applied and compared. The best results were obtained by a regression-kriging approach using the legacy measured soil profiles of the RRP. For the pH, organic carbon and the variables of texture (clay, silt, sand) the performances of prediction were of moderate quality but sufficient to allow the production of informative maps (R2 between 0.2 and 0.7). Conversely the soil properties with a too low density of profiles and/or that varied within too short distances (coarse fragment, soil Depth, CEC) could not be predicted. In a second step, methodologies were proposed and tested for better estimating uncertainties of predictions of soil properties. Concerning local uncertainties, a progress compared to the use of Regression Kriging was obtained with the use of Quantile Regression Tree. These local uncertainties could in addition be propagated in calculations of soil indicators characterizing the geographical entities of the area (example: districts). Finally a third stage was devoted to the setting in effective production of the new regional infrastructure of pedological data, which allowed the diffusion of the maps obtained in this thesis towards the users. The results of the thesis demonstrate the feasibility of a digital soil mapping approach at the regional scale that could be generalized over the French territory. Although some methodological obstacles have to be addressed (ex: models of prediction for censored data, soil covariate “lithology”), the low density of the pedological observations currently stored in regional databases represents the major limiting factor, which will have to be addressed in the future to obtain digital maps of soil properties with acceptable and known precision.
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Kévin Vaysse. Application et développement de méthodes de cartographie numérique des propriétés des sols à l'échelle régionale : cas du Languedoc-Roussillon. Sciences de la Terre. Montpellier SupAgro, 2015. Français. ⟨NNT : 2015NSAM0036⟩. ⟨tel-01399092⟩

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