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From carbon dating to hydrology, isotopic approaches have a wide range of applications, such as the study of climate change or nutrients cycling in soils. Water stable isotopic analysis is also a promising tool to better understand water dynamics in soil since it can be used to trace root water uptake at different depths. This MSc thesis had two objectives: one technical and one scientific. The first technical objective was a proof of concept in the field of a novel soil water vapour extraction method. This method had already been tested and calibrated in laboratory by Rothfuss et al. which considers isotopic thermodynamic equilibrium between soil liquid water and soil water vapour and the unique requirement of a temperature correction. The second scientific objective was the determination of the vertical root water uptake profile of winter barley crops from isotopic information. This has been done via a multi-sources mixing model (SIAR) that confronts stable isotopic compositions of soil water across depth, with stable isotopic compositions of crop xylem sap water. From a technical aspect, the gas-permeable tubing method is working properly in the field. It provides enough water quantities to be analysed off-line with a laser spectrometer. This study however outlines fractionation errors during the sampling. This work provides insight into RWU patterns of winter barley for its last stage development. Furthermore, this study underlines the importance of input data when using multi-sources mixing model. The sources isotopic signatures have to be discriminative so that the model can differentiate them. Extra parameters such as the root length density and soil water content are essential to provide thorough results.

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Le changement climatique va modifier drastiquement nos activités, notamment l’agriculture. Les agriculteurs doivent déjà faire face au défi de nourrir une population toujours grandissante avec des ressources toujours plus limitées, notamment en eau. L’amélioration de la gestion de cette ressource est donc essentielle pour réussir ce défi. A cette fin, il est nécessaire d’étudier la partition de l’évapotranspiration en ses composantes principales pour identifier la fraction productive de ce flux (c’est-à-dire la transpiration) et pour limiter l’évaporation. Dans le présent travail, Fluxpart, un récent modèle de partition, est utilisée. Une analyse de sensibilité aux paramètres principaux du modèle est réalisée, les performances de ce modèle sont évaluées et il est comparé à un autre modèle, lui bien établi et validé, AquaCrop. En plus de cela, l’impact du fait de considérer la transpiration à la place de l’évapotranspiration dans le calcul de la résistance de canopée via l’application de l’équation de Penman-Monteith est étudié. En effet, l’hypothèse d’égalité entre les flux de transpiration et d’évapotranspiration est souvent faite, mais peut mener à d’importantes erreurs. Ces dernières sont quantifiées dans le présent travail. Les résultats principaux de cette étude montrent que Fluxpart échoue à partitionner un nombre significatif de demi-heures. Le fait que la partition réussisse ou ne réussisse pas semble lié à la stabilité atmosphérique, mais de plus amples recherches sont nécessaires pour confirmer cette intuition. En ce qui concerne les performances de Fluxpart, bien que les dynamiques générales de la partition soient identiques pour les deux modèles et concordent avec la littérature, des différences significatives ont été observées entre l’importance des flux simulés par Fluxpart et par AquaCrop. Il est bon de noter que le nombre trop important de demi-heures pour lesquelles la partition a échoué est trop élevé que pour pouvoir implémenter Fluxpart en routine dans une station de mesure par eddy-covariance. De plus amples recherches devraient être menées sur Fluxpart, le succès de sa partition et son possible lien avec la stabilité atmosphérique. D’autres modèles récents devraient également être plus étudiés, comme le modèle développé par Li et ses collègues, qui a également l’air prometteur d’après la littérature. Climate change is expected to dramatically impact our activities, especially agriculture. Farmers have to face the challenge of feeding an ever-growing population with increasingly limited resources, notably water resources. Improving water management strategies and policies is therefore key to tackling this challenge. To this end, the study of the evapotranspiration (ET) partitioning into its main components is needed to identify the productive fraction of this flux (i.e., transpiration) and to limit evaporation (E). In this work, a recent partitioning model was used, Fluxpart. A sensitivity analysis to its main parameters was conducted, its partitioning performances were assessed, and the model was compared to another well-established and validated partitioning model, AquaCrop. In addition to this, the impact of considering transpiration (T) instead of ET in determining the canopy resistance via the application of the Penman-Monteith equation was investigated. Indeed, the assumption of equality between T and ET is often made but can lead to errors. These were quantified in the present work. The main outcomes of this study were that Fluxpart failed to partition a significant number of half-hours. The fact that the partition succeeded or not seemed to be linked to the stability parameter, but further research is needed to confirm this intuition. Concerning the performances of Fluxpart, although the general dynamics of the partitioning computed by the two models are identical and consistent with the literature, significant differences in the importance of the partitioned fluxes were observed between the Fluxpart’s partition and the AquaCrop’s partition. One should note that the number of half-hours, for which the partition failed was too important to implement Fluxpart routinely with an eddy-covariance station measurement. Further research should then be conducted on Fluxpart, its partitioning success and its possible link to the stability parameter. Other recent models should also be investigated, such as the model developed by Li and colleagues, which also looks very promising, according to the literature

Evapotranspiration --- Partitioning --- AquaCrop --- Fluxpart --- Canopy resistance --- évapotranspiration --- partition --- AquaCrop --- Fluxpart --- Résistance de canopée --- Sciences du vivant > Agriculture & agronomie --- Sciences du vivant > Sciences de l'environnement & écologie