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Caractérisation des échanges d’eau et d’énergie dans une oliveraie pluviale en zone aride : modélisation et intégration des données de télédétection

Abstract : The olive tree is a key perennial agrosystem for the economy of the Mediterranean basin with contrasting farming practices (rainfed sparse/intensive irrigated, co-planted ...). Known for its mechanisms of adaptation to water stress and its ability to survive for long periods under soil water limiting conditions, rainfed olive cultivation is dominant in this area, especially in Tunisia and the spacing between trees is a function of the distribution of the precipitation (from high density of plantation in the North (6 m) to very sparse in the South (25 m)) ensuring a volume of soil explored by the roots that is assumed to be enough to satisfy the water needs of the olive trees. However, it is subject to severe climatic pressure due to widespread dry spells, for which the frequency and intensity is likely to increase in the coming decades. The response of these sparse covers, where water is a limiting factor, to these climatic changes is unrecognized. In this context, this work aims to improve the current understanding of the physical processes governing the hydrological cycle and the development of vegetation in rainfed groves in central Tunisia. There is a twofold purpose: I / to test the vulnerability of these sparse olive groves to future conditions and ii / to make a decision support to farmers for the short (i.e., localized supplementary irrigation) and in the long term (in particular a decrease of the spacing between trees or the extension of irrigated area). To meet these goals, it is necessary to characterize the thermo-hydric functioning of olive trees, to identify the possibility of monitoring their current water status and to predict their responses to more severe future water conditions. As a first step, a dedicated experimental set up that includes eddy covariance and sap flow measurements has been installed. The collected data, which provide information on the exchange of energy and matter within the soil-plant-atmosphere continuum, were analyzed and their consistency was verified through a study of the different components of the water and energy balances at different spatial and time scales. In addition to direct measurements, proxidetection measurements are available and allow investigating the benefit of this powerful tool. Relationships between the estimated water stress indices (e.g., the difference between actual and potential transpiration) and the proxidetection indicators (surface temperature and photochemical reflectance index) were therefore established to detect critical thresholds beyond which a water supply becomes essential for the survival of the plant. In a second step, a soil-plant-atmosphere ISBA exchange model was calibrated and validated based on the large data set observed to reproduce the functioning of olive trees. This study emphasizes that the sparse nature of olive trees is nevertheless a challenge for the application of these tools because the fraction of vegetation cover (less than 7% for our Nasrallah site) is such that the surface is dominated by the bare soil functioning. Adjustments have therefore been proposed or even applied to these methods to adapt them to this low fraction vegetation cover. The database used in this study is published under the DOI: 10.6096 / MISTRALS-SICMED.1479.
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Submitted on : Monday, February 24, 2020 - 2:47:55 PM
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Chebbi Wafa. Caractérisation des échanges d’eau et d’énergie dans une oliveraie pluviale en zone aride : modélisation et intégration des données de télédétection. Hydrologie. Université Toulouse III - Paul Sabatier, 2020. Français. ⟨tel-02489472⟩



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