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PRISE EN COMPTE DE L'AGREGATION DES CULTURES DANS LA SIMULATION DU TRANSFERT RADIATIF : IMPORTANCE POUR L'ESTIMATION DE L'INDICE FOLIAIRE (LAI), DE LA PARCELLE AU PAYSAGE

Abstract : The leaf area index (LAI) is a key variable involved in many biophysical processes. Indirect measurements methods provide LAI estimations from in situ gap fraction measurements. Inversion of reflectance models provides LAI estimations from satellite data with high repetitivity on large areas; the models usually used are one-dimensional because few entry parameters are needed. In both cases, these models assume that the foliage elements are randomly distributed, that is not the case in most of the canopies.
The main objective of this work is to evaluate if the use of a clumping index in radiative transfer models could improve the simulations for heterogeneous canopies, in order to provide better LAI estimations.
In the first part of the work, we evaluate LAI estimations provided by the analysis of hemispherical photographs with the CAN_EYE software; it provides LAI estimations assuming leaves randomly distributed or taking into account the canopy heterogeneity with a clumping index. The evaluation is done by comparison with destructive measurements carried out over wheat, maize and sunflower crops. The main results show that the LAI estimations are improved when using the clumping index, but its calculation with the Lang and Xiang method (1986) must be done with great care according to the species. Possible improvements for its calculation are discussed.
The second part of the work concerns the analysis of BRDF simulations at field and landscape scales. Taking BRDF simulations with a 3D model as references (the DART model, Gastellu et al., 1996), we show first that the use of a clumping index in a one dimensional reflectance model generally improves field BRDF simulations for a maize canopy in the red spectral band. In the near infra red, we have best results when the canopy is considered as homogeneous. At the landscape scale, the changes in agricultural land use seem to be the main factor of heterogeneity and this heterogeneity could be partially taken into account with a clumping factor used to simulate BRDF in the visible spectral band. A preliminary study allows us to conclude that LAI estimations from reflectance models could be improved.
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Sylvie Duthoit. PRISE EN COMPTE DE L'AGREGATION DES CULTURES DANS LA SIMULATION DU TRANSFERT RADIATIF : IMPORTANCE POUR L'ESTIMATION DE L'INDICE FOLIAIRE (LAI), DE LA PARCELLE AU PAYSAGE. Autre. Université Paul Sabatier - Toulouse III, 2006. Français. ⟨tel-00135752⟩

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