Evaluation of Multiorbital SAR and Multisensor Optical Data for Empirical Estimation of Rapeseed Biophysical Parameters

This article aims to evaluate the potential of multitemporal and multiorbital remote sensing data acquired both in the microwave and optical domain to derive rapeseed biophysical parameters (crop height, dry mass, fresh mass, and plant water content). Dense temporal series of 98 Landsat-8 and Sentinel-2 images were used to derive normalized difference vegetation index (NDVI), green fraction cover (fCover), and green area index (GAI), while backscattering coefficients and radar vegetation index (RVI) were obtained from 231 mages acquired by synthetic aperture radar (SAR) onboard Sentinel-1 platform. Temporal signatures of these remote sensing indicators (RSI) were physically interpreted, compared with each other to ground measurements of biophysical parameters acquired over 14 winter rapeseed fields throughout the 2017–2018 crop season. We introduced new indicators based on the cumulative sum of each RSI that showed a significant improvement in their predictive power. Results particularly reveal the complementarity of SAR and optical data for rapeseed crop monitoring throughout its phenological cycle. They highlight the potential of the newly introduced indicator based on the VH polarized backscatter coefficient to estimate height (R2 = 0.87), plant water content (R2 = 0.77, from flowering to harvest), and fresh mass (R2 = 0.73) and RVI to estimate dry mass (R2 = 0.82). Results also demonstrate that multiorbital SAR data can be merged without significantly degrading the performance of SAR-based relationships while strongly increasing the temporal sampling of the monitoring. These results are promising in view of assimilating optical and SAR data into crop models for finer rapeseed monitoring

Estimation of evapotranspiration from remote sensing in West Africa : toward a better knowledge of this key variable for the region

L’Afrique de l’Ouest est particulièrement exposée aux changements climatiques et anthropiques qui exercent une pression croissante sur les ressources hydriques et végétales, dont la bonne gestion constitue un enjeu scientifique majeur. Plus particulièrement, il devient indispensable de mieux comprendre les échanges d’énergie et de matière au sein du continuum surface-atmosphère qui régissent une bonne partie du cycle hydrologique et du développement de la végétation. À cet égard, l’évapotranspiration constitue une variable clé à l’interface surface-atmosphère car elle recycle la plus grande partie des précipitations vers l’atmosphère et assure le couplage des cycles de l’eau et de l’énergie. En Afrique de l'Ouest, les connaissances actuelles sur ce processus restent limitées car elles reposent principalement sur des mesures de terrain représentatives de petites échelles spatiales, ou sur des modèles de surface complexes, dont l’importance des jeux de données qu’ils requièrent limite leur application. Dans ce contexte, ce travail de thèse vise à améliorer notre connaissance de la variabilité spatiotemporelle de l'évapotranspiration, en analysant le potentiel de la télédétection pour son estimation en Afrique de l'Ouest. L'approche proposée repose sur une comparaison des produits d'évapotranspiration par télédétection disponibles et la proposition d’une nouvelle méthode permettant la génération de nouveaux produits. L’étude a été menée sur trois sites de méso-échelle (~ 104 km2) fournissant un échantillon des conditions éco-climatiques rencontrées en Afrique de l’Ouest, avec du Nord au Sud : le Nord-Sahel (au centre-est Mali), le Sud-Sahel (au sud-ouest Niger) et la zone soudanienne (au nord Bénin). Une méthode d’estimation de l’évapotranspiration journalière par télédétection et de son incertitude épistémique, nommée EVASPA S-SEBI Sahel (E3S), a été élaborée. E3S a été appliqué sur les trois sites d’étude à partir de données issues des capteurs MODIS à bord des satellites TERRA et AQUA. Les estimations journalières d’évapotranspiration ont été évaluées par rapport aux observations pluriannuelles acquises par l’Observatoire AMMA-CATCH. Cette étude souligne le potentiel d’E3S pour l’estimation de l’évapotranspiration journalière en Afrique de l’Ouest. Toutefois, ces estimations restent soumises aux aléas de la mesure satellite (qualité des images, couvert nuageux, angle de visée trop grand) et sont donc ponctuées de lacunes. Ces travaux de thèse proposent également de nouvelles méthodes de reconstruction de séries d’évapotranspiration journalières via la combinaison d’estimations multi-résolution et multi-source. Cette étude montre la pertinence de ces approches de reconstruction par rapport aux méthodes d’interpolation standards utilisées dans la littérature. Les approches proposées permettent notamment de mieux retranscrire la réponse des surfaces terrestres aux séquences d’assèchement du sol entre deux épisodes pluvieux. Les nouveaux produits générés ont été introduit dans l’exercice d’inter-comparaison incluant huit autres produits disponibles en Afrique de l’Ouest à diverses résolutions spatio-temporelles. Ces produits ont été évalués à différentes échelles spatiales et temporelles par rapport aux mesures locales et aux simulations spatialisées de vingt modèles de surface générées dans le cadre de l’expérience ALMIP2. Cette étude met en évidence la forte variabilité inter-produit, en particulier au Sahel. Elle souligne par ailleurs l’importance de la prise en compte d’une information liée au stress hydrique dans la génération des produits d’évapotranspiration. La résolution kilométrique des produits E3S leur fournit un avantage indéniable concernant la description de la variabilité spatiale des flux d’évapotranspiration par rapport à d’autres produits à faible résolution. Les produits nouvellement générés présentent un potentiel évident pour de futures études à caractère éco-hydrologique et hydrogéologique au Sahel.West Africa is particularly vulnerable to climate and human-induced changes, exerting increasing pressure on water and plant resources. Sound management of the latter requires substantial scientific progress. In particular, it is essential to better understand energy and matter exchanges through the surface-atmosphere continuum, which are a major driver of the hydrological cycle and of vegetation development. In this respect, evapotranspiration is a key variable, as most of precipitation returns to the atmosphere by evapotranspiration and as it couples the water and energy cycles. In West Africa, current knowledge of this process is still limited because it is mainly based on field measurements that are representative of small spatial scales, or on land surface models that would require considerably more data than available in this region. In this context, this thesis work aims at improving our knowledge of the spatiotemporal variability of evapotranspiration, by analyzing the potential of remote sensing to estimate evapotranspiration in West Africa. The proposed approach is based on an inter-comparison of available remote sensing evapotranspiration products and on the proposal of a new method to generate new estimation products. The study was carried out over three mesoscale sites (~ 104 km2) providing a sample of the eco-climatic conditions encountered in West Africa, namely from North to South: the North-Sahel (in East-Central Mali), the South-Sahel (in South-West Niger) and the Sudanian zone (in North Benin). An improved remote sensing method for estimating daily evapotranspiration and its epistemic uncertainty named EVASPA S-SEBI Sahel (E3S) was developed. E3S was applied to the three study sites using data from the MODIS sensors onboard TERRA and AQUA satellites. Daily evapotranspiration estimates were evaluated against pluriannual observations acquired by the AMMA-CATCH Observatory. This study highlights the potential of E3S for estimating daily evapotranspiration in West Africa. However, these estimates are still subject to hazards of satellite measurements (image quality, cloud cover, large satellite view angle) and are therefore discontinuous. This thesis work also proposes new methods for reconstructing continuous daily evapotranspiration series by the combination of multi-resolution and multi-source estimations. The study shows the capabilities of these reconstruction approaches compared to the standard interpolation methods usually found in the literature. In particular, the proposed approaches allow better depicting the response of terrestrial surfaces to soil drying sequences between rainfall events. The newly generated evapotranspiration products were included in the product inter-comparison together with eight other products available in West Africa at various spatial and temporal resolutions. These products were evaluated at different spatial and temporal scales against local measurements and spatially distributed simulations by twenty land surface models from the ALMIP2 experiment. This study highlights the high inter-product variability especially in the Sahel. It also emphasizes the importance of taking into account information related to water stress in the generation of evapotranspiration products. The kilometric resolution of E3S products gives them a clear advantage in terms of description of the spatial variability of evapotranspiration flux compared to other coarse resolution products. The newly generated products show clear potential for future eco-hydrological and hydrogeological studies in the Sahel

Estimation de l’évapotranspiration par télédétection spatiale en Afrique de l’Ouest : vers une meilleure connaissance de cette variable clé pour la région

West Africa is particularly vulnerable to climate and human-induced changes, exerting increasing pressure on water and plant resources. Sound management of the latter requires substantial scientific progress. In particular, it is essential to better understand energy and matter exchanges through the surface-atmosphere continuum, which are a major driver of the hydrological cycle and of vegetation development. In this respect, evapotranspiration is a key variable, as most of precipitation returns to the atmosphere by evapotranspiration and as it couples the water and energy cycles. In West Africa, current knowledge of this process is still limited because it is mainly based on field measurements that are representative of small spatial scales, or on land surface models that would require considerably more data than available in this region. In this context, this thesis work aims at improving our knowledge of the spatiotemporal variability of evapotranspiration, by analyzing the potential of remote sensing to estimate evapotranspiration in West Africa. The proposed approach is based on an inter-comparison of available remote sensing evapotranspiration products and on the proposal of a new method to generate new estimation products. The study was carried out over three mesoscale sites (~ 104 km2) providing a sample of the eco-climatic conditions encountered in West Africa, namely from North to South: the North-Sahel (in East-Central Mali), the South-Sahel (in South-West Niger) and the Sudanian zone (in North Benin). An improved remote sensing method for estimating daily evapotranspiration and its epistemic uncertainty named EVASPA S-SEBI Sahel (E3S) was developed. E3S was applied to the three study sites using data from the MODIS sensors onboard TERRA and AQUA satellites. Daily evapotranspiration estimates were evaluated against pluriannual observations acquired by the AMMA-CATCH Observatory. This study highlights the potential of E3S for estimating daily evapotranspiration in West Africa. However, these estimates are still subject to hazards of satellite measurements (image quality, cloud cover, large satellite view angle) and are therefore discontinuous. This thesis work also proposes new methods for reconstructing continuous daily evapotranspiration series by the combination of multi-resolution and multi-source estimations. The study shows the capabilities of these reconstruction approaches compared to the standard interpolation methods usually found in the literature. In particular, the proposed approaches allow better depicting the response of terrestrial surfaces to soil drying sequences between rainfall events. The newly generated evapotranspiration products were included in the product inter-comparison together with eight other products available in West Africa at various spatial and temporal resolutions. These products were evaluated at different spatial and temporal scales against local measurements and spatially distributed simulations by twenty land surface models from the ALMIP2 experiment. This study highlights the high inter-product variability especially in the Sahel. It also emphasizes the importance of taking into account information related to water stress in the generation of evapotranspiration products. The kilometric resolution of E3S products gives them a clear advantage in terms of description of the spatial variability of evapotranspiration flux compared to other coarse resolution products. The newly generated products show clear potential for future eco-hydrological and hydrogeological studies in the Sahel.L’Afrique de l’Ouest est particulièrement exposée aux changements climatiques et anthropiques qui exercent une pression croissante sur les ressources hydriques et végétales, dont la bonne gestion constitue un enjeu scientifique majeur. Plus particulièrement, il devient indispensable de mieux comprendre les échanges d’énergie et de matière au sein du continuum surface-atmosphère qui régissent une bonne partie du cycle hydrologique et du développement de la végétation. À cet égard, l’évapotranspiration constitue une variable clé à l’interface surface-atmosphère car elle recycle la plus grande partie des précipitations vers l’atmosphère et assure le couplage des cycles de l’eau et de l’énergie. En Afrique de l'Ouest, les connaissances actuelles sur ce processus restent limitées car elles reposent principalement sur des mesures de terrain représentatives de petites échelles spatiales, ou sur des modèles de surface complexes, dont l’importance des jeux de données qu’ils requièrent limite leur application. Dans ce contexte, ce travail de thèse vise à améliorer notre connaissance de la variabilité spatiotemporelle de l'évapotranspiration, en analysant le potentiel de la télédétection pour son estimation en Afrique de l'Ouest. L'approche proposée repose sur une comparaison des produits d'évapotranspiration par télédétection disponibles et la proposition d’une nouvelle méthode permettant la génération de nouveaux produits. L’étude a été menée sur trois sites de méso-échelle (~ 104 km2) fournissant un échantillon des conditions éco-climatiques rencontrées en Afrique de l’Ouest, avec du Nord au Sud : le Nord-Sahel (au centre-est Mali), le Sud-Sahel (au sud-ouest Niger) et la zone soudanienne (au nord Bénin). Une méthode d’estimation de l’évapotranspiration journalière par télédétection et de son incertitude épistémique, nommée EVASPA S-SEBI Sahel (E3S), a été élaborée. E3S a été appliqué sur les trois sites d’étude à partir de données issues des capteurs MODIS à bord des satellites TERRA et AQUA. Les estimations journalières d’évapotranspiration ont été évaluées par rapport aux observations pluriannuelles acquises par l’Observatoire AMMA-CATCH. Cette étude souligne le potentiel d’E3S pour l’estimation de l’évapotranspiration journalière en Afrique de l’Ouest. Toutefois, ces estimations restent soumises aux aléas de la mesure satellite (qualité des images, couvert nuageux, angle de visée trop grand) et sont donc ponctuées de lacunes. Ces travaux de thèse proposent également de nouvelles méthodes de reconstruction de séries d’évapotranspiration journalières via la combinaison d’estimations multi-résolution et multi-source. Cette étude montre la pertinence de ces approches de reconstruction par rapport aux méthodes d’interpolation standards utilisées dans la littérature. Les approches proposées permettent notamment de mieux retranscrire la réponse des surfaces terrestres aux séquences d’assèchement du sol entre deux épisodes pluvieux. Les nouveaux produits générés ont été introduit dans l’exercice d’inter-comparaison incluant huit autres produits disponibles en Afrique de l’Ouest à diverses résolutions spatio-temporelles. Ces produits ont été évalués à différentes échelles spatiales et temporelles par rapport aux mesures locales et aux simulations spatialisées de vingt modèles de surface générées dans le cadre de l’expérience ALMIP2. Cette étude met en évidence la forte variabilité inter-produit, en particulier au Sahel. Elle souligne par ailleurs l’importance de la prise en compte d’une information liée au stress hydrique dans la génération des produits d’évapotranspiration. La résolution kilométrique des produits E3S leur fournit un avantage indéniable concernant la description de la variabilité spatiale des flux d’évapotranspiration par rapport à d’autres produits à faible résolution. Les produits nouvellement générés présentent un potentiel évident pour de futures études à caractère éco-hydrologique et hydrogéologique au Sahel

Exploring Sentinel-2 dense image time series to identify cover crop emergence and destruction dates in France: Towards the development of an approach for biomass estimation

International audienc

Estimating Winter Cover Crop Biomass in France Using Optical Sentinel-2 Dense Image Time Series and Machine Learning

Cover crops play a pivotal role in mitigating climate change by bolstering carbon sequestration through biomass production and soil integration. However, current methods for quantifying cover crop biomass lack spatial precision and objectivity. Thus, our research aimed to devise a remote-sensing-based approach to estimate cover crop biomass across various species and mixtures during fallow periods in France. Leveraging Sentinel-2 optical data and machine learning algorithms, we modeled biomass across 50 fields representative of France’s diverse cropping practices and climate types. Initial tests using traditional empirical relationships between vegetation indices/spectral bands and dry biomass revealed challenges in accurately estimating biomass for mixed cover crop categories due to spectral interference from grasses and weeds, underscoring the complexity of modeling diverse agricultural conditions. To address this challenge, we compared several machine learning algorithms (Support Vector Machine, Random Forest, and eXtreme Gradient Boosting) using spectral bands and vegetation indices from the latest available image before sampling as input. Additionally, we developed an approach that incorporates dense optical time series of Sentinel-2 data, generated using a Radial Basis Function for interpolation. Our findings demonstrated that a Random Forest model trained with dense time series data during the cover crop development period yielded promising results, with an average R-squared (r2) value of 0.75 and root mean square error (RMSE) of 0.73 t·ha−1, surpassing results obtained from methods using single-image snapshots (r2 of 0.55). Moreover, our approach exhibited robustness in accounting for factors such as crop species diversity, varied climatic conditions, and the presence of weed vegetation—essential for approximating real-world conditions. Importantly, its applicability extends beyond France, holding potential for global scalability. The availability of data for model calibration across diverse regions and timeframes could facilitate broader application

Estimating evapotranspiration from remote sensing: the case of Sahelian Africa.

Allies A., J. Demarty, A. Olioso, H. B.-A. Issoufou, I. Maïnassara, J.-P. Chazarin, M. Oï, C. Velluet, M. Bahir, B. Cappelaere, 2017. Estimating evapotranspiration from remote sensing: the case of Sahelian Africa. Abstract IAHS2017-164. IAHS 2017 Scientific Assembly, 10 – 14 July 2017, Port Elizabeth, South Africa. [Présentation orale par A. Allies]http://meetingorganizer.copernicus.org/IAHS2017/IAHS2017-164.pdfEstimating evapotranspiration from remote sensing: the case of Sahelian Africa. . IAHS 2017 Scientific Assembl

Evapotranspiration mapping from remote sensing data: uncertainties and ensemble estimates based on multimodel-multidata simulations

International audienc

Evapotranspiration mapping from remote sensing data: uncertainties and ensemble estimates based on multimodel-multidata simulations

International audienc

Mediterranean Temporary Lagoon: Proposal for a definition of this endangered habitat to improve its conservation

International audienceCoastal lagoons have been recognised as a priority habitat for conservation and have benefited from several conservation plans. Under the Mediterranean climate, some of these lagoons might dry out during summer due to drought events. We propose the term Mediterranean Temporary Lagoons (MTLs) for these ephemeral water bodies and discuss their definition and characteristics. This term emerged in France among its coastal zone managers, who now commonly use it for conservation purposes. It is used in both natural systems as well as most artificial salt ponds in abandoned saltworks.In Europe, two directives have integrated lagoons as key targets to be preserved. Nonetheless, a certain discrepancy in the different definitions of lagoons has constrained joint actions. Indeed, while institutional definitions were originally derived from the scientific concept, their legislative and managerials meanings have been gradually modified and nowadays often differ from the original concept to create difficulties in the field. In addition, while it has been recommended to consider MTLs as a coastal lagoon habitat in the European Habitat Directive, its interpretation among EU member states is unsettled. Thus, clarifying lagoon habitats' terminology is required to ensure better management, monitoring and planning, and coordinate conservation actions.We discuss the inclusion of MTLs in habitat 1150 by confronting scientific and institutional literature and propose a new framework to better delimitate lagoon habitat around the Mediterranean basin, integrating MTLs. MTLs represent a specific habitat that hosts a pool of stenoecious macrophytes of conservation interest like Althenia filiformis, Riella helicophylla or Tolypella salina