Water turbidity in West Africa : the Bagré (Take5) experiment

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The aim of "Bagré 2015" campaign was to closely monitor the spatio-temporal evolution of turbidity and Total Suspended Matters (TSM) in the water of Bagré Lake, the second biggest lake in Burkina Faso, where the concentrations are particularly heavy. Micro-organism find a favourable terrain in the suspended matters. The turbid waters which are absorbed or used by inhabitants to wash themselves or clothes or food, or to play, with a high risk of contamination. In Sub-Saharan Africa, many regions lack water quality networks and their health systems can be insufficient. Monitoring water quality can therefore contribute to mitigating this health hazard.

SPOT5 (Take5) images over Bagré Lake.

 

Monitoring of turbid waters


The SPOT5 (Take5) image time series shows the propagation of turbid waters after the start of the reain season, beginning inthe upstream part in june, the middle  part in July and August and the downstream part in September. It may be seen on the images above, acquired on the 11/04, 15/06, 05/07, 04/08, 19/08, 13/09, where the initially blue waters progressively turn yellow and green).

 

In order to asses our methods for turbidity and TSM monitoring, routine measurements were set-up from April the 16th, at the downstream end of the lake. A terrain campaign was also held between the 21st of July and the 5th of August 2015 (thanks to a French PNTS funding). Water samples were analysed to quantify turbidity (70 measures) and TSM (53 measures). 28 radiometric measurements (pictures 1 &2) and 12 absorption coefficients (Kd) (Picture 3 and map) were also recorded from a boat in all the lake parts (Picture 4).

Picture 1 :TriOs radiometer for reflectance measurements Picture 2: Reflectance measurements Picture 3 : TriOs Radiometer absorption coefficient measurement

Picture 4 : the boat used for water sampling and radiometric measurements

 

Preliminary results

The early results show nice relations between the NIR/RED surface reflectance ratio, and the in-situ turbidity and TSM. on a large range of values. The high resolution images will allow us to precisely document the lake dynamics, the contribution of each tributary and catchment, and the role of their land use. The measurements cruises also enabled to spot a few hippos, but the crocodiles did not show themselves...

 

Some samples were analysed with a scanning electro microscope to measure the size and type of the particles. Most of them have a diameter between 1 and 2 microns, and are mostly clays (kaolinite, illite and smectite).

 

All this work is done in the broader framework on continental water colour group within THEIA, in view of Sentinel-2 data use.

 

Elodie Robert, Manuela Grippa, Laurent Kergoat, Jean-Michel Martinez, Sylvain Pinet, Laetitia Gal, Nogmana Soumaguel

Turbidité des eaux en Afrique de l'Ouest: la campagne "Take 5" Bagré 2015.

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L’objectif de la campagne 'Bagré 2015' était de suivre finement l’évolution spatio-temporelle de la turbidité et des matières en suspension (MES) dans l’eau du lac de Bagré, deuxième plus grand réservoir du Burkina-Faso, dans un contexte où les valeurs de ces paramètres sont particulièrement élevées. Les matières en suspensions sont des vecteurs des contaminants microbiologiques, les microorganismes se développant particulièrement sur les particules en suspension. Les eaux fortement turbides, qui sont consommées directement ou avec lesquelles les habitants sont en contact (lessive, bain, lavage des légumes, jeux) constituent donc un risque important pour la santé. En Afrique subsaharienne, de nombreuses régions ne bénéficient pas de réseaux de suivi de la qualité de l'eau, et disposent de systèmes de santé parfois insuffisants. Le suivi de la turbidité est donc important pour quantifier cet aléa sanitaire.

 

Arrivée des eaux turbides


La série d'images SPOT5 (Take5) montre une propagation des eaux fortement turbides avec l'arrivée de la saison des pluies, depuis l’amont du lac en juin, les zones médianes en juillet-août jusqu’à l’aval en septembre (voir les images ci-dessus acquises les 11/04, 15/06, 05/07, 04/08, 19/08, 13/09). Vous pouvez voir le changement de couleur des eaux de ce lac avec un passage du bleu au marron-beige.

 

Pour valider l’algorithme à retenir pour suivre la turbidité et les MES, des relevés de routine ont été mis en place depuis le 16 avril 2015 sur un point localisé à l’aval du réservoir. Une mission terrain s’est également déroulée entre le 21 juillet et le 5 août 2015 (financement du PNTS). Des relevés d’eau ont été effectués mesurer la turbidité et les MES (70 et 53 mesures). Nous avons aussi pris 28 mesures radiométriques de terrain (photos 1 et 2) et 12 mesures de coefficient d’absorption (Kd) (photo 3 et carte). Ces mesures se sont déroulées à l’aide d’un bateau dans les différentes zones du lac de Bagré depuis l’amont jusqu’à l’aval (photo 4).

Photo 1 : Radiomètre TriOs, mesures de réflectance Photo 2: Mesures de réflectance avec les radiomètres TriOs Photo 3 : Radiomètre TriOs pour la mesure du coefficient d'absorption

Photo 4 : Bateau utilisé pour les prélèvements d’eau et les mesures radiométriques

 

Résultats préliminaires de la campagne

Les résultats en cours d'analyse montrent de très bonnes relations entre les rapports des réflectances PIR/rouge in situ et les MES et la turbidité, sur une gamme importante de variation. La haute résolution permettra de décrire finement le fonctionnement du lac, le rôle des différents affluents, des différents bassins versant et de l'usage des sols associé.  Les relevés de terrain ont permis de détecter également quelques hippopotames, mais les crocodiles sont restés invisibles...

 

Des échantillons ont été analysés au microscope électronique à balayage afin d'identifier la taille et les types de particules rencontrées et détectées. La majorité des particules a une taille variant entre 1 et 2 microns. Il s’agit principalement d'argiles (kaolinite, d’illite et de smectite).

 

Ce travail participe à une action plus large sur la couleur des eaux continentales (CES Couleurs des eaux continentales - Theia) et l'utilisation de Sentinel 2 en particulier.

 

Elodie Robert, Manuela Grippa, Laurent Kergoat, Jean-Michel Martinez, Sylvain Pinet, Laetitia Gal, Nogmana Soumaguel

Sentinel-2A first data gap

Sentinel-2 is a brand new and complex system, it is not a full surprise if it is experiencing some glitches. ESA is publishing weekly status reports, and the last two ones quote an issue with the mass memory.

 

From weekly report #11, 24 Oct – 30 Oct 2015:

The spacecraft has experienced this week an anomaly of the Mass Memory and Formatting Unit (MMFU), which is currently under investigation with the full support of the satellite prime contractor and the MMFU manufacturer

 

This anomaly was not yet solved at the end of last week, as said in weekly report #12 : 31 Oct–6 Nov 2015 :
The anomaly of the Mass Memory and Formatting Unit (MMFU) experienced since last week still persists. Analysis is ongoing with the full support of the satellite prime contractor and the MMFU manufacturer. An MMFU application software fix is being prepared and trouble-shooted on Engineering Functional Model and the Sentinel-2B platform.

 

I do not have much more information, but if the issue can be solved by a software modification, it means that all the equipments are fine. The ESA teams do not seem to be worried, as the outlook is :

Resolution of the MMFU anomaly and return to nominal S2A ramp-up activities
However, the release of Sentinel-2A preliminary product seems to be postponed a little, as the outlook also tells :
Opening of S2A core products access to all users –pending resolution of the ongoing on-board anomaly

 

To get more information here is where the status reports are provided, and we should thank ESA for sharing the information in such an open manner.

Les coefficients SMAC pour la correction atmosphérique de Sentinel-2A sont disponibles

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Les coefficients SMAC pour la correction atmosphérique des images Sentinel-2A ont été calculés et ajoutés à la collection du CESBIO. Cette fois, les coefficients ont été calculés par le service Physique de la Mesure Optique du CNES. Si vous les utilisez, n'oubliez donc pas de remercier le CNES et le CESBIO.

 

Comme je l'ai dit dans un ancien article :

Le Simplifié Modèle d'Atmosphérique Correction (SMAC) est parfaitement adapté à l'implémentation rapide et moyennement précise de corrections atmosphériques. Il utilise des fonctions analytiques dérivées du modèle 5S. Les 49 coefficients de ce modèle sont ajustés à partir de simulations de transfert radiatif obtenues avec le modèle 6S (l'ancienne version, pas la récente version vectorielle). SMAC n'est pas un modèle très précis (beaucoup moins que MACCS), et il faut lui fournir des données auxiliaires pour l'épaisseur optique des aérosols ou pour les contenus atmosphériques en ozone et vapeur d'eau. Quand ces données sont précisément connues, la précision des simulations est en général meilleure que deux à trois pour cent, sauf parfois pour les grands angles (au dessus de 70°) ou dans de fortes bandes d'absorption et si on ne prend pas en compte les effets d'environnement et les effets de pente.

 

Nous mettons aussi à disposition un code en python qui utilise ces coefficients. Ce code est décrit et distribué ici.


References

[1] Rahman, H., & Dedieu, G. (1994). SMAC: a simplified method for the atmospheric correction of satellite measurements in the solar spectrum. REMOTE SENSING, 15(1), 123-143.
"[2]"Tanré, D., Deroo, C., Duhaut, P., Herman, M., Morcrette, J. J., Perbos, J., & Deschamps, P. Y. (1990). Technical note Description of a computer code to simulate the satellite signal in the solar spectrum: the 5S code. International Journal of Remote Sensing, 11(4), 659-668.
"[3]"Vermote, E. F., Tanré, D., Deuze, J. L., Herman, M., & Morcette, J. J. (1997). Second simulation of the satellite signal in the solar spectrum, 6S: An overview. Geoscience and Remote Sensing, IEEE Transactions on, 35(3), 675-686.>
"[4]"Kotchenova, S. Y., Vermote, E. F., Matarrese, R., & Klemm Jr, F. J. (2006). Validation of a vector version of the 6S radiative transfer code for atmospheric correction of satellite data. Part I: Path radiance. Applied Optics, 45(26), 6762-6774.
"[5]"Kotchenova, S. Y., & Vermote, E. F. (2007). Validation of a vector version of the 6S radiative transfer code for atmospheric correction of satellite data. Part II. Homogeneous Lambertian and anisotropic surfaces. Applied Optics, 46(20), 4455-4464.

SMAC coefficients for quick Sentinel-2A atmospheric correction

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The SMAC coefficients for Sentinel-2A are now available, and have been added to the CESBIO repository for SMAC coefficients. These coefficients were computed and checked by CNES colleagues in the Optical Measurement Physics service. If you happen to use these coefficients in your work, please thank CNES and CESBIO for providing them.

As already said in an earlier post :

The Simplified Model for Atmospheric Correction (SMAC) is the perfect model to perform easy, quick and not too dirty atmospheric corrections. It is based on very simple analytic formulas, based on the 5S model. The 49 coefficients of this model are fitted using a large number of radiative transfer simulations with the 6S model (the old historic version, not the recent vector version). This software is not very accurate (much less than MACCS), and it requires in-situ measurements for the aerosol optical thickness, and weather analyses for ozone and water vapour. If these data are available,  in most cases, its accuracy is within 2 and 3 percent, if we do not account for adjacency effects and slope effects, and it may be worse for large viewing and solar angles (above 70°) or within strong absorption bands.

 

We are also providing a python code to use these coefficients. This code is described and available here.

References

[1] Rahman, H., & Dedieu, G. (1994). SMAC: a simplified method for the atmospheric correction of satellite measurements in the solar spectrum. REMOTE SENSING15(1), 123-143.

 

Real time production of land cover maps without terrain data of the current time period.

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With the new availability of repetitive image time series after atmospheric correction over France from the Theia Land Data Centre, it is now possible to imagine the automatic production of land cover maps continuously with the availability of new images.

 

In the framework of the SYRHIUS project, a prototype was developed at CESBIO to assess the results of this kind of classification method at the scale of a medium scale catchment. The study zone is the Fresquel catchment (937 km2), close to the famous medieval city of Carcassonne. The main crops present in this catchment are cereals, sunflower and vineyards, and also some corn and rapeseed.
A supervised classification is used, based on Support Vector Machines, but for which the learning data base is not derived from terrain surveys held during the time period to process, as in the classical supervised methods. The learning data base which is used is created from previous years observations and from terrains data acquired in the past. Such a method has the advantage of needing no terrain data on the present period, knowing that these data often come too late to allow a real time processing, but it requires a very large data volume from several years. In case of a time period with an exceptional climate, errors might arise if the training data base does not contain the necessary information to recognise the crops.


View of the real time land cover processor

 

To test this approach, we used the Common Agriculture Policy plot data base, for years 2011 and 2012, for the Fresquel catchment, along with LANDSAT5/7 time series, which allow a time evolution of reflectances for the plots in the data base. Both data sources were used to create the learning data base. which was then use to classify the data of 2013, 2014 and 2015 for the Fresquel catchment.

 

THEIA LANDSAT8 Level 2A (corrected from atmospheric effects and provided with a cloud mask) are used as input or the processor. Due to the late availability of the Commpon Agriculture Policy data base, we are not able to provide validation figures, but previous campaigns provided Kappa in the 0.65-0.7 range for Midi Pyrénées region.
Of course, at the beginning of the crop season, the available information is not complete and the accuracy might be reduced. For that reason, the nomenclature and the number of classes evolves with the number of available LANDSAT dates. Three key dates are used : end of March, end of July and end of year. For each of the dates a new land cover map is computed with an increased detail level, as shown in next figure.

 

Three land cover maps are produces along the year, first one (S1) in March, Second one in July (S2), and the last one at the end of the year with an increasing number of classes.

We will however stress the fact that steady observations are necessary, and that on certain years, the cloud cover might degrade the quality of the results, as in the case of spring 2013, for which the LANDSAT observations only started in April. In 2013, some parts of the Area where only observed 3 times along the whole year. The results at the beginning of season are quite bad, but they enhance along the year. For the subsequent years, results are better and should further enhance with the availability of Sentinel-2 and its far better observation frequency.

The SIRHYUS project

The SIRHYUS project aims at developping and setting operationnal services related to managing water resources thanks to the integration, assimilaton and valorisation of satellite earth observation  : Veolia Environnement Recherche&Innovations, Veolia Eau, EDF, G2C environnement, Acri ST, l’UMR TETIS-IRSTEA, le CNES, VERI et le CESBIO. It was funded by the 12th Fonds Unique Interministériel, by the ministry in charge of water  and by the Provence and Languedoc-Roussillon, and the aeronautics and space foundation.

The aim is to provide new services, based on the know how of experience companies. In this framework, CESBIO implemented or enhances methods for 4 products : snow cover, land cover, evaop-transpiration and soil water content. In the future, these products will be applied to Sentinel-2. In this framework, two posts will be published on this blog : tis one, and a second related to evapotranspiration estimates in this same catchment.

 

 

 

Yoann Moreau et Isabelle Soleihavoup