Human impact on erosion and sedimentation

Cet article présente les premiers résultats obtenus sur les transports solides en suspension de trois petits bassins versants au Mali, en zone de savane humide. Les flux annuels de matières en suspension y sont relativement faibles, compris entre 4 et 45 t km-2 an-1, et varient dans le même sens que la pluviométrie et les écoulements annuels. Les transports solides varient également au cours de la saison des pluies, quelques crues importantes pouvant assurer l'essentiel du tonnage annuel exporté. L'influence de la mise en culture sur l'érosion est étudiée au pas de temps de la crue sur les deux bassins dont les occupations du sol sont les plus contrastées. Le bassin le plus cultivé s'avère être le plus ruisselant (en terme d'intensité maximale), ce qui favorise l'exportation de matières en suspension. (Résumé d’auteur

Etude par capteur optique du dépôt formé lors de la filtration d'une suspension de bentonite sur fibre creuse

Lors de la filtration sur fibres creuses de suspensions de bentonite, il a été mis en évidence que le facteur limitant est la formation d'un dépôt à la surface de la fibre.Parlant de ces considérations, un dispositif optique a été mis au point pour suivre l'évolution spatio-temporelle du dépôt en cours de filtration sur fibre creuse à peau externe.Ainsi, il a pu être mis en évidence que l'influence de la vitesse d'écoulement n'est sensible qu'après un certain temps de filtration ou un certain volume filtré.Dans le domaine testé des faibles vitesses d'écoulement, il existe une période initiale où la loi de filtration sur gâteau est applicable.Dans le cas de fibres creuses à peau externe, l'optimum technique (productivité maximale) ou économique (coût minimal du m3 traité) serait donc plus à rechercher dans une optimisation de la séquence filtration-lavage que dans une augmentation de la vitesse d'écoulement.L'efficacité du réentraînement du dépôt lors du lavage peut être suivie grâce à l'équipement et au capteur développés.Use of membranes (MF or UF) for water treatment is now developed at an industrial scale. This approach is considered as a good solution to deal with the periodic strong increases in suspended solids contents which occur in karstic aquifers.Under these conditions, the main contribution to the growth of hydraulic resistance is related to the formation of a deposit on the membrane surface.Thus, an experimental study was performed at the laboratory scale in order to perform on line characterization of deposit formation during the filtration of a bentonite suspension through a hollow fiber.The filtration module is transparent and contains only one hollow fiber with its external skin.An optical device formerly developed was used as a sensor : a horizontal laser beam is focussed at the surface of the hollow fiber which is held perpendicular to the beam and can be moved step by step in the micrometric range.Experiments were performed with a bentonite suspension (concentration 0.375 kg/m3) and under different operating conditions :- dead end filtration and crossflow filtration with velocities up to 0,30 m/s in the annular space (which means Re ≈ 2000, velocity gradient ≈ 1000 S-1, shear stress ≈ 1 Pa) ;-transmembrane pressure between 85 and 185 Kpa.One of the main conclusions of this study is that influence of the velocity appears only after an initial period during which the growth of the deposit and the filtrate flowrate variation with time are in agreement with the equations of dead end filtration at constant pressure.Thus a combination of dead end filtration and periodic backwash for removing the deposit might be the best solution for getting the highest productivity with minimal energy consumption.These optimal conditions may be simulated using the equations of dead end filtration at constant pressure and making the assumption that the backwashing is fully efficicent.The experimental study of backwashing, using the optical sensor for measuring the diameter of the fiber after each backwashing, proves that this assumption is correct and that it is possible to get a steady mean filtrate flux by periodic backwashing.From the theoretical study, the optimal duration of the filtration period is rather short (about 10 minutes) but the decline after the maximum is very malt, which means that the fiitration period may be longer without significative variation of the productivity.Thus the method presented here is able, if applied to actual raw water, to give technical and economical elements of comparison between dead end filtration with periodic backwashing and crossflow filtration

Influence des couvertures lithologiques et végétales sur les régimes et la qualité des eaux des affluents congolais du fleuve Congo

Les affluents et sous affluents congolais du fleuve Congo se partagent en deux systèmes hydrologiques, qui, tout en étant proches dans l'espace, sont sensiblement différents dans leur fonctionnement. Cette distinction repose sur la différenciation manifeste entre les deux régions naturelles que sont les "Plateaux Tékés" d'une part et la "Cuvette congolaise" d'autre part, nettement contrastées du point de vue du couvert végétal et lithologique et subsidiairement du climatLa classification hydrologique que nous présentons est basée sur l'étude des coefficients d'écoulement interannuel (rapport de la lame d'eau écoulée sur la lame d'eau précipitée, exprimé en pourcentage) et sur le coefficient d'irrégularité saisonnière moyen des débits qui est le rapport moyen sur les chroniques disponibles, entre débits mensuels maximum et débits mensuels minimum par année hydrologique. Pour les rivières Tékés, le premier paramètre présente des valeurs comprises entre 45% et 60% contre 20% à 30% pour la "Cuvette Congolaise", alors que le deuxième paramètre varie respectivement pour ces deux régions de 1,1 à 1,5 et de 2,5 à 5,5.Cette classification présente l'avantage d'être à la fois adaptée à ces deux contextes régionaux voisins mais aussi de mettre en évidence le rôle prépondérant des formations géologiques et des couvertures végétales sur les écoulements des cours d'eau concernés, ainsi que sur la qualité des eaux.En effet, après son passage au travers d'un important aquifère gréseux, les eaux des Plateaux Tékés à couverture de savane font partie des "eaux claires", à très faible minéralisation, marquée essentiellement par la présence de silice dissoute. En revanche, les " eaux noires " couleur de thé qui drainent la Cuvette Congolaise, couverte de forêts, sont très riches en matière organique et très acidesThe Congolese tributaries and sub-tributaries of the Congo river come from two adjacent regions that have similar annual rainfall volumes (1,700 and 1,900 mm.year-1) but very different physiographic characteristics. These latter characteristics are the main reasons accounting for the differences in their hydrological behavior. The purpose of this work is to introduce a regional hydrological classification - adapted to the particularities of the regions that are crossed by these tributaries - that points out the major influence of the geological formations and the vegetative cover on the runoff of the rivers, as well as on their water qualities.The two natural regions concerned are the Téké plateaus and the Congolese basin. The first region, centered on Congo but also spread into Gabon in its western part and into Congo on its eastern side, consists of sand-gritty formations of the Tertiary going from 200 to 400 m in thickness, covered with savanna. The second region builds up a continuation of the wide Congo depression that occupies the whole center of the Congo river basin. It comprises Quaternary alluvial deposits, covered with swamps and dense equatorial rain forests. The hydrological characterization we present here is based on two hydrological coefficients that do not have physical linkages but illustrate the differences in the regimes of their respective flows. These are the interannual runoff coefficient (KE) and the average seasonal variability of the discharges (CIS) observed over a period of forty years. Concerning the Téké rivers, the first parameter (KE) has values ranging between 45 % and 60 % versus 20 % o 30 % for the Congolese basin. The second parameter (CIS) has values ranging from 1.1 to 1.5 for the Téké rivers, versus 2.5 to 5.5 for the Congolese basin. The specific discharges of the first region vary from 25 to 35 l.s-1.km-2 versus 10 and 15 l.s-1.km-2 for the second region.Concerning the Téké plateaus, the weak average seasonal variations of the discharges can only be explained by a great capacity for storage, and consequently for flow regulation, of the sand-gritty aquifer. Infiltration of the rainfall waters is important and they are mostly replaced by this important aquifer which attenuates the floods and helps minimize the droughts. This is also the reason why it is in this region that some of the most regular rivers of the planet are encountered.Concerning the Congolese basin, the lower permeability of the soils, the interception of rain waters, the evapotranspiration of the forest cover, and the direct evaporation on the floodable areas as well as the swamps, lead to an important water deficit. As there is no aquifer, at least none as important and regulating as in the Téké plateaus, the hydrological regime is more similar to the regional rainfall rhythm. The Téké hydrological regimes are for their part practically independent of the regional rainfall regime.From a qualitative point of view, these waters are also very distinct. Indeed, after crossing the important gritty aquifer, the waters of the Téké plateau - which is covered with savanna - consist of highly diluted clear waters, relatively rich in dissolved silica. These waters, with dissolved inorganic matter ranging from 1 to 3 mg.l-1 (not taking into account the dissolved silica), are similar in composition to rainfall waters. These are among the most dilute surface waters of the world, the soluble ions of the mentioned formations having been almost totally leached. On the other hand, waters draining the Congolese basin originate from a long residence time under the forest cover. They are very rich in organic matter (up to 44 % of particulate organic carbon) and very acid (pH can be lower than 4). They are truly qualified as "black rivers". Their mineralization, although low (11 to 30 mg.l-1, without dissolved silica), can be considered high when compared to the plateaus' waters, and also of a greater diversity. For both regions, the content in dissolved silica is identical to the world wide average, varying between 8 and 10 mg.l-1.If the specific export rates in dissolved matter are low, they are nevertheless identical in the two geographical domains despite the important differences in concentrations. For matter of mineral and organic origins, we calculate respectively 10 t.km-2.year-1 in the basin versus 9.7 t.km-2.year-1 on the plateaus, and 19.3 t.km-2.year-1 in the forest versus 13.5 t.km-2.year-1 in the savanna. This apparent similitude is obviously linked to the compensation by the specific discharges of the Plateau rivers, more than twice as much as in the basin

X Chromosome Inactivation and Differentiation Occur Readily in ES Cells Doubly-Deficient for MacroH2A1 and MacroH2A2

Macrohistones (mH2As) are unusual histone variants found exclusively in vertebrate chromatin. In mice, the H2afy gene encodes two splice variants, mH2A1.1 and mH2A1.2 and a second gene, H2afy2, encodes an additional mH2A2 protein. Both mH2A isoforms have been found enriched on the inactive X chromosome (Xi) in differentiated mammalian female cells, and are incorporated into the chromatin of developmentally-regulated genes. To investigate the functional significance of mH2A isoforms for X chromosome inactivation (XCI), we produced male and female embryonic stem cell (ESC) lines with stably-integrated shRNA constructs that simultaneously target both mH2A1 and mH2A2. Surprisingly, we find that female ESCs deficient for both mH2A1 and mH2A2 readily execute and maintain XCI upon differentiation. Furthermore, male and female mH2A-deficient ESCs proliferate normally under pluripotency culture conditions, and respond to several standard differentiation procedures efficiently. Our results show that XCI can readily proceed with substantially reduced total mH2A content

A Large Fraction of Extragenic RNA Pol II Transcription Sites Overlap Enhancers

A substantial fraction of extragenic Pol II transcription sites coincides with transcriptional enhancers, which may be relevant for functional annotation of mammalian genomes

Evolutionary diversity and developmental regulation of X-chromosome inactivation

X-chromosome inactivation (XCI) results in the transcriptional silencing of one X-chromosome in females to attain gene dosage parity between XX female and XY male mammals. Mammals appear to have developed rather diverse strategies to initiate XCI in early development. In placental mammals XCI depends on the regulatory noncoding RNA X-inactive specific transcript (Xist), which is absent in marsupials and monotremes. Surprisingly, even placental mammals show differences in the initiation of XCI in terms of Xist regulation and the timing to acquire dosage compensation. Despite this, all placental mammals achieve chromosome-wide gene silencing at some point in development, and this is maintained by epigenetic marks such as chromatin modifications and DNA methylation. In this review, we will summarise recent findings concerning the events that occur downstream of Xist RNA coating of the inactive X-chromosome (Xi) to ensure its heterochromatinization and the maintenance of the inactive state in the mouse and highlight similarities and differences between mammals

Spécial Erosion

Dans le contexte climatique de la sécheresse, la maîtrise de l'eau et la conservation des sols représentent un objectif prioritaire pour le Burkina Faso. Les procédés de lutte antiérosive peuvent s'inspirer aujourd'hui de toute une gamme de techniques traditionnelles, fruit d'initiatives paysannes spontanées. Il s'agit soit de procédés mécaniques (cordons de pierres, fascinage, plantations d'herbes ou d'arbustes), soit de pratiques biologiques ou culturales (cultures associées, paillage, buttage). Depuis le début des années 1970, deux organismes publics ont lancé des programmes de conservation faisant appel à la technique des bandes d'arrêt (A.V.V. : Aménagement des Vallées des Voltas) ou des terrasses de diversion (F.E.E.R. : Fonds de l'Eau et de l'Equipement Rural). Dans le cas du F.E.E.R. la participation paysanne est effective durant la phase d'élaboration. En revanche, l'entretien des dispositifs (47 000 hectares traités en 1977 et 1985) reste très aléatoire du fait de contraintes variées et complexes. Sur le plan technique, des enseignements utiles, du moins pour les régions septentrionales, peuvent être tirés des expérimentations animées par certaines organisations non gouvernementales. Toutefois toutes les stratégies conservatoires dépendent probablement davantage d'éventuelles réformes foncières et de mesures économiques propres à enrayer l'émigration. (Résumé d'auteur