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

Recent Budget of Hydroclimatology and Hydrosedimentology of the Congo River in Central Africa

Although the Congo Basin is still one of the least studied river basins in the world, this paper attempts to provide a multidisciplinary but non-exhaustive synthesis on the general hydrology of the Congo River by highlighting some points of interest and some particular results obtained over a century of surveys and scientific studies. The Congo River is especially marked by its hydrological regularity only interrupted by the wet decade of 1960, which is its major anomaly over nearly 120 years of daily observations. Its interannual flow is 40,500 m3 s−1. This great flow regularity should not hide important spatial variations. As an example, we can cite the Ubangi basin, which is the most northern and the most affected by a reduction in flow, which has been a cause for concern since 1970 and constitutes a serious hindrance for river navigation. With regard to material fluxes, nearly 88 × 106 tonnes of material are exported annually from the Congo Basin to the Atlantic Ocean, composed of 33.6 × 106 tonnes of TSS, 38.1 × 106 tonnes of TDS and 16.2 × 106 tonnes of DOC. In this ancient flat basin, the absence of mountains chains and the extent of its coverage by dense rainforest explains that chemical weathering (10.6 t km−2 year−1 of TDS) slightly predominates physical erosion (9.3 t km−2 year−1 of TSS), followed by organic production (4.5 t km−2 year−1 of DOC). As the interannual mean discharges are similar, it can be assumed that these interannual averages of material fluxes, calculated over the longest period (2006–2017) of monthly monitoring of its sedimentology and bio-physical-chemistry, are therefore representative of the flow record available since 1902 (with the exception of the wet decade of 1960). Spatial heterogeneity within the Congo Basin has made it possible to establish an original hydrological classification of right bank tributaries, which takes into account vegetation cover and lithology to explain their hydrological regimes. Those of the Batéké plateau present a hydroclimatic paradox with hydrological regimes that are among the most stable on the planet, but also with some of the most pristine waters as a result of the intense drainage of an immense sandy-sandstone aquifer. This aquifer contributes to the regularity of the Congo River flows, as does the buffer role of the mysterious “Cuvette Centrale”. As the study of this last one sector can only be done indirectly, this paper presents its first hydrological regime calculated by inter-gauging station water balance. Without neglecting the indispensable in situ work, the contributions of remote sensing and numerical modelling should be increasingly used to try to circumvent the dramatic lack of field data that persists in this basin

Congo Basin peatlands: threats and conservation priorities

The recent publication of the first spatially explicit map of peatlands in the Cuvette Centrale, central Congo Basin, reveals it to be the most extensive tropical peatland complex, at ca. 145,500 km2. With an estimated 30.6 Pg of carbon stored in these peatlands, there are now questions about whether these carbon stocks are under threat and, if so, what can be done to protect them. Here, we analyse the potential threats to Congo Basin peat carbon stocks and identify knowledge gaps in relation to these threats, and to how the peatland systems might respond. Climate change emerges as a particularly pressing concern, given its potential to destabilise carbon stocks across the whole area. Socio-economic developments are increasing across central Africa and, whilst much of the peatland area is protected on paper by some form of conservation designation, the potential exists for hydrocarbon exploration, logging, plantations and other forms of disturbance to significantly damage the peatland ecosystems. The low level of human intervention at present suggests that the opportunity still exists to protect the peatlands in a largely intact state, possibly drawing on climate change mitigation funding, which can be used not only to protect the peat carbon pool but also to improve the livelihoods of people living in and around these peatlands

Age, extent and carbon storage of the central Congo Basin peatland complex

Peatlands are carbon-rich ecosystems that cover just three per cent of Earth's land surface, but store one-third of soil carbon. Peat soils are formed by the build-up of partially decomposed organic matter under waterlogged anoxic conditions. Most peat is found in cool climatic regions where unimpeded decomposition is slower, but deposits are also found under some tropical swamp forests. Here we present field measurements from one of the world's most extensive regions of swamp forest, the Cuvette Centrale depression in the central Congo Basin. We find extensive peat deposits beneath the swamp forest vegetation (peat defined as material with an organic matter content of at least 65 per cent to a depth of at least 0.3 metres). Radiocarbon dates indicate that peat began accumulating from about 10,600 years ago, coincident with the onset of more humid conditions in central Africa at the beginning of the Holocene. The peatlands occupy large interfluvial basins, and seem to be largely rain-fed and ombrotrophic-like (of low nutrient status) systems. Although the peat layer is relatively shallow (with a maximum depth of 5.9 metres and a median depth of 2.0 metres), by combining in situ and remotely sensed data, we estimate the area of peat to be approximately 145,500 square kilometres (95 per cent confidence interval of 131,900-156,400 square kilometres), making the Cuvette Centrale the most extensive peatland complex in the tropics. This area is more than five times the maximum possible area reported for the Congo Basin in a recent synthesis of pantropical peat extent. We estimate that the peatlands store approximately 30.6 petagrams (30.6 × 10(15) grams) of carbon belowground (95 per cent confidence interval of 6.3-46.8 petagrams of carbon)-a quantity that is similar to the above-ground carbon stocks of the tropical forests of the entire Congo Basin. Our result for the Cuvette Centrale increases the best estimate of global tropical peatland carbon stocks by 36 per cent, to 104.7 petagrams of carbon (minimum estimate of 69.6 petagrams of carbon; maximum estimate of 129.8 petagrams of carbon). This stored carbon is vulnerable to land-use change and any future reduction in precipitation

Proliferation of Hydroelectric Dams in the Andean Amazon and Implications for Andes-Amazon Connectivity

Due to rising energy demands and abundant untapped potential, hydropower projects are rapidly increasing in the Neotropics. This is especially true in the wet and rugged Andean Amazon, where regional governments are prioritizing new hydroelectric dams as the centerpiece of long-term energy plans. However, the current planning for hydropower lacks adequate regional and basin-scale assessment of potential ecological impacts. This lack of strategic planning is particularly problematic given the intimate link between the Andes and Amazonian flood plain, together one of the most species rich zones on Earth. We examined the potential ecological impacts, in terms of river connectivity and forest loss, of the planned proliferation of hydroelectric dams across all Andean tributaries of the Amazon River. Considering data on the full portfolios of existing and planned dams, along with data on roads and transmission line systems, we developed a new conceptual framework to estimate the relative impacts of all planned dams. There are plans for 151 new dams greater than 2 MW over the next 20 years, more than a 300% increase. These dams would include five of the six major Andean tributaries of the Amazon. Our ecological impact analysis classified 47% of the potential new dams as high impact and just 19% as low impact. Sixty percent of the dams would cause the first major break in connectivity between protected Andean headwaters and the lowland Amazon. More than 80% would drive deforestation due to new roads, transmission lines, or inundation. We conclude with a discussion of three major policy implications of these findings. 1) There is a critical need for further strategic regional and basin scale evaluation of dams. 2) There is an urgent need for a strategic plan to maintain Andes-Amazon connectivity. 3) Reconsideration of hydropower as a low-impact energy source in the Neotropics

Recommendations for the diagnosis of pediatric tuberculosis

Tuberculosis (TB) is still the world's second most frequent cause of death due to infectious diseases after HIV infection, and this has aroused greater interest in identifying and managing exposed subjects, whether they are simply infected or have developed one of the clinical variants of the disease. Unfortunately, not even the latest laboratory techniques are always successful in identifying affected children because they are more likely to have negative cultures and tuberculin skin test results, equivocal chest X-ray findings, and atypical clinical manifestations than adults. Furthermore, they are at greater risk of progressing from infection to active disease, particularly if they are very young. Consequently, pediatricians have to use different diagnostic strategies that specifically address the needs of children. This document describes the recommendations of a group of scientific societies concerning the signs and symptoms suggesting pediatric TB, and the diagnostic approach towards children with suspected disease

The severity of pandemic H1N1 influenza in the United States, from April to July 2009: A Bayesian analysis

Background: Accurate measures of the severity of pandemic (H1N1) 2009 influenza (pH1N1) are needed to assess the likely impact of an anticipated resurgence in the autumn in the Northern Hemisphere. Severity has been difficult to measure because jurisdictions with large numbers of deaths and other severe outcomes have had too many cases to assess the total number with confidence. Also, detection of severe cases may be more likely, resulting in overestimation of the severity of an average case. We sought to estimate the probabilities that symptomatic infection would lead to hospitalization, ICU admission, and death by combining data from multiple sources. Methods and Findings: We used complementary data from two US cities: Milwaukee attempted to identify cases of medically attended infection whether or not they required hospitalization, while New York City focused on the identification of hospitalizations, intensive care admission or mechanical ventilation (hereafter, ICU), and deaths. New York data were used to estimate numerators for ICU and death, and two sources of data - medically attended cases in Milwaukee or self-reported influenza-like illness (ILI) in New York - were used to estimate ratios of symptomatic cases to hospitalizations. Combining these data with estimates of the fraction detected for each level of severity, we estimated the proportion of symptomatic patients who died (symptomatic case-fatality ratio, sCFR), required ICU (sCIR), and required hospitalization (sCHR), overall and by age category. Evidence, prior information, and associated uncertainty were analyzed in a Bayesian evidence synthesis framework. Using medically attended cases and estimates of the proportion of symptomatic cases medically attended, we estimated an sCFR of 0.048% (95% credible interval [CI] 0.026%-0.096%), sCIR of 0.239% (0.134%-0.458%), and sCHR of 1.44% (0.83%-2.64%). Using self-reported ILI, we obtained estimates approximately 7-96lower. sCFR and sCIR appear to be highest in persons aged 18 y and older, and lowest in children aged 5-17 y. sCHR appears to be lowest in persons aged 5-17; our data were too sparse to allow us to determine the group in which it was the highest. Conclusions: These estimates suggest that an autumn-winter pandemic wave of pH1N1 with comparable severity per case could lead to a number of deaths in the range from considerably below that associated with seasonal influenza to slightly higher, but with the greatest impact in children aged 0-4 and adults 18-64. These estimates of impact depend on assumptions about total incidence of infection and would be larger if incidence of symptomatic infection were higher or shifted toward adults, if viral virulence increased, or if suboptimal treatment resulted from stress on the health care system; numbers would decrease if the total proportion of the population symptomatically infected were lower than assumed.published_or_final_versio