Analyse des écoulements dans le bassin du fleuve Sénégal de 1960 à 2008

L’hydrologie du fleuve Sénégal est tributaire de l’influence cumulée de la variabilité climatique et des barrages de Diama et de Manantali depuis la mise en service de ces derniers. Aujourd’hui, dans la vallée du fleuve, on assiste à une recrudescence des inondations. C’est dans ce contexte que nous nous proposons d’étudier l’évolution du régime hydrologique du fleuve Sénégal afin d’appréhender, à la fois, l’effet de la variabilité climatique et des barrages sur les écoulements. Une approche statistique associant plusieurs méthodes complémentaires (indices centrés et réduits, tests de détection de ruptures des moyennes et procédure de segmentation des séries) a été utilisée. Nos analyses nous ont permis de constater, sur le bassin du fleuve Sénégal, la rupture climatique des années 1970 qui s’est traduite par une diminution de l’écoulement moyen annuel mais aussi des cotes maximales annuelles et minimales annuelles. Cependant, une reprise significative des écoulements moyens annuels est observée à partir des environs de l’année 1994, ce qui atteste l’entrée dans une nouvelle période climatique plus humide que celle des décennies 1970 et 1980 à l’échelle du bassin du fleuve Sénégal. En outre, dans la vallée du fleuve, l’écoulement moyen annuel est renforcé, depuis la fin des années 1980 et le début des années 1990, par l’effet cumulé des barrages (par leurs impacts sur l’occupation du sol) et de la variabilité climatique. En effet, les barrages ont entraîné une hausse des crues maximales annuelles à la station de Bakel depuis 1994 et un soutien considérable des étiages dans toute la vallée du fleuve Sénégal.Since the commissioning of Diama and Manantali dams, the hydrological regime of the Senegal river depends on the cumulative impact of climate variability and dams. Nowadays, in the valley of the river there is an increase in flooding. The aim of this research was to analyze the evolution of the hydrological regime of the Senegal River in order to determine the effect of both climate variability and the dams on stream flows. A statistical approach combining several complementary methods (standard scores indices, tests for statistical ruptures and a segmentation series procedure) was used. Our analysis revealed that in the Senegal River Basin, a climatic break in the 1970s resulted in a decrease in the mean annual discharge and also in the annual maximum and minimum water levels. However, a significant recovery of the average annual flow is observed from around 1994, demonstrating the occurrence of a new climatic period, wetter than the 1970s and 1980s, across the Senegal River basin. In addition, in the river valley, the yearly mean of the flow is supported, since the late 1980s and early 1990s, by the cumulative effect of the dams (by their impact on land use) and climate variability. Furthermore, dams have led to an increase of the maximum annual flood levels at the Bakel station since 1994, and to a considerable support of low flows across the entire Senegal River Valley

Hydrological responses to rainfall variability and dam construction: a case study of the upper Senegal River basin

International audienceThe understanding of the spatial and temporal dynamic of river systems is essential for developing sustainable water resource management plan. For the Senegal River, this subject is very complex according to the context of (1) transboundary basin, (2) several contrasted climatic zones (Guinea, South Sudanian, North Sudanian and Sahelian) with high rainfall variability and (3) high human pressures (dam construction and water uses). From 1954 to 2000, 80% (mean value) of the Senegal River flows recorded downstream part of the basin are provided by three majors tributaries (Bafing, Bakoye and Faléme) located in the upstream part. Then, in our study, this upper Senegal River basin was chosen in order to investigate the hydrological responses to rainfall variability and dam construction. Two nonparametric statistical methods, Mann–Kendall and Hubert test, were used to detect the long-term changes in the time series of precipitation and water discharge (1954–2000) at the annual and seasonal scales. The continuous wavelet transform (Morlet Wavelet) was employed to characterize the different mode in the water discharge variability. Flow duration curve and cumulative curve methods were used to assess the impact of dams on the hydrological regime of the Senegal River. Results showed that the Senegal River flows have been changing under the influence of both rainfall variation and dam construction. The long-term evolution of water discharge depend on long-term rainfall variability: The wet periods of the 1950s and 1960s correspond to periods of higher river flows, while the droughts of the 1970s and 1980s led to unprecedented river flows deficits. The new period, since 1994, show a high inter-annual variability of rainfall and discharge without clear trend. At seasonal scale, the results showed also a strong relationship between rainfall and runoff (R 2 > 0.8) resulting from alternating wet and dry seasons and rapid hydrological responses according to annual rainfall. Nevertheless, the observed flows during dry seasons highlighted the influence of water storage and restitution of infiltrated waters in soils and surficial formations during wet seasons. In the dry seasons, the water budget of the three upstream tributaries showed a water deficit at the downstream gauging station. This deficit was characterized by water loss to underlying aquifers and highlighted the influence of geological setting on water balance. However, in this context, water restitution during the dry season remained dependent on climatic zone and on the total annual rainfall volume during the previous wet season. The results have highlighted an impact of the Manantali dam previously obscured: The dam has no effect on the regulation of high river flows. That is what explains that since its construction in 1988, flooding of coastal cities, like Saint-Louis, by seasonal river floods has not ceased. The flooding risk in coastal cities is not avoided, and the dams caused hyper-salinization of the Senegal lower estuary. The breach created in the coastal barrier of the Langue of Barbary in October 2003 promotes direct export of excess floodwater to the sea and reduces this risk of flooding in the delta area. But, this solution led to considerable loss of potential water resources, and the authors recommend a new water management plan with a global focus. However, this study shows the positives impacts of the two dams. They allow the availability of freshwater in order to support agricultural irrigation in the valley and delta zone, in particular during low flows periods

Additional file 10: Figure S8. of Epidemiology of influenza in West Africa after the 2009 influenza A(H1N1) pandemic, 2010–2012

Aggregate of influenza surveillance for influenza-like illness (ILI) and severe acute respiratory illness (SARI) data in Togo: 2010–2012. (DOCX 40 kb

Additional file 4: Figure S2. of Epidemiology of influenza in West Africa after the 2009 influenza A(H1N1) pandemic, 2010–2012

Aggregate of influenza surveillance for influenza-like illness (ILI) and severe acute respiratory illness (SARI) data in Cote d’Ivoire: 2010–2012. (DOCX 20 kb

Additional file 8: Figure S6. of Epidemiology of influenza in West Africa after the 2009 influenza A(H1N1) pandemic, 2010–2012

Aggregate of influenza surveillance for influenza-like illness (ILI) and severe acute respiratory illness (SARI) data in Nigeria: 2010–2012. (DOCX 18 kb

Additional file 2: of Epidemiology of influenza in West Africa after the 2009 influenza A(H1N1) pandemic, 2010–2012

Supplemental Information ILI/SARI Case Definitions and sample strategy for ILI by country. (DOCX 11 kb

Additional file 6: Figure S4. of Epidemiology of influenza in West Africa after the 2009 influenza A(H1N1) pandemic, 2010–2012

Aggregate of influenza surveillance for influenza-like illness (ILI) and severe acute respiratory illness (SARI) data in Mauritania: 2010–2012. (DOCX 17 kb

Additional file 9: Figure S7. of Epidemiology of influenza in West Africa after the 2009 influenza A(H1N1) pandemic, 2010–2012

Aggregate of influenza surveillance for influenza-like illness (ILI) and severe acute respiratory illness (SARI) data in Sierra Leone: 2010–2012. (DOCX 37 kb

Additional file 3: Figure S1. of Epidemiology of influenza in West Africa after the 2009 influenza A(H1N1) pandemic, 2010–2012

Aggregate of influenza surveillance for influenza-like illness (ILI) and severe acute respiratory illness (SARI) data in Burkina Faso: 2010–2012. (DOCX 39 kb