Analysis of flood downstream from the Nangbeto dam on the Mono River (Togo and Benin)

peer reviewedThe analysis of risk of flood downstream from the hydroelectric dam of Nangbeto in the Mono River was about the survey of interannual variability of rainfall and rivers flows from 1955 to 1999, the population vulnerability, the hydrological modification and the impacts downstream in Togo and Benin from the dam constructed in 1987. The analysis of the mean annuel discharge has revealed no tendency, showing thus that the change of land use, the increase of waterproof areas and even the functioning of the hydroelectric dam have not modified in a significant manner the river's rate of flow. Apart from the return to the improvement in rainfull as far back as the end of years 80, the anthropic pressure and the demographic explosion have best explained the magnitude and frequency of flooding downstream from the hydroelectric dam of Nangbeto. The soil degradation, the deforestation of the floodplain, the increase in the number of human settlements in the river boundary increases the vulnerability of population faced with flooding.L'analyse des risques d'inondation en aval du barrage hydroélectrique de Nangbéto sur le fleuve Mono a porté sur les variabilités interannuelles des précipitations et débits de 1955 à 1999, la vulnérabilité des populations, les modifications hydrologiques et les effets en aval au Togo et au Bénin du barrage construit en 1987. L'analyse des modules annuels n'a révélé aucune tendance, montrant ainsi que les changements intervenus dans l'affectation du sol, l'accroissement des surfaces imperméables et même le fonctionnement du barrage n'ont pas modifié de manière significative la variation interannuelle des débits annuels. Outre le retour à l'amélioration des pluviométries dès la fin des années 80, les pressions anthropiques et la poussée démographique expliquent mieux l'ampleur et la fréquence élevée des inondations en aval du barrage hydroélectrique de Nangbéto. La dégradation des sols, le déboisement des berges du fleuve et des zones inondables, l'augmentation des habitations dans le lit majeur même du fleuve ne font que fragiliser les populations riveraines et accroître leur vulnérabilité face aux inondations

Dynamic of carbon fluxes between the atmosphere and some West African ecosystems: a case study of forests and svannah under a Sudanian climate in Benin

In West Africa as elsewhere in other parts of the world, natural and anthropogenic ecosystems are the main sources for energy, building poles and livestock fodder. They probably behave like some nets carbon sinks, but there are only few studies that have focused on their carbon exchanges with the atmosphere. Preserving terrestrial ecosystems has proven essential to any emissions mitigation policy of green house gases, notably CO2. This thesis focuses on the analysis of the CO2 fluxes in three contrasting types of the ecosystem under a Sudanian climate in Benin. Specifically, it was a cultivated forest, a protected forest and a cultivated savannah. Turbulent CO2 fluxes were measured by an eddy-covariance system placed above the ecosystems. The measurements were made during variable periods of 17 months (cultivated forest), 18 months (protected forest) and 29 months (cultivated savannah). The three studied sites have been equipped by the hydro-meteorological observatory AMMA-CATCH and the national project Ouémé 2025. The CO2 fluxes data were completed by some meteorological measurements and by an inventory of dominating species the three sites. Fluxes were examined on the basis of the two main seasons imposed by the cycle of West African Monsoon. The spatial (local and regional) and temporal (hour, seasonal, annual and inter-annual) scales were considered according to the sites. Water was the main factor controlling the dynamic of the terrestrial West African ecosystems. At the daily scales, the radiation was the main driving variable of the net CO2 fluxes. Besides that radiation control, a clear relationship was observed between the net CO2 fluxes and the canopy conductance. A limited impact of saturation deficit was observed, notably during the afternoon. The soil moisture was the main factor governing the ecosystem respiration. However, no clear dependency of the ecosystem respiration on the temperature was observed at hourly scale. After long dry periods, the ecosystem respiration increased immediately in response to the first significant rains. The CO2 fluxes and the characteristics of the protected forest were always higher than those of the cultivated savannah at the daily and seasonal scales. At the annual scale, while the protected forest (Bellefoungou) and cultivated savannah (Nalohou) sequestered an average of 640 ± 50 and 232 ± 27 g C m-2, respectively considering the period of the measurements, the cultivated forest was in equilibrium with 29 ± 16 g C m-2. Overall, the three studied ecosystems were more efficient during morning and wet season than during afternoon and dry season. Finally, this study revealed that the inter-annual variability of the carbon sequestration by the cultivated savannah was mainly controlled by the ecosystem respiration variability in relation to the soil water content variation

CO2 measurement and carbon sequestration in West African terrestrial ecosystems. A review

En Afrique de l’Ouest comme ailleurs dans d’autres régions du monde, la préservation des écosystèmes terrestres s’avère primordiale pour toute politique d’atténuation des émissions de gaz à effet de serre, en particulier du dioxyde de carbone. Une évaluation pertinente de la contribution d’une région donnée au cycle global du carbone implique la nécessité de raisonner en termes de bilan en considérant dans l’analyse tous les flux d’exportation et d’importation dans les écosystèmes.In West Africa, as elsewhere in other parts of the world, preserving terrestrial ecosystems has proven essential to any emissions mitigation policy for greenhouse gases, notably carbon dioxide. A meaningful assessment of the contribution of a given region to the global carbon cycle implies the need to think in terms of balance, taking into account the totality of the carbon export and import within the ecosystem

Carbon dioxide fluxes from a degraded woodland in West Africa and their responses to main environmental factors

Background : In West Africa, natural ecosystems such as woodlands are the main source for energy, building poles and livestock fodder. They probably behave like net carbon sinks, but there are only few studies focusing on their carbon exchange with the atmosphere. Here, we have analyzed CO2 fluxes measured for 17 months by an eddy-covariance system over a degraded woodland in northern Benin. Specially, temporal evolution of the fluxes and their relationships with the main environmental factors were investigated between the seasons. Results : This study shows a clear response of CO2 absorption to photosynthetic photon flux density (Qp), but it varies according to the seasons. After a significant and long dry period, the ecosystem respiration (R) has increased immediately to the first significant rains. No clear dependency of ecosystem respiration on temperature has been observed. The degraded woodlands are probably the “carbon neutral” at the annual scale. The net ecosystem exchange (NEE) was negative during wet season and positive during dry season, and its annual accumulation was equal to +29 ± 16 g C m−2. The ecosystem appears to be more efficient in the morning and during the wet season than in the afternoon and during the dry season. Conclusions : This study shows diurnal and seasonal contrasted variations in the CO2 fluxes in relation to the alternation between dry and wet seasons. The Nangatchori site is close to the equilibrium state according to its carbon exchanges with the atmosphere. The length of the observation period was too short to justify the hypothesis about the “carbon neutrality” of the degraded woodlands at the annual scale in West Africa. Besides, the annual net ecosystem exchange depends on the intensity of disturbances due to the site management system. Further research works are needed to define a woodland management policy that might keep these ecosystems as carbon sinks

Diurnal and seasonal variability of CO2 fluxes over a degraded Woodland under a Sudanian climate in Northern Benin, West Africa

Turbulent CO2 exchanges over a degraded woodland were measured during 17 months (from November 2005 to March 2007) by an eddy-covariance system at Nangatchori in the northern part of Benin, West Africa. The site (Lat 9.65°N, Long 1.74°E, Alt: 432 m), under a Sudanian climate, is one of the sites that were equipped in the framework of the international AMMA-CATH program. The site was highly disturbed during preceding years by illegal tree logging, agricultural activities, cattle pasture, and bushfire. The footprint area is mainly formed by herbs and crops with some sparse shrubs and trees. Fluxes data were completed during the same period by meteorological measurements made at the Nalohou site located approximately 20 km from Nangatchori, and by an inventory of dominating species on 1km2 area around the tower during the wet season. Fluxes response to climatic variables was analyzed. The annual drought and moisture cycle was found to be the main controlling factor of the ecosystem dynamics. A very clear response of CO2 fluxes to PPFD appears, but is different according to seasons. During wet season, CO2 uptake increases with increasing PPFD following a typical curvilinear function and saturates for high PPFD (PPFD > 1000 µmol m-2 s-1), while during dry season, a very weak linear response of CO2 fluxes was observed. No clear dependency of the total ecosystem respiration on temperature was observed. At an annual scale (from November 1st 2005 to October 31st 2006), net carbon sequestered by the ecosystem was 18 +- 5 g C m-2. Finally, with respect to the water use the ecosystem appeared to be more efficient during morning and wet season than during afternoon and dry period

Response of CO2 fluxes and productivity to water availability in two contrasting ecosystems in northern Benin (West Africa)

CO (2) fluxes were measured during 18 months in a forest and a savannah in northern Benin. Higher values of carbon fluxes were found during the wet season at each site. A strong dependency of carbon fluxes on water relations was found in two contrasting sites. The forest sequestered 640 +/- 50 and the savannah 190 +/- 40 g C m (-2) year (-1) . In West Africa, the main mechanisms or factors governing the dynamics of ecosystems, especially the dynamics of the carbon fluxes and productivity, still remain less known. This study reports the carbon fluxes over two contrasting ecosystems, notably a protected forest (lat 9.79A degrees N, long 1.72A degrees E, alt 414 m) and a cultivated savannah (lat 9.74A degrees N, long 1.60A degrees E, alt 449 m) in northern Benin. The two sites were among those equipped by the AMMA-CATCH observatory and Ou,m, 2025 project. Flux data were analyzed at the daily and seasonal scales in order to understand their controlling variables. We discussed the patterns of CO2 fluxes and the characteristics of the two ecosystems. The study also focused on the different water usage strategies developed by the two ecosystems since the alternation between dry and wet seasons highly influenced the seasonal dynamics. Finally, the annual carbon sequestration was estimated together with its uncertainty. The carbon fluxes were measured during 18 months (July 2008-December 2009) by an eddy-covariance system over two contrasting sites in northern Benin. Fluxes data were computed following the standard procedure. The responses of CO2 fluxes to the principal climatic and edaphic factors, and the canopy conductance were studied. A clear CO2 fluxes response to main environmental factors was observed, however with difference according to the seasons and vegetation types. The ecosystem respiration showed the highest values during the wet season and a progressive decrease from wet to dry periods. Also, the carbon uptake values were high during the wet period, but low during the dry period. However, the CO2 fluxes for the protected forest were always higher than that for the cultivated savannah within each defined period. This was due to the seasonal changes not only in phenology and physiology but also to the acclimation to environmental conditions, especially to the soil water availability. The water use efficiency was influenced by VPD during the day conditions for two ecosystems. However, the VPD response curve of water usage was relatively constant for the protected forest during the transitional and wet seasons. In contrary, for the cultivated savannah the VPD response decreased about 46 % from transitional dry-wet to wet seasons and remained relatively constant until transitional wet-dry season. The close relationships between the net CO2 assimilation and the canopy conductance were found for the two ecosystems. This suggests a regulation of the stomata by a partial stomatal closure besides the radiation control. Finally, the forest sequestered at the annual scale 640 +/- 50 and the savannah 190 +/- 40 g C m(-2) year(-1). We conclude with a strong relation between the carbon fluxes and water in the two investigated ecosystems. Apart from the radiation control, the stomata also play an important role in the regulation of the CO2 assimilation in the two ecosystems