Floodplain wetlands in tropical river basins play a disproportionate role in carbon cycling, yet their contribution to greenhouse-gas emissions in Central Africa remains poorly constrained. We quantified dissolved carbon, CO2 and CH4 concentrations, together with floating-chamber fluxes, across small streams, flooded forests, lakes, and the main channel of the Lower Ogooué River floodplain (Gabon) during two representative wet-season campaigns. Dissolved organic carbon (5–50 mg L−1), pCO2 (1500–16,000 µatm), and dissolved CH4 showed their highest observed values in small tributaries and flooded forests, whereas the main stem generally exhibited lower values. The highest observed chamber-measured CO2 fluxes occurred in tributaries, although differences among water body types were not statistically significant. Despite 5–10-fold spatial variability in concentrations, CO2 fluxes varied by only ∼2-fold among water body types and showed little diel or between-campaign variability. Dissolved oxygen was strongly associated with CO2 distributions, whereas its relationship with CH4 was weaker and season-dependent. First-order area-based upscaling suggests that flooded forests and floodplain lakes were the principal contributors to wet-season CO2 evasion, whereas the main river contributed a smaller share. Limited chamber-based CH4 flux observations (n = 5, one campaign) suggest that flooded forests may be methane-emission hotspots, but do not support quantitative floodplain-scale partitioning. Scenario calculations applying wet-season CO2 emission factors to assumed regional water coverage suggest that aquatic evasion could exceed downstream dissolved-carbon export by several-fold; however, these values represent order-of-magnitude illustrations rather than measured annual budgets. These results highlight the need to explicitly account for floodplain hydrology, wetland extent, and aquatic-landscape heterogeneity when constraining carbon budgets and projecting responses of humid tropical river basins to climate-driven changes in flooding dynamics.

Abstract This chapter explores the hydrology of the Ogooué (Gabon) and Sanaga (Cameroon) river basins—two critical but understudied watersheds within the broader Central African hydro-ecological system. While both rivers discharge independently into the Atlantic Ocean, they are closely tied to the Congo Basin Forest through shared climatic drivers, ecological continuity, and significant contributions to regional water cycles and biodiversity. The chapter provides an in-depth analysis of their geomorphology, hydrological regimes, and socioeconomic importance, highlighting the Ogooué’s largely pristine, forest-dominated catchment and the Sanaga’s heavily regulated hydrosystem dominated by hydropower infrastructure. Long-term observational and satellite data reveal trends of declining rainfall and altered discharge regimes in both basins since the 1970s, driven by climate change and land use transitions. Advanced hydrological modeling—using satellite altimetry, GRACE, and distributed models like SWAT and MGB—demonstrates spatial variability in hydro-climatic responses and emphasizes the need for integrated, context-specific water resource management strategies. The chapter also stresses the ecological significance of these rivers, which host diverse fauna and flora, and underscores the role of traditional ecological knowledge in managing dynamic wetland systems. A call is made for increased scientific investment in Central Africa, where hydrological data scarcity and low international funding hinder evidence-based conservation and development planning. Recognizing the Ogooué and Sanaga as vital components of the Central African hydrological mosaic is key to enhancing resilience and sustainability in the face of rapid environmental change.

Environmental and climatic changes interact within specific socio-ecological contexts shaped by historical human-landscape interactions. This study explores the interconnection between long- and short-term drivers of environmental change and current landscape uses in the Gabon's Bas-Ogoou & eacute; Ramsar site. Working in five villages, we combined georeferenced data of human settlements (n = 166) with individual and group interviews and household surveys (n = 108 informants), to map landscape dynamics over time. We found that the current landscape reflects four periods of human mobility shaped by shifts in land governance and resource management practices. Documenting 30 reported local indicators of environmental change and their respective perceived drivers, we analyzed their interwoven and cascading effects on the social-ecological system. Recent impacts of conservation policies, combined with environmental changes, challenge local livelihoods and the balance of the social-ecological system. These insights advance African historical ecology and emphasize the need for inclusive, context-sensitive strategies to achieve sustainable conservation.

This study investigates teleconnections between rainfall in the Ogooué River Basin (ORB) and sea surface temperature (SST) in the tropical ocean basins. The Maximum Covariance Analysis (MCA) is used to determine coupled patterns of SST in the tropical oceans and rainfall in the ORB, depicting regions and modes of SST dynamics that influence rainfall in the ORB. The application of MCA to rainfall and SST fields results in three coupled patterns with squared covariance fractions of 84.5%, 76.5%, and 77.5% for the Atlantic, Pacific, and Indian tropical basins, respectively. Computation of the correlations of the Savitzky–Golay-filtered resulting expansion coefficients reached 0.65, 0.5 and 0.72, respectively. The SST variation modes identified in this study can be related to the Atlantic Meridional Mode for the tropical Atlantic and the El Niño Southern Oscillation for the tropical Pacific. Over the Indian Ocean, it is a homogeneous mode over the entire basin, instead of the popular dipole mode. Then, the time-dependent correlation method is used to remove any ambiguity on the relationships established from the MCA.

Despite the importance of inland waters of African Equatorial Belt (humid tropics) in carbon (C) and greenhouse gas control, geochemical transfer of C, major and trace elements in fluvial systems of this region remains poorly investigated. Towards filling this gap, this study investigates the Ogooue River Basin (ORB), a major tropical river system in Gabon (Western Central Africa, WCA), focusing on the dissolved and colloidal transport, fractionation, and mobility of organic carbon, major, and trace elements. Water samples were collected using a twophase approach. First, snapshot sampling was conducted during two cruise campaigns in the wetland-dominated Lower Ogooue Ramsar Zone during rainy seasons in 2022 and 2023, targeting several main river locations and its major tributaries, together with lakes and organic-rich hotspots of stagnant waters, including submerged shorelines and ancient raised fields. A time-series sampling was also carried out with 11 samples from the main stem at Lambarene (Lower Ogooue) and 24 fortnightly samples from Franceville (upper reaches) over a year. Analyses included major and trace elements, dissolved organic and inorganic carbon in <0.22 mu m filtrates, and stepwise size fractionation using 3.1 mu m and 0.22 mu m filters, together with dialysis through 25 kDa and 1 kDa membranes. The Ogooue River exhibits high elemental yield (watershed area - normalized export) compared to other organic-rich fluvial systems, with aluminum 37 times, iron 3 times, and dissolved organic carbon (DOC) 4 times higher than those of boreal and sub-Arctic rivers. DOC specific fluxes are also 2.8 times higher than the Congo River and comparable to the blackwater Nyong River. Colloidal transport of solutes dominates the overall export, with organic carbon primarily carried in high molecular weight (HMW) colloids and trace elements such as Fe and Al associated with coarser colloidal fractions. Seasonal flooding and short-term discharge variations strongly influence annual transport, as demonstrated during the high-discharge period of 2022-2023, when DOC concentrations ranged from 5 to 15 mg/L and DOC yields reached 9.3 tC/km2/year. These increases are attributed to the mobilization of organic-rich matter from submerged wetlands and riparian zones. The Ogooue's colloidal behavior aligns with boreal systems for elements like Fe and Al but differs in the transport of alkaline and alkaline earth elements, reflecting unique biogeochemical traits among tropical rivers. Its significant contribution to regional and global carbon and trace element cycles underscores its critical ecological and geochemical importance. This study highlights the need for high resolution hydrometeorological monitoring to better understand the Ogooue's elemental transport dynamics and sensitivity to environmental changes.

We measured the long-term physical denudation of the Ogooue River catchment using Be-10 produced in situ by cosmic rays. These measurements are averaged over 25-200 ka (average 40 ka), depending on the physical denudation rate. The denudation rate of the Ogooue River catchment is slow (38 t/km(2)/a, 15 m/Ma), slightly higher than in Equatorial West Africa (from Senegal to Angola, 26 t/km(2)/a, 10 m/Ma). Physical denudation and chemical weathering fall within the same order of magnitude. Thus, although low, there is substantial chemical weathering compared to physical denudation, that likely contributes over 30 % of the total denudation. Denudation rates are spatially variable (from 10 to 60 t/km(2)/a) within the large Ogooue River catchment. Over the long term, physical denudation and chemical weathering roughly match, except in the Bateke Plateaux area, because the plateaus are made up of already weathered detrital material and therefore their modern flux of solutes is very low (similar to 9.5 t/km(2)/a). The spatial distribution is similar to the one described in the work of Moquet et al. (2021) on the basis of solute fluxes, i.e. the southern part of the catchment is denuding twice as fast as the northern part. We show here that the whole picture did not vary much since 100 ka, as shown by both methods which give consistent results. Faster denudation in the southern part of the catchment may be related to more uplift than in the northern part caused by the southern African "superswell".

The Sanaga is Cameroon’s most important river, supplying almost 90

Study regionMbakaou and Bamendjing basins (Sanaga River sub-basins).Study focusIn this study, the availability of water resources was assessed over the period 2002–2019, based on the SWAT (Soil and Water Assessment Tool) hydrological model and certain meteorological and spatial reference data available for the region (Merra2, Landsat, etc.). Forecasts of its evolution were then made with the same tool (SWAT) over two futures periods (near 2024–2035 and medium: 2036–205) based on data from four (04) regional climate models (RCMs) (CCCma, HIRHAM5, RCA4 and REMO) and future land use and land cover (LULC) data simulated using the CA-Markov procedure. To separate the impact of climate variability (CV) and land use and use and land cover changes (LULCCs) on future water resources, two evolution scenarios (experiments) were established: (1) the impact of the CV, by associating future climate data with LULC from the historical period; (2) the impact of LULCCs, by combining future LULC maps with climate data from the historical period.New hydrological insights for the regionThe performances of the SWAT model are satisfactory in calibration and validation on the two basins with R2, NSE and KGE greater than 0.68. Two models (CCCma and REMO) predict a decline in water resources in these basins, and two others (HIRHAM5 and RCA4) the opposite. The REMO model seems the most reliable. It predicts a drop in precipitation and runoff (SURQ) in the two basins that do not respectively exceed –19% and –31%. CV is the only forcing whose impact will be visible in the dynamics of future water resources, given the insignificant changes expected in the evolution of LULC patterns. The results of this study could contribute to improving the management of water resources in the studied basins and the region.

communication systems have developed significantly over the last few decades. Due to the saturation of lower frequencies of microwave spectrum (3-30 GHz) and the increasing need for high speed, emerging systems for consumer or professional use are progressively shifting to upper microwave and millimeter waves. Our study proposes a methodology for evaluating and classifying losses on a vertically polarized millimeter wave link at 80 GHz. To achieve this, we simulated the link budget of a Nokia 80UBT millimeter wave link operating in its real propagation space (with overground) with Pathloss 5.1 Design tool. Then we built a 3.58 km full-scale link in the Tongo-Bassa watershed of the coastal city of Douala in Cameroon. Analysing data collected over the period from December 06, 2020 to December 16, 2021 under Power BI allowed us to characterize the response of the millimeter signal in free space, during dry and rainy seasons. We then challenge ITU-R P.837-7 and ITU-R.P.838-3 Recommendations on statistical models of rainfall for propagation modeling, especially for millimeter signals propagated in an equatorial climate with heavy rainfalls. The study estimated a rainfall rate for 0.01% of the time at 110.1 mm/h, with a millimeter link cut-off for a rainfall rate greater than 64.8 mm/h, with a specific attenuation due to rain of 6.5 dB/km.

A major flood event occurred on 21 August 2020 in the densely populated Makèpè Missokè neighborhood in the city of Douala (Cameroon, Africa). Nearly 2210 buildings and 12,376 victims spread over 82 hectares were affected. A 2D HEC-RAS model is applied to simulate and characterize this event. A cross analysis of flood depth and flow velocity is used to classify the flood risk and identify areas exposed from low to high hazard. The simulations provide detailed information on the flood characteristics (extent, depth, velocity, arrival time, and duration). The simulated maximum water surface profiles are consistent with the floods marks with differences ranging from 0.02 m to 0.44 m, indicating a good agreement between the observed and simulated water levels at the peak flow (NSE = 0.94, Erel = 0.92, RMSE = 0.21 m). The maximum inundation level is 4.48 m and the flow velocity is globally low at less than 1 m/s. The average flood arrival time and duration are 5 h and 26 h, respectively, for a threshold height of 0.5 m. These results indicate a fast mobilization of the major river channel for the evacuation of this flood. The level of accuracy of the developed model of the 21 August 2020 flood event is appropriate for flood hazard assessment in the city of Douala and is designed to find operational application in future events.

Ce chapitre décrit la variabilité des précipitations et des débits des rivières dans le bassin de l'Ogooué (ORB) au cours des dernières décennies (depuis 1940). En raison de sa situation géographique, traversant l'équateur, l'ORB reçoit des précipitations abondantes qui maintiennent l'un des écosystèmes les mieux préservés au monde. Contrairement aux bassins forestiers voisins qui ont été gravement dégradés par la déforestation, l'extraction des ressources minières, l'extension des zones agricoles et le transport fluvial, qui constitue une alternative cruciale au manque cruel d'infrastructures routières, l'ORB expérimente une politique de conservation exceptionnelle dans la région. Par exemple, le taux de pénétration des zones rurales au Gabon est d'environ 1 habitant par km 2 et de nombreuses études font état d'un taux de déforestation proche de 0 %, avec même une régénération naturelle. Cependant, les fluctuations de l'indice d'anomalie standardisé des précipitations dans l'ORB montrent trois phases principales de variations: la première phase humide a été caractérisée par des précipitations abondantes de 1940 à 1970, la deuxième phase de sécheresse douce à long terme s'est prolongée dans les années 1970 et 1980 et la troisième phase finale a présenté un léger retour de l'abondance des précipitations. Même si la gravité de la sécheresse dans l'ORB était principalement faible, ses effets sur les rejets des rivières étaient très sensibles aux échelles saisonnière et interannuelle. Le régime équatorial pur de l'ORB, caractérisé par des crues maximales égales au printemps et à l'automne, a changé de manière significative, passant de la différence entre les deux débits maximaux de 13,5 % dans les années 1960 à 27.0 %, 38.4 %, 33.9 % et 26.7 % pour les années 1970, 1980, 2000 et 2010 respectivement. Une brève comparaison entre l'ORB et le bassin du fleuve Congo a montré que l'évolution de l'ORB s'inscrit dans un processus régional que connaît l'Afrique centrale avec quelques hétérogénéités spatiales.

This chapter describes the variability of rainfall and river discharges in the Ogooué River Basin (ORB) in recent decades (since 1940). Due to its location crossing the Equator, the ORB receives abundant precipitation, which maintains one of the world's best-preserved ecosystems. In contrast to neighboring forest basins that have been severely degraded because of deforestation, mining resource extraction, extension of agricultural areas, and river transport, which is a crucial alternative to the cruel lack of road infrastructures, the ORB is experimenting with an exceptional conservation policy in the region. For example, the rural penetration rate in Gabon is about 1 inhabitant per km 2 and many studies report a deforestation rate close to 0%, with even full natural regeneration. However, the fluctuations of the standardized anomaly index of rainfall in the ORB show three main phases of variations: the first wet phase was characterized by abundant precipitations from 1940 to 1970, the second phase of long-term mild drought was extended in 1970s and 1980s, and the final third phase presented a slight return of abundance in precipitation. Even though drought severity in the ORB was mainly weak, its effects in river discharges were very sensitive in seasonal and interannual scales. The pure equatorial regime of the ORB, characterized by equal maximum floods in spring and autumn, changed significantly from the difference between both maximum discharges of 13.5% during the 1960s to 27.0%, 38.4%, 33.9%, and 26.7% for the 1970s, 1980s, 2000s, and 2010s, respectively. A brief comparison between the ORB and the Congo River Basin showed that changes in the ORB are part of a regional process that Central Africa is undergoing with some spatial heterogeneities.

This paper investigates links between rainfall variability in the Ogooué River Basin (ORB) and El Niño Southern Oscillation (ENSO) in the Pacific Ocean. Recent hydroclimatology studies of the ORB and surrounding areas resulting in contrasting conclusions about links between rainfall variability and ENSO. Thus, to make the issue clearer, this study investigates the links between ENSO and rainfall in the ORB over the period 1940–1999. The principal component analysis of monthly rainfall in the ORB was done. The temporal mode of the first component corresponds to the interannual variations of rainfall on the ORB. Also, the pattern of the spatial mode of the first component shows that the ORB is a homogeneous hydroclimatic zone. However, no leading mode is significantly correlated to the ENSO index. A cross-wavelet analysis of the time series of basin-scale rainfall and the ENSO index was therefore carried out. The result is a set of periodogram structures corresponding to some ENSO episodes recorded over the study period. And wavelet coherence analysis of both time series confirms that there are significant links between ENSO and rainfall in the ORB.

The Bay of Bonny, located along the Cameroon coastline, is home to diverse ecosystems. It is under significant pressure from large human activity, but remains very little studied, like much of the Gulf of Guinea. In order to understand its long-term shoreline variations and the role the wave regime plays in the evolution of the coastline, a study was conducted on the basis of optical image archives from Landsat 5/7/8 and Sentinel-2A/2B satellite missions acquired between October 1986 and May 2020, coupled with daily ERA-Interim wave re-analysis data covering the period from January 1986 to August 2019. Overall, the results show that the evolution of the coast is highly variable in space and time, as indicated by different levels of erosion (30.55 %), and accretion (27.7 %) on the decadal-scale, with the most significant variations occurring in estuarine areas. Nevertheless, 41.75% of the Cameroon shoreline remains stable during the study period. Three main periods (1986-1994; 1995-2005; 2006-2020) during which the coast underwent significant changes a different location were identified, reaching a retreat rate of up to -10 m/year in the northern section during the first period 1986-1994. The annual trend of significant wave heights anomaly along the coast (-5.6 to -4.1 mm/year) with wave height maxima estimated at 1.46 +/- 0.65 m, where observed during the summer months (July-August). Monthly shoreline changes are inversely correlated with wave climate in some segments. Eigenvalue orthogonal decomposition analyses (Mode 1, Empirical Orthogonal Function) show that 76.3% of the observed variability would be due to the relatively strong local influence of erosion and accretion. This local influence can be related to wave regime at shorter timescale (monthly to seasonal) and the sediment variability from the source inland to the coast at longer timescale (seasonal to decadal). These observations explain a complex pattern of shoreline changes with an almost continuous retreat shoreline during the period 1986-2013 and a reversal in trend towards accretion during the period 2013-2020. EOF mode 2 explains 23.7% of variability, which can be potentially associated with the synergetic relationship between tidal currents and wave-induced longshore current, tidal currents and possibly pockets of human activity. This is attributed to an overall decrease in the supply of sediment via the coastal transport system that prevails in the Gulf of Guinea. (C) 2021 Elsevier B.V. All rights reserved.

Despite the absence of tectonic activity, cratonic environments are characterized by strongly variable, and in places significant, rock weathering rates. This is shown here through an exploration of the weathering rates in two inter-tropical river basins from the Atlantic Central Africa: the Ogooué and Mbei River basins, Gabon. We analyzed the elemental and strontium isotope composition of 24 water samples collected throughout these basins. Based on the determination of the major element sources we estimate that the Ogooué and Mbei rivers total dissolved solids (TDS) mainly derive from silicate chemical weathering. The chemical composition of the dissolved load and the area-normalized solute fluxes at the outlet of the Ogooué are similar to those of other West African rivers (e.g., Niger, Nyong, or Congo). However, chemical weathering rates (TZsil+ rate expressed as the release rate of the sum of cations by silicate chemical weathering) span the entire range of chemical weathering intensities hitherto recorded in worldwide cratonic environments. In the Ogooué-Mbei systems, three regions can be distinguished: (i) the Eastern sub-basins draining the Plateaux Batéké underlain by quartz-rich sandstones exhibit the lowest TZsil+ rates, (ii) the Northern sub-basins and the Mbei sub-basins, which drain the southern edge of the tectonically quiescent South Cameroon Plateau, show intermediate TZsil+ rates and (iii) the Southern sub-basins characterized by steeper slopes record the highest TZsil+ rates. In region (ii), higher DOC concentrations are associated with enrichment of elements expected to form insoluble hydrolysates in natural waters (e.g., Fe, Al, Th, REEs) suggesting enhanced transport of these elements in the colloidal phase. In region (iii), we suggest that a combination of mantle-induced dynamic uplift and lithospheric destabilization affecting the rim of the Congo Cuvette induces slow base level lowering thereby enhancing soil erosion, exhumation of fresh primary minerals, and thus weathering rates. The study points out that erosion of lateritic covers in cratonic areas can significantly enhance chemical weathering rates by bringing fresh minerals in contact with meteoric water. The heterogeneity of weathering rates amongst cratonic regions thus need to be considered for reconstructing the global, long-term carbon cycle and its control on Earth climate.

Hydrological models are important tools for the simulation of water storage and hydrological fluxes in large basins and complex river systems. The hydrological models can compensate the lack of observed data in ungauged basins. In this study, the hydrological model of large basins MGB (for Model of Large Basins in Portuguese) is used to evaluate the hydrological processes of the Ogooue' River Basin (ORB), which has been mostly unmonitored for about three decades. Simulations were carried out over an 18-year period from 1998 to 2015 using TRMM 3B42 daily rainfall data from the Tropical Rainfall Measurement Mission (TRMM) as forcing and in situ and altimetry-based river discharges from Envisat, Saral Altika and Jason-2 for calibration and validation. The results of the model were in good agreement with the flows measured at stations upstream and downstream of the Ogooue' basin (Nash-Sutcliffe Efficiency (NSE) > 0.56 for all calibration gauges). The MGB model efficiently describes the seasonal and interannual variations of the flow in the Ogooue ' River and its major tributaries which were found to be highly correlated to the rainfall (r ranging from 0.72 to 0.90 and 0.56 to 0.87 at seasonal and interannual time-scales respectively). Interannual variations of precipitation and river discharge of the ORB are linked to the El Nino Southern Oscillation (ENSO) in the tropical eastern Pacific Ocean and southeastern tropical Atlantic Nino. Also, the Ogooue' River discharge was found to be strongly correlated with Sea Surface Temperature (SST) at annual and semi-annual time-scales. (C) 2020 COSPAR. Published by Elsevier Ltd. All rights reserved.

At the global scale and on geological time scales, mechanical erosion and chemical weathering budgets are linked. Together, these processes contribute to the formation and the degradation of the Earth’s critical zone and to the biogeochemical cycles of elements. While the weathering of hot and humid shields areas exhibit low weathering rates because of the depth of the mature depleted soil mantle there, shields areas dominate the continents areas over intertropical regions and, therefore, represent a significant proportion of the global delivery of dissolved matter to the oceans. In addition, these environments are under supply-limited conditions (the weathering rate is limited by the low rates of the erosion) and thus particularly sensitive to long-term variability erosion rates. Despite this importance, weathering-erosion budgets and rates estimation in these environments is sparse, and generally performed at a local scale (soil profiles) or, when performed at a larger catchment scale, the intra cratonic characteristics variabilities (e. g. the diversity of mechanical erosional regimes) are usually not singled out. In the present study, we explored the variability of the weathering intensity of the Ogooué sub-basins (Western central Africa, Gabon) as a function of their geomorphologic, tectonic and lithological setting variability. We analyzed major and trace elements concentration and the strontium and neodymium isotopes of water, suspended matter sediments and bedload sampled in 24 Ogooué tributaries (September 2017 campaign). Our results show that shield areas exhibit a high variability of chemical weathering intensity, which follows the erosional regime characteristics of the studied sub-basins, likely related to their tectonic activity. Three regions can be distinguished: The Bateke plateau (East sub-basins - PB), is composed of pure sandstones (quartz) and is inert in term of tectonic activity and therefore in term of erosion and weathering budget; the northern sub-basins (NB) are subjected to low tectonic activity and exhibit slightly higher erosion and weathering intensity than PB region and, by comparison, southern sub-basins (SB) exhibits uplift activity which is traduced by more intensive erosion and weathering processes. The annual dissolved solid budget of the Ogooué basin is ~2.52 t.yr-1 for a rate of 11.7 t.km-2.yr-1. According to the source discrimination method performed based on the geochemical analysis, the atmospheric inputs contributes to around 20% to the TDS, the silicate weathering contribution dominates the dissolved exports throughout 70% of its production while the carbonates weathering lowly contributes to the TDS production. By comparison to the other large shields rivers, this basin exhibit a lower range of chemical silicate weathering rate than most of the world’s large rivers, with values similar to those of the Congo River. This new dataset provides a key information to complete the World River chemistry database, which is limited for inter-tropical regions, especially in tectonically quiescent environments. Moreover, this study provides new data for tropical shields contexts allowing for the exploration of the interactions between erosion rates and climate in the control of continental weathering rates, and their relationships with long-term carbon cycle and short-term biogeochemical cycles.