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La sécheresse observée en Afrique tropicale vers la fin des années 1960, a également affecté le bassin de la Bénoué en Afrique centrale, avec une persistance remarquable qui s'est répercutée sur les écoulements. Les ruptures à la baisse ont été mises en évidence dans les séries hydropluviométriques de ce bassin au pas de temps annuel en 1970–1971 (pluies) et 1971–1972 (débits). Les déficits associés à cette rupture sont de -2,9 % pour les pluies et -14,2 % pour les débits, par rapport à la moyenne climatologique (1950–1951 à 2014–2015). La saison humide a connu des évolutions pratiquement identiques. Cependant, depuis la décennie 1990, il est observé un relèvement significatif dans les écoulements moyens annuels de ce cours d'eau, et cela coïncide avec le retour des pluies. Le maintien de cette hausse au cours des récentes décennies pourrait être envisagé en raison de l'accroissement des espaces imperméabilisés dans le bassin, qui compenseraient le déficit engendré par la rechute des pluies après la décennie 1990 via une accentuation du ruissèlement. De plus, depuis la mise en eau du barrage de Lagdo en 1983, il est observé non seulement une augmentation de l'ensemble des gammes de débits minima suivant des taux allant de +57,8 % (minimum sur 1 jour) à +70,1 % (minimum sur 90 jours), mais aussi un accroissement général de la variabilité de l'ensemble des débits extrêmes (minima et maxima). L'augmentation progressive du stockage en saison de pluies à la retenue de Lagdo, pour fournir l'électricité et l'eau d'irrigation durant la saison sèche à une population grandissante, rendent imperceptibles les effets de l'accroissement des espaces imperméabilisés non seulement sur les écoulements de la saison humide, mais aussi sur les écoulements maxima. Au demeurant, l'opérationnalisation du barrage de Lagdo a eu un impact significatif sur le régime de la Bénoué, se traduisant essentiellement par le caractère désormais permanent de l'écoulement tout au long de l'année hydrologique et un important fléchissement de la crue d'août à octobre. Abstract. A tendency to drought conditions appeared in tropical Africa from the end of 1960s, also affect the Benoue watershed in central Africa, with a remarkable persistence in streamflow. Negative breakpoints are found in the annual hydroclimatic time series of the Benoue catchment at annual time step in 1970–1971 for rainfall, and in 1971–1972 for discharges. Compared to the climatological mean (1950–1951 to 2014–2015), the deficits associated with these negative breakpoints are approximating −2.9 % for rainfall, and −14.2 % for discharges. The wet season shows similar developments. However, from the 1990s, there has been a significant increase in the mean annual discharges of the Benoue River, which coincides with that of the rainfall during the same decade. The maintenance of this increase over the recent decades could also be expected in response to an increase in impervious surface areas (ISA) in the catchment area, which could compensate the deficit generated by the post-1990s rainfall deficit through increasing in runoff. From the watering of the Lagdo dam in 1983, an increase in all ranges of minimum flow, ranging from +57.8 % (1 d minimum) to +70.1 % (90 d minimum), as well as an increase in the variability of extreme flows (minima and maxima), has been detected. During the wet season, the increase in water storage capacity at the Lagdo reservoir, which is used to provide electricity and irrigation water to a growing population, mitigates the increase in runoff, especially in maximum runoff. Moreover, the operationalization of the Lagdo dam has a significant impact on the Benoue hydrological regime, resulting mainly in a new perennial nature of the flow, and a significant decrease in flood from August to October.

L'objectif de cet article est de faire une évaluation des tendances récentes des grandeurs hydroclimatiques dans le bassin de l'Ogooué en contexte de changement climatique. Pour cela, les données de pluies et de débits de ce bassin ont été analysées au moyen du test de Pettitt. Les résultats de cette étude révèlent une diminution statistiquement significative des modules annuels que le test de Pettitt situe en 1972–1973, mais rien de tel pour les pluies à ce même pas de temps. Les écarts des moyennes décennales à la moyenne interannuelle montrent cependant des liens plus nets entre ces variables, se traduisant essentiellement par une diminution synchrone au cours des décennies 1970 et 1980. Cette diminution est suivie d'une reprise au cours des deux décennies d'après (1990 et 2000), laquelle s'estompe à nouveau au cours de la décennie 2010. Les écoulements des saisons pluvieuses (printemps et automne) ont enregistré les modifications les plus importantes, suite aux variations des régimes pluviométriques des saisons sèches (hiver et été) qui les précèdent. La saison sèche d'hiver a connu entre les décennies 1970–1990 une diminution importante des précipitations qui a provoquée une chute des écoulements du printemps. Le rehaussement et la flexion respectivement notés au cours des décennies 2000 et 2010 s'accompagnent des mêmes tendances dans les écoulements du printemps. A l'inverse, entre les décennies 1980–1990, il a été noté une hausse conjointe des pluies d'été et des écoulements d'automne. La flexion des pluies d'été notée depuis la décennie 2000 est également perceptible dans les écoulements d'automne. Ces résultats pourraient servir à renforcer les capacités de gestion des ressources en eau dans le bassin versant concerné et dans la région. Ils apportent également de nouveaux éléments pour étudier et comprendre la variation saisonnière et la disponibilité de l'eau douce en aval, dans les estuaires et les zones côtières des rivières régionales. The objective of this article is to assess recent trends of hydroclimatic quantities in Ogooue basin in the context of climate change. For this, the rainfall and discharges data of this basin were analyzed using the Pettitt test. The results of this study reveal a statistically significant decrease in runoff that the Pettitt test situates in 1972–1973, but nothing like that for rainfall at this same time scale. The decadal deviations from the interannual average, however show links between these variables essentially marked by a synchronous decrease in the 1970s and 1980s. This decrease is followed by a recovery in the two decades following (1990 and 2000), which stops again during the 2010s. The flow of the rainy seasons (Spring and Autumn) recorded the most important modifications, following variations in the rainfall regimes of the dry seasons (Winter and Summer) which precede them. The dry Winter season experienced a significant decrease in precipitation between the 1970s and 1990s, which caused a decrease in Spring flows. The increase and flexion noted respectively during the 2000 and 2010 decades are accompanied by the same trends in Spring flows. Conversely, between the 1980s and the 1990s, there was a joint increase in Summer rainfall and Autumn flows. The decrease of Summer rainfall noted since the 2000s is also noticeable in the Autumn flows. These results could be used to strengthen water resources management capacities in the watershed concerned and the region. They also provide new insights to study and understand seasonal variation and availability of freshwater downstream, in estuaries and coastal areas of regional rivers.

Le comblement et l'eutrophisation des retenues d'eau constituent des contraintes majeures pour les usages qu'en font les populations riveraines. L'étude bathymétrique a révélé une diminution moyenne de 0,116 m/an de la profondeur sur sept (07) ans. Ce niveau élevé de comblement s'explique par l'érosion des terres due à la dégradation du couvert végétal et le surpâturage. Les mesures directes de la qualité physique : le pH, la température et l'oxygène dissous avec le multi paramètre PC HORIBA WATER QUALITY CHECKER U-10; les paramètres chimiques par la méthode chromatographique ICS 1000 et de dosage de la chlorophylle a avec le spectrophotomètre DR 5000 par la Norme AFNOR T90-117 ont permis d'évaluer le risque d'eutrophisation par la grille de diagnostic d'Ifremer (2000). Les eaux sont dans un état eutrophe. Le comblement et l'eutrophisation de la retenue constituent des menaces potentielles à court terme pour la pisciculture, l'irrigation et la consommation tandis que la disponibilité quantitative de la ressource est menacée à moyen terme. Des relevés bathymétriques périodiques et des actions d'aménagement du bassin versant de la retenue sont nécessaires à la gestion intégrée de la retenue d'eau.

Nowadays, special attention is paid to hydroelectric production because it is an efficient, reliable, and renewable source of energy, especially in developing countries like Cameroon, where hydropower potential is the main source of electricity production. It also represents a useful tool to reduce the atmospheric greenhouse gas concentrations caused by human activities. However, it is the most sensitive industry to global warming, mainly because climate change will directly affect the quality, quantity of water resources (streamflow and runoff), which are the important drivers of hydropower potential. This study examined the response of hydropower potential to climate change on the Lagdo dam located in the Benue River Basin, Northern Cameroon. Hydropower potential was computed based on streamflow simulated using HBV-Light hydrological model with dynamically downscaled temperature and precipitation from the regional climate model REMO. These data were obtained using the boundary conditions of two general circulation models (GCMs): the Europe-wide Consortium Earth System Model (EC-Earth) and the Max Planck Institute-Earth System Model (MPI-ESM) under three Representative Concentrations Pathways (RCP2.6, RCP4.5 and RCP8.5). The results suggest that, the combination of decreased precipitation and streamflow, increased PET will negatively impact the hydropower potential in the Lagdo dam under climate change scenarios, models and future periods.

Conceptual rainfall-runoff models are commonly used in many hydrological applications in support of water resources management practices. They provide an advantage in data scarce areas due to their ability to use limited data and generate sufficiently reliable information. The main challenge with this type of hydrological models remains the ability to establish an optimal model parameter space due to a number of sources of uncertainties. This study is conducted in the Upper Benue River Basin (UBRB) in Cameroon with the aim to establish a rainfall-runoff model that is fit in the context of hydro-climate characteristics of the basin. The UBRB is the second-largest river in Cameroon and one of the most important water resources in northern Cameroon from both a water supply and hydropower generation perspective. A Monte Carlo procedure was implemented to calibrate the HYMOD conceptual rainfall-runoff model using five statistical performance measures: the Absolute percent bias (PBIAS), the ratio between root mean square error (RMSE) and standard deviation of observed data (STDEVobs) (RSR), the Pearson correlation coefficient (r), the Nash–Sutcliffe efficiency (NSE) and the Kling–Gupta efficiency (KGE). The results reveal that the model performance varies from good to very good with NSE greater than 0.78 and RSR less than 0.46 during the calibration and validation periods. Therefore, this model can be used to support various water resources management initiatives in the basin.

Coastal areas are areas where a very large number of laguno-marine species reproduce, grow or transit, which are most often the subject of fishing exploitation. The purpose of this study is to analyse the risk of contamination of fisheries ecosystems and its consequences on the evolution of aquatic resources in the coastal zone and more specifically from south-west Benin to south-east Togo.The methodology adopted is based on the collection and processing of raw data such as: hydrometric data, particle size data, climatological data, data from fauna and flora inventories, supplemented by those from in situ measurements and those from laboratory analyses made from sediment samples and some species caught during fisheries during the different hydrological seasons. Analysis of the results revealed that pollution of the lagoon channel in Cd, As, Pb and some major ions such as Cl -, NO3- and PO43- are responsible for its eutrophication. In addition, organic matter and MES in high concentrations lead to the covering of aquatic habitats, particularly spawning grounds and suffocation of eggs by reducing essential metabolic exchanges (supply of oxygen and nutrients, evacuation of metabolic waste) through the decrease in the concentration of dissolved oxygen within the substrate. Analysis of trace elements in the flesh of samples of Captain fish (Polydactylus quadrifilis), white grouper (Epinephelus aeneus), Tilapia zillii, and white catfish revealed their rate of contamination with metallic trace elements. Behavioural effects (abandonment of shelter areas and avoidance reaction) are thus recorded; subletal effects (short-term reduction in feeding rate; minor physiological stress; increased cough frequency; increased breathing rate; moderate physiological stress; major; delayed outbreak; reduction of population density; mortality). According to 89% of fishermen, the decline in fish species caught is recorded during dry periods, when the channel has a high rate of contamination with heavy metals and major ions.

The city of Cotonou (Benin) in the delta of the River Ouémé and the city of Lomé (Togo) closed to the River Zio and depending lagoons have hydrosystems include aquifers and urban ecology under pressure of annual floods and discharge. So, bad sanitation and hygiene, poor physicochemical and bacteriological groundwater quality and occurrence of health risks (malaria, gastroenteritis, diarrheal) are observed ecohydrological characteristics. Well water quality data, sanitation and hygiene related public health data linking to the flow data of referred rivers are used to highlight a complexity of the ecohydrological state of the cities. With regard to the global agenda 2030 specifically the SDG 3, 6 and 11, an ecohealth model was developed to contribute in improvement of well water quality and to promote sustainable healthy living in the study area. Elaborated ecohealth model supports pathways on how improved human health could be attained from better groundwater and waste management in the whole west African coastal cities. In Cotonou and Lomé, recurrent floods, controlled by hydrological variation on the depending river basin affects urban ecology and human settlements. The footprint of the interactions is deterioration of environment patterns, of groundwater quality and higher prevalence of waterborne diseases. Referring to knowledge on public health (current and expected status), the model ecohealth is generated by establishing a relationship between floods, aquifers (hydrosystem) and sanitation and hygiene systems (disposal). Given that the urban population faces problems of unsafe sanitation and hygiene, and well water quality degradation, adequate information-education-communication (IEC) for social change approach was used to help in adoption of socio-ecological sanitation systems for water security and its accessibility. Dry toilets, above ground and watertight pit latrines and polyethylene septic tanks are the proposed sanitation disposals. Effective implementation of developed ecohealth model could have great social impacts and help in sustainable well-being in the coastal West African cities.Keywords: River delta, coastal city, groundwater quality, floods, ecohealth model

Due to the recurrence of floods in recent years through the Niger River watershed in Benin, it’s necessary to assess the damage caused in order to consider preventive measures. The main goal of this paper is to evaluate and compare floods impact from 2010 and 2020, which are years of major floods in the Niger basin in Benin, particularly in the districts of Karimama and Malanville. The data used in this study are daily flows and were collected through Malanville hygrometric station (from 1981 to 2020), Gaya synoptic station (from 1981 to 2020) and the sentinel images (September 2010 and September 2020) which are used to calculate satellite indices and produce land use maps. Rainfall and streamflow data, that are factors in the occurrence of floods were analyzed using the reduced centered anomaly method.The satellite data were used to calculate the NDPI (Normalised Difference Pond Index), NDWI (Normalised Difference Water Index) and land cover maps for the same dates. The NDPI is used to delineate dry and water surfaces with greater discrimination, and the NDWI is intended to extract water masses, as it maximizes the reflectance of water in the green wavelengths. The results show that precipitation increased significantly in both years, especially during the months of July, August and September, but the increase in flow was more significant, with more pronounced values in 2020. The floods of 2020 affected 20276.53 ha of fields and 2987 ha of settlements against 17812.28 ha and 2012.17 ha in 2010. Monitoring the level of flood variation by remote sensing would increase the capacity of action of the early warning and risk management system and disasters associated with floods increasingly recurrent in the context of climate change.Keywords: North-Benin; Niger Basin; floods; remote sensing.

Low flows and irrigation in Mono River basin at Athiéme outlet (West -Africa)1Coffi Justin Noumon, 2Domiho Japhet Kodja, 2Ernest Amoussou, 1Luc O. Sintondji,1 Euloge K. Agbossou.1Laboratoire d’Hydraulique et de Maîtrise de l’Eau (LHME), Institut National de l’Eau, Université d’Abomey Calavi 01 BP 526 Cotonou (Bénin).2Laboratoire Pierre Pagney Climat Eau Ecosystèmes et Développement (LACEEDE), Institut de Géographie et Aménagement du Territoire et de l’Environnement, Université d’Abomey Calavi 01 BP 526 Cotonou (Bénin).Auteur correspondant: 1Coffi Justin NOUMON (justinoum2001@yahoo.fr)AbstractThe basic goal of this survey is the assessment of water resources of Mono River at Athiémé outlet in low flow period where it availability for agriculture uses is problematic. Therefore, daily data (precipitations, ETP, flows) from 1971 to 2016 are collected. It allows to simulate low discharge with GR4J model. Field survey permits the characterization of agriculture uses and the assessment of water requirement for irrigation. Low flows of Mono River at Athiémé outlet simulation with GR4J showed a good performance of the model to simulate low discharges before the construction of Nangbéto dam in 1988. Criteria values of Nash-Sutcliffe before dam establishment are 69% (calibration) and 71% (validation). The performance of the model is less for the period after the dam establishment: The change in the hydrological regime of the river due to water jets from Nangbéto dam can explain this low performance. Water requirement for irrigated areas estimation has given 42 587 596 m3 in 2020 and 73 007 308 m3 in 2030 mainly for the constraint period production which came from february to may. The comparison of water availability/water requirement shows that the water needs are closely covered monthly but a fine decade period analysis revealed some deficit decades. These results involve a great attention and consequent investments for deep knowledge of the resource in order to plan it sustainable management.Key words: Mono basin, Low flow, GR4J, Irrigation.

This study aims to investigate drought and flooding area in West Africa during the pre-industrial and projected future periods. The datasets used consist of CHIRPS-2 and CRU for observed reanalysis and CORDEX/CMIP6 for climate model scenarios. The CORDEX/ CMIP6’s potential evapotranspiration (PET) and precipitation correlate well with observation dataset over the study area with respective KGE up to 0.75 and 0.9. The pre-industrial period used in this research covers 1971-2000, while the projection period is defined according to the global warming level and the RCPs. Results showed that, the spatial and temporal repartition of SPEIs was elucidated by applying the Empirical Orthogonal Function (EOF) analysis, which showed that during the pre-industrial period, the wettest areas were located at the eastern and southern part of West Africa. The analyses on projected scenarios reveals that most of the coastal countries in Gulf of Guinea will be globally wetter under all the GWLs studied, with some interleaving as coastal countries (Senegal, Ivory-Coast, Benin, Togo and Nigeria). Some years will experiment extreme drought, whilst Savanna will get positive changes, which will lead them to become less dry compare to period 1971-2000. An investigation on the spatial concentration of precipitation using precipitation concentration index (PCI), have involved this study to highline an uniform spatial distribution of annual and wet season precipitations over gulf of Guinea and Savanna, follow by an increasing of uniform and moderate spatial distribution of precipitation under RCP4.5, and negative changes over eastern and western part of the study area for projection RCP8.5, explaining the high variability of concentration of precipitation over these regions.Keywords: Drought and flood area, climate model, West Africa, global warming

This study was conducted in the White Bandama watershed (WBW) in Côte d’Ivoire (West Africa). The objective is to assess the impacts of future climate change (CC) on the hydropower potential (HPP) of the WBW. The methodology is based on coupling the SWAT (Soil and Water Assessment Tool) hydrological model with the Geographic Information System (GIS) QGIS to assess HPP on streams and evaluate the impacts of future CC on HPP of the watershed. Historical and climate projection data (precipitation, minimum and maximum temperature) for a set of three Regional Climate Models (RCM) from CORDEX-AFRICA (CCCma-CanRCM4, CCLM4-8-17 and REMO 2009) under RCP 4.5 were used. The biases of the ensemble mean were corrected by the Delta-change method. The relative change of streamflow discharge and HPP was assessed as the relative difference between the projection periods (2041-2070 and 2071-2100) and the reference period (1976-2005). The results showed a total of 22 future hydropower potential sites in the watershed. These sites were identified, geolocated and classified according to their potential capacity of generation in 82% as small (1 MW-25 MW potential capacity), 9% as medium (25 MW-100 MW potential capacity) and 9% as large (more than 100 MW potential capacity) hydropower. The climate models’ ensemble projected an upward trend for both the annual mean discharge of rivers and HPP of the WBW according to RCP 4.5 for the periods 2041-2070 and 2071-2100. The variation in flow rates is projected to have a positive impact on the HPP of the WBW. On the annual cycle, the months of August and September will record the highest monthly mean flows between 150 and 200 m3/s while the months from November to April will record low monthly mean flows on the WBW.

The streamflow is important for planning et decision-making of development activities related to water resource. The hydrological mathematical models are frequently used to simulate the transformation of rainfall into streamflow as well as Artificial Neural Networks (ANNs. In order to contribute to the establishment of an adequate framework for water resource management in the Davo river basin, this study presents an intercomparison between streamflow estimations from conventional hydrological modelling and ANN analysis. The Davo River basin is located in the southwestern forest of Côte d'Ivoire. The hydrological models used are the HEC-HMS model (Hydrologic Engineering Center-Hydrologic Modeling System) and the GR4J model (Rural Engineering with four daily parameters), while the data-driven ANN model is developed in MATLAB. The three tools receive the same data and their parameters are calibrated using the same objective functions. The quality of the simulations is measured in the control phase using several statistical criteria. For the assesment of the impacts of climate change on hydrological regime of the Davo river basin, the AFRICA-CORDEX (Coordinated Downscaling Experiment for Africa) data under two RCP (representative concentration pathway) scenarios RCP4.5 and RCP8.5 are integrated in the three tools of modelling. The approach developed in this study allows a considerable improvement of the outils performances developed in calibration and validation. In addition, the results of the current study provide valuable feedback for water resources’ planners in Côte d’Ivoire and the developing regions.Keywords: Climate change, Streamflow, Artificial Neural Networks, HEC-HMS, GR4J, Davo river basin, Côte d’Ivoire

The paper explores how a project implemented in the framework of the LIRA2030 Africa programme is contributing to the achievement of the Sustainable Development Goals (SDG) on water, sanitation and...

In the coastal cities of West Africa, land use change, rapid population growth, bad sanitation systems and poor environmental governance degrade the quality of groundwater. This study aimed to assess alternative, acceptable, affordable sanitation disposal and practices for groundwater quality rehabilitation in the cities of Cotonou and Lome. The study was based on the participatory transdisciplinary approach, field surveys, feedback from interactions with stakeholders, experiences of the practitioners and institutional consultations. This multi-stakeholder approach helped to appreciate ecological aspects of sanitation disposal and its implications on water quality improvement. SWOT model was used to analyze the relevance of assessed ecological system. Well water quality is deteriorated by traditional waste management disposal. Ecological sanitation systems are septic tanks on polyethylene, above-ground latrines and phytoremediation technique in the swamp areas. Collected wastes are used for composting and biogas production. Based on the optimist scenario at 2030 horizon, kind success factors of groundwater security are participation of citizens, existence of sanitation market, valorization of the waste by category, low-cost disposal adaptable to the individual, household and neighborhood’s scales. The strategic directions rely on funding and public policies for WASH, ecological sanitation disposal, cultural environment and good sanitation practices for emergence of new sanitation system to secure and sustain well water quality. But the social acceptability of ecological disposal is limited by the society’s multicultural heritage. These findings could help in decision-making concerning urban groundwater quality protection in the African coastal cities.

Urban metabolism, a process of material, energy and water flows, consumption or transformation in the cities and outflows of wastes, has impacts on the shallow groundwater. This study addressed the relationship between the urban metabolic system and well water physicochemical and bacteriological quality in the coastal cities of Cotonou and Lome. A participatory transdisciplinary approach, involving non-academics and academics stakeholders was used to analyse urban inflows and outflows related to groundwater quality. Standard methods were used to measure well water quality from 100 seasonal samples. Waste management revealed poor sanitation and hence a linear urban metabolism with no solid and liquid wastes and excreta recycling. This form of city metabolism reinforced by the seawater inflow is the main source of groundwater quality deterioration in Cotonou and Lome. The principal water types Ca2+–Mg2+–Cl-– SO2-4 (48.21%), Na+–K+–Cl-–SO2-4 (65.9%) and the Gibbs diagram suggested water-rock interactions and dominance of cations exchange on the hydrogeological compositions, also seasonally controlled by saltwater intrusion or anthropogenic salinization. The water quality index on the range of 35.90 to 169.60 in Cotonou and 82.94 to 364.68 in Lome indicated well water very poor quality to unsuitable for drinking. Moreover, the bacteriological quality was bad due to total coliforms (112 - 1812; 1 - 1000 UFC/100 mL respectively in Cotonou and Lome), Escherichia coli (40 - 780; 1 - 112 UFC/100 mL), faecal enterococci/streptococcus (18 - 736; 1 - 118 UFC/100 mL). The findings could help to sustain groundwater quality by controlling the pollution sources linked to urban metabolism.

This study was conducted in the Bandama watershed (BW) in Cote d’Ivoire (West Africa). The objective is to analyze hydrological dynamics and hydropower generation in BW in a context of climate change. The methodology is based on the Inverse Distance Weighted (IDW) used to interpolate rainfall and on statistical tests (Normality, Buishand, Pettitt and Hubert) applied on hydrometeorological and hydrometric data to analyze the hydrological functioning of BW. Hydropower generation at Kossou dam was analyzed with Pearson Chi square independence method according to Representative Concentration Pathways (RCP) 4.5 and 8.5 scenarios of CMIP5 (coupled model intercomparison project phase 5) and CORDEX-AFRICA (coordinated regional climate downscaling experiment). The results showed the variability of rainfall as well monthly, seasonal as annual from 1980 to 2013; the hydroclimatic variability in the basin characterized by the presence of breaks in stationarity in 1998 and 2008, and the variability of flow. The Buishand and Pettitt tests described these two ruptures. The segmentation of Hubert specified three sub-periods. The results also showed that there is an alternation of wet and dry periods followed by a slight resumption. The climate projection scenarios RCP 4.5 and 8.5 have shown that the change in the variability of rainfall and flow in the BW will have a negative impact on hydropower generation at Kossou from 2030 to 2050.

Satellite precipitation products are widely used in different domain, in area where there is a lack in observation. These have different spatio-temporal resolutions consequently resulting in different precipitation amounts depending on the product. The present study validates three satellite products, namely the Climate Hazard group Infrared Precipitation with Stations (CHIRPS), the Climate Research Unit (CRU) and the Global Precipitation Climatology Project (GPCP) over Bandama and Mono river basins for 1981-2005 and 1981-2016 respectively by comparing them to the observation precipitation of the basin. The available studies are focused on the regional scale but not on a watershed scale for hydrological studies. The analysis reveals that all the products are strongly correlated to each other as well as to the observed data at basin level. The Lamb coefficient test shows that most all the chosen basin namely Bandama and Mono presents the same climatic indices. All the products present the same variability and trend as the observation at basins scale. By comparing those products to observation, CHIRPS product following by GPCP give the lowest mean absolute error (MAE) at annual and seasonal time scales while CHIRPS is followed by CRU at monthly scale. Overall, all products overestimate the precipitation at Bandama basin while they underestimate it over Mono river basin. The comparison over 1981-2017 period of the total annual precipitation increasing southern ward (from Sahel to the coastal zone) for all the three studied products which varies from 300 mm to 2400 mm/year. All the three products are not significantly different from one another and they all highlight the same areas of hotspot rainfall in the region. The same conclusion is made at monthly and seasonal scales. Therefore, any of these products especially CHIRPS can be used for study in this region due to its lowest bias and MAE.

Hydropower energy, the main renewable energy source in West Africa, contributes to more than half of the Togo and Benin National electrification. This resource highly depends on water availability in rivers or reservoirs. The water availability heavily relies on climate patterns of the area. In the climate change context, the sustainability of hydropower plants is at risk. This work aims to assess the sensitivity of the Nangbeto hydropower plant to multiyear climate variability using statistical analysis. The results show that energy generation at Nangbeto hydropower is more modulated by four main variables namely inflow to reservoir, water level, rainfall of the actual and the previous year. The energy generation is found to be strongly and significantly correlated to inflow to reservoir, water level, and rainfall. Overall, the Nangbeto hydropower generation is more sensitive to inflow which is controlled by climate variables (rainfall, temperature) and land use/cover change. Therefore, the probable future change in these variables is suggested to be deeply investigated.

International audience; The present work reports on the study and the comparison of the performance of three “Génie rural” (GR) and two Water Balance (WB) models. The calibration and robustness performances are analysed in the light of the hydro-climatic conditions. The study shows that the GR models are much more efficient and robust than the WB models. The behaviour of calibration performance as a function of hydroclimatic variables varies according to the model and goodness-of-fit criteria (GOFC). The GR models are more robust in terms of NSE(Q) and NSE(√Q) and the WB models in terms of KGE. For more robustness of the models, it is better to transfer the parameters to wetter periods or periods with a lower Potential EvapoTranspiration (PET) than the calibration period. For a loss of robustness of less than 20% for GR and 30% for WB, the variation between calibration and rain validation/PET periods must be around ±15%/±1.5%.

The FRIEND-Water program (FWP) is the oldest and the most transverse program (i.e. Flagship Initiative) within the Hydrological Intergovernmental Program (IHP) from UNESCO. FRIEND means “Flow Regimes from International and Experimental Network Data”. The FWP is dedicated to allow large communities of hydrologists and associated disciplines to collaborate across borders on common shared data and scientific topics which are addressed through large world regions. The program has evolved in its 35 years of existence. There are 8 large regions in the world which gather tenths of hundreds of researchers, and organize events following several research topics defined according to each region priorities. The FWP is chaired by a scientific committee gathering regional coordinators and thematic experts. Each region gives its research priorities which evolve according to the projections given by the member States during the IHP council every year. The future activities of the FWP are defined by the upcoming IHP IX program and with the support of the newly created Montpellier UNESCO Category II Center ICIREWARD, acting as FWP Secretariat.

The African continent is probably the one with the lowest density of hydrometric stations currently measuring river discharge, despite the fact that the number of operating stations was quite important until the 70s. This new African Database of Hydrometric Indices (ADHI) is compiling data from different sources carefully checked for quality control. It includes about 1500 stations with at least 10 years of daily discharge data over the period 1950–2018. The average record length is 19 years and for over 100 stations complete records are available over 50 years. With this dataset spanning most regions of the African continent, several hydrometric indices have been computed, representing mean flow characteristics and extremes (low flows and floods), and are made accessible to the scientific community. The database will be updated on a regular basis to include more hydrometric stations and longer time series of river discharge. The ADHI database is available for download at: https://doi.org/10.23708/LXGXQ9 (Tramblay and Rouché, 2020).

Climate change is expected to significantly impact on the availability of water resources in West and Central Africa through changes in rainfall, temperature and evapotranspiration. Understanding these changes in this region, where surface water is fundamental for economic activity and ecosystem services, is of paramount importance. In this study, we examine the potential impacts of climate variability and change on hydrological systems by the mid-21st century in West and Central Africa, as well as the uncertainties in the different climate-impact modelling pathways. Simulations from nine global climate models downscaled using the Rossby Centre Regional Climate model (RCA4) are evaluated and subsequently bias-corrected using a nonparametric trend-preserving quantile mapping approach. We then use two conceptual hydrological models (GR2M and IHACRES), and a regression-based model built upon multi-timescale sea surface temperatures and streamflow teleconnections, to understand hydrological processes at the subcontinental scale and provide hydrological predictions for the near-term future (2020-2050) under the RCP4.5 emission scenario. The results highlight a zonal contrast in future precipitation between western (dry) and eastern (wet) Sahel, and a robust signal in rising temperature, suggesting an increase in potential evapotranspiration, across the multi-model ensemble. Overall, across the region, a significant increase in discharge (similar to + 5%) is expected by the mid-21st century, albeit with high uncertainties reported over most of Central Equatorial Africa inherent to climate models and gridded observation data quality. Interestingly, in this region, teleconnections-based regression models tend to be an alternative to hydrological models.

This study characterizes the future changes in extreme rainfall and air temperature in the Mono river basin where the main economic activity is weather dependent and local populations are highly vulnerable to natural hazards, including flood inundations. Daily precipitation and temperature from observational datasets and Regional Climate Models (RCMs) output from REMO, RegCM, HadRM3, and RCA were used to analyze climatic variations in space and time, and fit a GEV model to investigate the extreme rainfalls and their return periods. The results indicate that the realism of the simulated climate in this domain is mainly controlled by the choice of the RCMs. These RCMs projected a 1 to 1.5 &deg, C temperature increase by 2050 while the projected trends for cumulated precipitation are null or very moderate and diverge among models. Contrasting results were obtained for the intense rainfall events, with RegCM and HadRM3 pointing to a significant increase in the intensity of extreme rainfall events. The GEV model is well suited for the prediction of heavy rainfall events although there are uncertainties beyond the 90th percentile. The annual maxima of daily precipitation will also increase by 2050 and could be of benefit to the ecosystem services and socioeconomic activities in the Mono river basin but could also be a threat.

Over recent decades, regions of West and Central Africa have experienced different and significant changes in climatic patterns, which have significantly impacted hydrological regimes. Such impacts, however, are not fully understood at the regional scale, largely because of scarce hydroclimatic data. Therefore, the aim of this study is to (a) assemble a new, robust, reconstructed streamflow dataset of 152 gauging stations; (b) quantify changes in streamflow over 1950–2005 period, using these newly reconstructed datasets; (c) significantly reveal trends and variability in streamflow over West and Central Africa based on new reconstructions; and (d) assess the robustness of this dataset by comparing the results with those identified in key climatic drivers (e.g. precipitation and temperature) over the region. Gap filling methods applied to monthly time series (1950–2005) yielded robust results (median Kling-Gupta Efficiency >0.75). The study underlines a good agreement between precipitation and streamflow trends and reveals contrasts between western Africa (negative trends) and Central Africa (positive trends) in the 1950s and 1960s. Homogenous dry conditions of the 1970s and 1980s, characterized by reduced significant negative trends resulting from quasi-decadal modulations of the trend, are replaced by wetter conditions in the recent period (1993–2005). The effect of this rainfall recovery (which extends to West and Central Africa) on increased river flows are further amplified by land use change in some Sahelian basins. This is partially offset, however, by higher potential evapotranspiration rates over parts of Niger and Nigeria. Crucially, the new reconstructed streamflow datasets presented here will be available for both the scientific community and water resource managers.

This study aims to analyze hydroclimatic variation (meaning rainfall and flow decrease or increase, regime irregularity, …) in Mono River basin and flood risk for ecological conservation of Naglanou (Benin) and Akissa (Togo) hydrosystems. Climate, hydrology and planimetry data were used for descriptive statistical and spatial interpolation to determine rain/flow relationship, climate balance (rainfall less potential evapotranspiration), flow coefficient (relationship between rainfall and flow), flood risk thresholds (base on Standardized Precipitation Index) return period. Naglanou and Akissa forest areas record heavy rainfall ranging 161 to 277 mm in June and 90 to 130 mm in October, representing respectively 16 and 13% of annual rainfall (1961-2015). This unequal spatiotemporal rainfall distribution determines surface flows and moisture of these forest sites during water level rise periods. Moreover, increase of flow rate by 20.38% over 1961-2015 and 14% over 1961-2000 linked to rainfall since 1990 and impoundment of Nangbeto dam since 1987. Flood hazard thresholds are limited (424.8 m3.s-1), moderate (609.3 m3.s-1), significant (709.1 m3.s-1), and critical (824.1 m3.s-1). Return periods correspond to 2, 10, 20 and 50 years. Key words: Naglanou and Akissa Forest, biodiversity, mono River Delta, standardized precipitation index, flood risk threshold.

Climate and land use/cover changes are potential drivers of change in hydrology and water use. Incidences of these factors on Bandama hydrological basin and Kossou hydropower generation (1981&ndash, 2016) in West Africa are assessed in this present work. Using Landsat products of United Stated Geological Survey, results show that water bodies areas and land use have increased by 1.89%/year and 11.56%/year respectively, whereas herbaceous savanna, savanna, forest and evergreen forest coverage have been reduced by 1.39%/year, 0.02%/year, 2.39%/year and 3.33%/year respectively from 1988 to 2016. Hydroclimatic analysis reveals that streamflow presents greater change in magnitude compared to rainfall though both increasing trends are not statistically significant at annual scale. Streamflow varies at least four (two) times greatly than the rainfall (monthly and seasonally) annually except during driest months probably due to land use/cover change. In contrast, Kossou hydropower generation is significantly decreasing (p-value 0.007) at both monthly and annual scales possibly due to water abstraction at upstream. Further works are required to elucidate the combined effects of land use/cover and climate changes on hydrological system as well as water abstraction on Kossou generation.

This paper performs non-parametric Mann Kendall (MK) trend analysis of historical hydroclimatic data (1961-2016), an ensemble climate model validation and a computation of 16 Expert Team on Climate Change Detection and Indices (ETCCDI) temperature and rainfall extremes indices. The climate indices are evaluated using MK test and annual trend analysis for two Representative Concentration Pathways (RCP4.5 & RCP8.5) future scenarios from 2020 to 2045 over Mono River Basin (MRB) in Togo. The annual and seasonal trend analyses are assessed on historical potential evapotranspiration, mean temperature, rainfall and discharge data. Results show positive and negative trends of hydroclimatic data over MRB from1961 to 2016. Mean temperatures increase significantly in most of the stations while a negative non-significant trend is noticed for rainfall. Meanwhile, the discharge presents a significant seasonal and annual trend for three gauge stations (Corrokope, Nangbéto and Athiémé). Validation of the ensemble climate models reveals that the model under-estimates observations at Sokode, Atkakpamé and Tabligbo stations, however linear regression and spatial correlation coefficients are higher than 0.6. Moreover, the percentage of bias between climate model and observations are less than 15% at most of the stations. Finally, the computation of extreme climatic indices under RCP4.5 and RCP8.5 scenarios shows a significant annual trend of some extreme climatic indices of rainfall and temperature at selected stations between 2020 and 2045 in the MRB. Therefore, relevant governmental politics are needed to elaborate strategies and measures to cope with projected climate changes impacts in the country.

International audience; Droughts and floods are responsible for ~ 80% of fatalities, and ~70% of economic losses related to natural hazards in Sub-Saharan Africa. Rainfall variability which is driven by interannual to multi-decadal climate fluctuations, here underpins these issues but is also crucial to agriculture and livelihoods. However, due to data scarcity, little is known about the impact of these climatic fluctuations and catchment properties on streamflow variability. Therefore, in this study, using a newly reconstructed streamflow dataset, we aim to extend understanding of hydrological variability through investigation of key large-scale controls which modulate climate-river flows relationships at the subcontinental scale. Modes of variability are extracted using continuous wavelet transform and maximum overlap discrete wavelet transform. We first assess the relative importance of catchment properties in modulating streamflow and modes of variability, before examining teleconnections with climate variables from different datasets (ERSST.v5 and 20CR.v2). The results underline the importance of interannual components of river flow along the Gulf of Guinea and the coastal regions of Central Africa where annual rainfall amounts are higher, and runoff is mostly generated from surface and sub-surface processes. In contrast, river flows along the Sahelian band show stronger multi-decadal components: this partly reflects the prominent role of geological formations in modulating high frequency rainfall signals. These modes of variability (also significantly detected in rainfall) are likely related to anomalies in sea-surface temperature (SST) anomaly patterns and associated atmospheric circulations, which together modulate the West African monsoon. Clarifying the picture of streamflow variability in western and Central Africa thus opens up the prospect for improved future predictions for streamflow and water resources in data scarce environments.

West African basins play a vital role in the socio-economic development of the region. They are mostly trans-boundary and sources of different land use practices. This work attempts to assess the spatio-temporal land use and land cover changes over three South Western African basins (Volta, Mono and Sassandra basins) and their influence on discharge. The land use and land cover maps of each basin were developed for 1988, 2002 and 2016. The results show that all the studied basins present an increase in water bodies, built-up, agricultural land and a decline in vegetative areas. These increases in water bodies and land use are as a result of an increase in small reservoirs, of dugouts and of dam constructions. However, the decline in some vegetative clusters could be attributed to the demographic and socio-economic growth as expressed by the expansion of agriculture and urbanization. The basic statistical analysis of precipitation and discharge data reveals that the mean annual discharge varies much more than the total annual precipitation at the three basins. For instance, in the entire Volta basin, the annual precipitation coefficient of variation (CV) is 10% while the annual discharge CV of Nawuni, Saboba and Bui are 43.6%, 36.51% and 47.43%, respectively. In Mono basin, the annual precipitation CV is 11.5% while the Nangbeto and Athieme annual discharge CV are 37.15% and 46.60%, respectively. The annual precipitation CV in Sassandra basin is 7.64% while the annual discharge CV of Soubre and Dakpadou are 29.41% and 37%, respectively. The discharge varies at least three times much more than the precipitation in the studied basins. The same conclusion was found for all months except the driest months (December and January). We showed that this great variation in discharge is mainly due to land use and land cover changes. Beside the hydrological modification of the land use and land cover changes, the climate of the region as well as the water quality and availability and the hydropower generation may be impacted by these changes in land surfaces conditions. Therefore, these impacts should be further assessed to implement appropriate climate services and measures for a sustainable land use and water management.

Le couvert vegetal de la commune de Bante subit de forte pression anthropique. L’objectif vise a ete d’analyser l’influence de la pression humaine sur l’evolution spatio-temporelle des unites paysagiques. Les donnees utilisees concernent les statistiques demographiques de l’INSAE des annees1979, 1992, 2002 et 2013, l’historique de l’installation des populations dans le temps et dans l’espace, les cartes planimetriques et les images Landsat TM de 1986, Landsat ETM+ de 2000 et Landsat OLI-TIRS de 2016 de la commune de Bante. La cartographie des unites d’occupation des terres sur la base des donnees de la teledetection et leur etude diachronique ont ete les principales methodes utilisees. La cartographie des changements spatio-temporels du couvert vegetal a revele que la superficie des formations vegetales naturelles est passee de 93,63% dans la Commune en 1986 a 92,92% en 2000 et a 38,83% en 2016 au profit des champs et des jacheres dont la superficie est passee de 6,19% en 1986 a 6,68% en 2000 et a 33,15% en 2016. Entre ces trois dates, les formations saxicoles ont completement disparu au profit des mosaiques de champs et jacheres. Dans le meme temps, le phenomene de colonisation agricole s’est accru avec la creation de nombreux campements peuhls et fermes faisant passer le nombre de 100 a pres de 200 lieux habites. En somme, la pression humaine sur les terres a engendre une degradation des formations naturelles au profit des formations anthropiques, necessitant ainsi, une gestion rationnelle de l’espace communal par les autorites locales en charge. Mots cles : Benin, Bante, pression anthropique, couvert vegetal, peuplement, migration.

The study aims to analyze the performance criteria of the GR4J model to reproduce high water flows in the Ouémé watershed at Bonou's outlet which has been vulnerable to climate change in recent decades. The methodology focused on the use of daily climatological and hydrometric data extracted from files of National Directorate of Meteorology, and General Directorate of Water; they were supplemented by those of SIEREM/HSM dataset over the period 1961-2015. The rainfall was regionalized using Thiessen method. The performance of the GR4J model was assessed with NSE, RMSE and KGE criteria. The results indicate that the study area is marked by rainfall variabilities and detection of two breakpoints (1968 and 1987) which divide the series into three sub-periods; these discontinuities have repercussions on the streamflow. It's found that GR4J model overestimates the streamflow during the low water period and underestimates them in high water. However, the efficiency and performance criteria NSE, RMSE and KGE calculated on high water flow rates are better in calibration than in validation. The KGE values are range between 83-85% in calibration and 56-68% during validation, which gives to GR4J model the efficiency and performance to reproduce high flow rates in the study area

Les regions soudano-saheliennes sont confrontees aux defis de l’explosion demographique et des changements climatiques qui induisent des modifications inevitables dans les systemes de gestion des terres. Les processus naturels et humains ont des consequences sur les capacites des systemes de production. Ce phenomene conduit dans plusieurs regions a la degradation des ressources naturelles, a la faible productivite des terres et a la perte de la biodiversite comme c’est le cas deja dans le bassin du fleuve Niger au Benin. L’analyse s’appuie sur les donnees climatologiques de 1951-2010, les statistiques de production agricole de Banikoara de 1978 a 2010. Ces donnees sont completees par celles des enquetes de terrain et des resultats d’une seance de travail avec les acteurs de gestion des ressources naturelles dans le bassin. La rarete croissante des ressources naturelles et l’augmentation des besoins ont entraine une forte competition et divers conflits entre les utilisateurs du bassin. En effet, les strategies les plus envisageables sont : la culture de contre-saison dans les bas-fonds, la reduction des superficies consacrees a la culture du coton, l’utilisation des plantes veneneuses pour droguer les poissons, le reboisement en amont des retenues d’eau et sur les berges. Mots cles : Bassin beninois, Niger, ecosystemes sensibles, changements environnementaux, gestion.

Les inondations des dernières années en Afrique de l'Ouest rendent nécessaire la prévision hydro-logique pour réduire la vulnérabilité des populations. Nous avons étudié la dynamique des crues dans le bassin du Mono en amont du barrage de Nangbéto (Togo et Bénin) par des méthodes statistiques et déterministes. L'augmentation des précipitations au cours de la décennie 1990 et la mise en eau du barrage de Nangbéto en 1988 ont modifié le régime du Mono, dont les débits d'étiages ont augmenté. Les débits entrants et sortants du barrage sont équivalents, même lors des crues. Les maximums annuels des débits entrants peuvent être modélisés par une distribution des valeurs extrêmes (GEV) de type Fréchet, tandis que ceux des débits sortants, suivent une loi GEV de type Weibull (bornée). Le mécanisme dominant de génération des crues dans le bassin est la montée progressive des eaux associée à l'augmentation de la saturation des sols. L'analyse des débits de crue entre 1988 et 2010 n'a pas montré d'augmentation de la magnitude ou de la fréquence des crues. Le modèle GR4J reproduit bien les débits de crues en période calage, mais ces critères de performance se dégradent lorsque l'on s'en éloigne. Dans ces conditions, l'utilisation de ce modèle pour évaluer les risques futurs de crues serait hasardeuse et il est nécessaire d'envisager d'autres types d'approches.

International audience

It is likely that climate change in the coming decades, even very few, will produce more important impacts on thesocio-economic systems than in the past, due to the growth in world population and the increasing sophistication of technology and development issues (Issa, 1995). According to UNESCO (2006), climate change will be responsible for 20% of the worsening water scarcity in the world. This looming water crisis will be induced by changes in the parameters that determine the water cycle (Totin, 2005). The study is based on analysis of a baseline from which the main meteorological parameters (rainfall, flow, etc.) are projected to 2025 time horizon. It corresponds to the horizon of the prospective study for 2025 in Benin. From these basic parameters, the main elements to characterize water resources (such as infiltration, the volume of water available water bodies, etc.) were determined for each time horizon. Research carried out in different regions of the Benin on climate issues, surface and ground water (Gbatcho 1992; Boko, 1998; Boukari, 1998 Linsoussi, 2000; Sadji, 2004; Amoussou, 2003 and 2005; Totin, 2003 and 2005; Tossa, 2005; Vissin et al., 2006 ; Vissin, 2001 and 2007 ; Alassane and Boukari, 2007; Idiéti, 2009 ; Totin, 2009) attest ruptures occurred in hydro-climatic chronicles during the 1970s and indicate a downward trend in superficial water resource two to four folds greater than that of rainfall in key watersheds. Similarly, the aquifers record deficit on refill aquifers (Boukari, 1998; Totin, 2005) and a variation of hydro-chemical caracteristics (Alassane, 2004). If this climate and hydrological trend continues, hydrological drought will lead to a considerable reduction of water resources and a potential shortage. Similarly, the water quality will be affected in various ways. Very wet weather events would induce an increased transfer of pollutants to the water bodies as opposed to dryness that would accentuate deterioration especially of the bacteriological quality of water resources. Facing to the quantitative and qualitative degradation of both surface and groundwater should develop strategies to increase the potential availability of the resource. To this end, it is important above all to protect areas of groundwater recharge, stabilize stream banks and water bodies. Improving the quality of water resources in a changing climate context implies that people are aware of the risks of shortage and of agricultural good practices (no use of chemical fertilizers and synthetic pesticides), management of solid and liquid waste. The water law must be adapted to the context of climate change and vulnerability of the water resource.