ANALYTICAL ABSTRACT
West Africa, particularly the Sudano-Sahelian zone, has experienced unprecedented climate variability in recent decades. Despite some periods of respite, the statistics do not really plead for a return to better climatic conditions, precisely rainfall. Beyond the structural aspect of this climate variability, many effects have been observed on socio-economic activities and also on socio-cultural practices. This situation has a dramatic impact on water resources and in particular on the hydrology of West African transboundary basins such as those of Senegal and Gambia. The overall objective of this study is to contribute to a better understanding of climate variability and risks and their impacts on the water resources availability in transboundary watersheds in West Africa. It aims to create up-to-date knowledge bases for climate variability analysis and risk assessment as well as the search for sustainable solutions to overcome the environmental and societal vulnerability. In this perspective, the updating of the West African climate database and its extension to shared basins (Senegal and Gambia) should make it possible to carry out relevant analyses of climate variability and its impact on the environment and more particularly on the water resources availability. The Senegal and Gambia River basins are selected as sites for this study, which focuses on analyzing the climate vulnerability and its effects on water resource variability in West Africa. These two transboundary tropical basins cover respectively an area of about 300,000 km2 and 77,054 km2. The Senegal River is 1,800 km long and its basin is shared between Guinea (7%), Senegal (8%), Mali (35%) and Mauritania (50%). About 3.5 million people live in this basin, 85% of whom live near watercourses. The Gambia River basin is divided between Senegal (70.9%), Guinea (15.4%), Gambia (13.7%) and Guinea Bissau (0.021%). The Gambia River is 1,150 km long, 205 km of which are in Guinea, 485 km in Senegal and 490 km in Gambia. The project's objectives include the following: - to update the climate database and cover all the two basins of Senegal and Gambia; - to analyze the climate variability and trends across West Africa with a focus on the Senegal and Gambia river basins; - to identify climate impacts on hydrology in West Africa based on data from monitoring stations, metadata, existing modelling tools and assessments and even climate re-analyses if necessary; - to build materials for tailor-made training sessions. Precipitation remains the main conditional factor in the hydrological regime in West Africa and constitutes the largest part of the climate data collected from these different sources. Other climatic factors (temperature, sunshine, humidity, wind regime) that have a much less direct influence and are less likely to have changed the regime were also collected. In general, the available data range from the origin of the measures until 2016. However, it must be considered that time series are rarely continuous; this can sometimes be a limitation for the implementation of analyses. These two basins (Senegal and Gambia) straddle very contrasting climatic zones: the wellwatered Guinean and Sudanese areas and the semi-arid and arid Sahelian areas. The average rainfall of the Senegal River Basin is 550 mm.yr-1, varying between more than 1500 mm.yr-1 at the source in Fouta Djalon Mountain, to less than 200 mm.yr-1 in the most northern part of Senegal. In the Gambia River Basin, the rain measurement varies considerably in each riverside country: from 1200 to 4500 mm in Guinea; from 1200 mm in the north to 2400 mm in the south in Bissau Guinea and about 500 to 1000 mm in Senegal and in Gambia. The rainfall regime as well as the hydrological regime of these two rivers are historically marked by strong interannual and seasonal variability. From the early 1970s to the end of the 1990s, the basins had suffered from chronic rainfall and water deficits. Over the past two decades, significant improvements in rainfall and average river water conditions have been observed. But on a scale of 50 to 100 years, we are still in a generally dry sequence, marked by a strong temporal and spatial variability. The dams built on the Senegal River, particularly the Manantali dam, reduce the seasonal variability of flows but do not eliminate the very marked unimodal nature of the river regime, with most of the flows concentrated over a short period of the year (from August to October). The region is therefore facing major hydro-climatic challenges, with strong repercussions on the biophysical environment but also on the social and economic activities of these basins. Concerning of the environment, the landscapes of the Senegal and Gambia River Basins remain highly contrasted and closely linked to climate zoning. The upper basins of Senegal and Gambia, the Fouta Djalon area, have a relatively dense vegetation cover and are home to most of the wildlife of these basins. There is here a high value protected area, the Niokolo-Koba Park, a UNESCO World Heritage Site. In the Sahel region, semi-desert landscapes are sparser, with gallery forests and wetlands containing relatively high concentrations of flora and fauna species. Overall, the physical environment of the two basins is in a fairly advanced state of degradation despite the fact that areas of rich biodiversity remain in various places. These are the Fouta Djalon Massif, the Bafing Fauna Reserve, the Boucle du Baoule Biosphere Reserve, the floodplain in the middle valley of Senegal, natural lakes such as Lake Guiers and Lake R'kiz, wetlands classified as Ramsar sites (2 in Guinea, 1 in Mali, 2 in Mauritania and 4 in Senegal), and several classified forests. To this must be added the dam reservoirs: the Manantali and Diama dams on the Senegal River, which has been in operation for some thirty years; the Felou reservoir, which dates from 2013. These reservoirs increasingly play ecological functions similar to those of natural wetlands. However, their ecosystem potential remains undervalued. In general, the environment of the basin -including the areas of high biodiversity value mentioned above- is subject to various pressures and threats resulting from a combination of factors such as deteriorating hydro-climatic conditions and very high population growth. In view of this, the challenges of water and environmental management in these basins are therefore enormous, despite the existence of two basin organizations (OMVS for the Senegal River Basin and OMVG for the Gambia) responsible for establishing coordinated and concerted management of water resources and ensuring social peace and an institutional climate favorable to the development of riparian States. The first results of this study confirmed the knowledge previously acquired on the main fluctuations in rainfall and hydrological regime in the Senegal and Gambia river basins. The use of hydrological indices makes it possible to visualize and subdivide the chronicles studied into several intervals according to dry or wet conditions and thus to characterize the extent of dry periods as well as their intensity. Examination of the rain-flow relationship confirmed their synchronization throughout the study period. Hydrological drought indices indicate that the most severe droughts have occurred since the 1970s in the two basins. Hydrological modelling with the SWAT tool applied to the Bafing catchment area has given encouraging results as to the applicability of the model with a very high satisfaction degree. By joining efforts with the team of JRC, the model will also be extended to the entire basin of Senegal. The next step in the modelling component of this study will be to test the WEAP model over the basins of Senegal and Basin. Keywords: climate variability; transboundary basins; hydro-rainfall data; drought; hydrological modelling