ABSTRACTHydroclimatic variability manifests as abrupt shifts, trends, runs, and recurrent cyclical phenomena, collectively referred to as components. In this article, we tested for the presence and magnitude of each component in the Niger Basin (West Africa), using the 0.5° by 0.5° gridded annual rainfall and temperature data produced by the Climate Research Unit (CRU) at the University of East Anglia, UK, for the period 1901–2006. The streamflow data was also analysed for different sub‐basins of the Niger Basin. Abrupt shifts were tested using a Bayesian and nonparametric approach. Trends were analysed using the Mann‐Kendall trend test. Runs were extracted for dry, neutral, and wet conditions, simulated 1000 times based on the skew normal distribution, and used to investigate various run characteristics. Cyclical behaviour was investigated using continuous wavelet analysis. The results show that an abrupt change point occurred in 1969 in the rainfall and streamflow (but not temperature) time series in all subwatersheds of the Niger Basin. The magnitude of the shift in the mean rainfall varied between 16 and 24%. The temperature time series exhibit strong positive trends in all watersheds. Post change point, the rainfall and streamflow time series show positive, though statistically non‐significant trends at α = 0.1. In contrast, disregarding the change point, all subwatersheds show significant negative trends at α = 0.05 and the maximum run lengths are about 4 years long for both dry and wet conditions. Finally, wavelet analysis showed that both rainfall and streamflow in the Niger Basin fluctuate on cycles that are predominantly 2–4 years long, with a few occurrences in the 6–8 years range. Wavelet activity diminished noticeably when a series appeared strongly dominated by trends. The results provide a more comprehensive view of climatic variability than would be obtained from only one or a few components.

The nine member states (Benin, Burkina Faso, Cameroon, Ivory Coast, Guinea, Mali, Niger, Nigeria and Chad) sharing the Niger River Basin have early recognized the concerted management of water resources by creating the Niger River Commission in 1963 and the Niger Basin Authority (NBA) in 1980.The NBA, in the aim to lead a concerted, sustainable, equitable management of the basin water resources, has developed a shared vision with the support of the 9 Member States and signed a Declaration called "Declaration of Paris". This statement sets out the management principles and good governance for sustainable and shared development of the Niger Basin. The implementation of the Shared Vision has developed legal, institutional and technical instruments for the planning and good governance of water resources.- Legal tools : The agreement creating NBA signed in 1980 by nine member states and the Water Charter adopted in 2008 and ratified by member States.- Institutional Bodies tools of good governance of the basin are: The Summit of Heads of State and Government, The Council of Ministers, the Technical Committee of Experts, the Executive Secretariat with the Niger Basin Observatory, the National Focal Structures, the Permanent Technical Committee, the Panel of Experts and the Sub-basins Commissions.- Technical tools for planning and management for the development of the basin resources A network of 105 hydrometric stations and a database which serves to support the models of hydrological forecasts and allocation of water resources, an Action Plan for Sustainable Development (APSD) by 2027, an Investment Program by 2027, a Hydraulic model for allocation and water resources management and a model for hydrological forecasting.These models are used to analyze the impacts of new development on the basin and alert in case of imbalance. As a tool for decision support, they can anticipate emergencies and deficits, plan achievements and manage hydraulics structures.

The River Niger is the third longest river in Africa, with a stream length of 4200 km, a drainage basin of 2 170 500 km (of which 1 500 000km(2) is an active basin), and an average discharge of about 6000 m(3)/s. The natural variability of its rainfall and discharge are analysed for several major sub-basins, in the context of the West African drought, which has lasted for nearly 40 years. Two paradoxes are shown: the increase of Sahelian runoff since the beginning of the drought due to land degradation, and the steep decrease of Sudanian runoff over the same period, substantiated by the long-lasting decrease of the groundwater tables. The cooperation between the nine countries sharing the basin is organized through the Niger Basin Authority (NBA), which needs to propose and administer plans to meet objectives for all development projects throughout the basin. Much information about the water resources available in the basin is collected and analysed by the NBA, which is summarized in this paper, including surface and groundwater resources, rainfall and evaporation over the basin, existing and projected dams and water consumption for irrigation. From the standpoint of water resources, the Niger basin can be divided into four zones with different physical and geographical characteristics: (i) the Upper Niger basin, in Mali, Guinea, and Ivory Coast, covering a surface of 257 000 km(2), with only one large dam in Mali (Selingue, 1.7 MMm(3)); (ii) the inner delta, in Mali, which seasonally flooded surface can reach 35 000 km(2), and where the water losses vary between 24% and 48%; (iii) the Middle Niger basin, in Mali, Niger and Benin, where local flows from Sahelian and Sudanian areas join the residual outflow from the inner delta; and (iv) the Lower Niger basin, between Cameroon, Nigeria and Chad, where high rainfall increases the River discharge, and where large dams for hydro-electric power production and irrigation are built. The River Niger is deficient in dams to control water, especially in its upper and middle basins. Nigeria has many dams, including large dams, while Burkina-Faso has many small dams, but there are only a few dams upstream of the River Niger in Mali/Guinea/Ivory Coast. It is therefore likely that several dams will be built in the Niger basin in the coming years, and several are in the project phase: among which three are large ones in Guinea (Fomi), Mali (Tossaye) and Niger (Kandadji). All of these will have a large impact on the River Niger regime and the environment, especially the Fomi dam, which will significantly change the river regime upstream of the inner delta, inducing an important reduction of the flooded area, and the Tossaye dam on the Saharan border of Mali, which could promote a very significant level of evaporation, while the Kandadji dam will have less impact downstream. It is very important before building these dams to take into account the past years variability of climate and river regime. It is particularly important to take into account the very large runoff decrease in the tropical humid sub-basins, and the runoff increase in the Sahelian sub-basins.

Abundant information is available on West African drought and its hydrological and environmental impacts. Land-use and climatic changes have greatly modified the conditions of Sudanian and Sahelian hydrology, impacting the regime and discharge of the main rivers. Human pressure on the environment (significant increase in crops and disappearance of natural bushes and landscapes, for example) has led to severe soil crusting and desertification throughout Sahelian regions.Despite recent increases in rainfall, the drought has not ended, resulting in two different hydrological evolutions. In the Sudanian areas, stream flows have been reduced, sometimes as much as twice the rainfall reduction rate. In the Sahelian regions, runoff coefficients have increased to such a degree that discharges are increasing, in spite of the reduced rainfall.The main goal of this paper is to synthesize the recent advances in the Sahelian and Sudano-Sahelian West African hydrology. The other objectives are two fold: First, to discuss the "Sahelian Paradox" (the increase in runoff in most of the Sahel during the drought, at least during the 1968-1995 period, as described in the 1980s) and paradox of groundwater highlighted in the square degree of Niamey (the rise in water table levels in some endorheic areas during the same drought, evidenced in the 1990s), and second, to attempt to define the application of their respective geographical areas.The land-use changes act as a general factor of hydrological evolution of soils and basins, while some spatial factors explain the great variability in the response to environmental evolution, such as endorheism, geological context, latitudinal climate gradient, and local hydrodynamic behaviour of environment.This paper is literature-based, and incorporates current research advances in the field, as well as a prospective focused on resources and socio-economic impacts. (C) 2009 Elsevier B.V. All rights reserved.

The annual average of rile groundwater levels in 27 wells is connected to the annual climate impact on the groundwater resources in the Bani river basin (Douna, 101 600 square kilometers). The groundwater level variations are well correlated to rainfall variations, and for the lowest levels of groundwater, the runoff stops in the dry season at Douna. The humid year of 1994 caused a rise in the groundwater level to the same level as 1981, together with a reduction of the depletion coefficient. But the runoff is still weak, showing that the rainfall-runoff relationships are modified over the long term. (C) 2000 Academic des sciences/Editions scientifiques et medicales Elsevier SAS.

Nous avons etudie les fluctuations interannuelles des niveaux piezometriques minimums sur 30 puits du bassin versant du Bani a Douna au Mali, installes a partir de 1981 dans les deux aquiferes du Socle et de l'Infracambrien schisto-greseux. La moyenne annuelle sur la surface du bassin des niveaux minimums est representative de l'impact annuel du climat sur les ressources en eaux souterraines du bassin. Les variations du niveau de la nappe sont bien correlees avec les variations de pluie, et les annees de debit d'etiage nul a Douna correspondent aux annees de niveau de nappe le plus bas de la serie de 15 annees d'observations. L'effet de la bonne annee pluviometrique 1994 est tres sensible sur le niveau de la nappe, avec une remontee au niveau de 1981, ce qui est concordant avec le coefficient de tarissement le plus bas depuis 1982.