Basins and tributaries
The Niger River (4 180 km) is the longest river in West Africa and the third longest on the African continent. From its source in the highlands of Guinea, the river passes through Mali, Niger and Nigeria where it flows into the Gulf of Guinea in the Atlantic Ocean. The river drains a catchment area of over 2 million km2 and provides water to more than 100 million people.
The Niger and its tributaries rise in the Guinea Highlands. Next to the Niger proper there are five main tributaries, the Tinkisso, Niandan, Milo and Sankarani in Guinea and the Bani in Mali (see viewer). The most northerly branch, the Tinkisso, originates in the Fouta-Djalon. The main tributary of the Niger is the Bani, which drains southernmost Mali and the north-eastern corner of Ivory Coast. After the Bani flows into the Niger near Mopti, at the southern edge of the Inner Niger Delta, there is no further run-off from eastern Mali and Niger. Consequently, evaporation gradually diminishes the river flow.
The total catchment area of the Bani (129 000 km2) is nearly as large as the rest of the Upper Niger Basin (147 000 km2). Yet the discharge of the Bani is less than half that of the Niger, because the Bani receives less rainfall than the other sub-basins of the Upper Niger.
Rainfall and discharge
Long term rainfall data and monitoring of discharge enables to show how the river flow is related to rainfall. This relationship has been investigated by Zwarts et al. (2005, 2009). The data of 28 long-term rainfall stations on the Upper Bani were related to the discharge at Douna. 17 long-term rainfall stations are available for the Upper Niger Basin. These data are compared with the discharge at Koulikoro. Rainfall and hydrometric stations are shown on the map.
The Niger Basin, showing four dams (Selingue and Markala on the Upper Niger, Talo on the Bani and Kainji on the Lower Niger), the hydrological stations (Koulikoro and Douna) and 45 long term meteorological stations on the Upper Niger. From Zwarts et al. (2009).
The annual rainfall in the Bani catchment area usually varies between 1000 and 1200 mm. Wet and dry periods are reflected in water flow. The Bani’s flow in September fell back from 3000 m3/s to only 250 m3/s during the Great drought in the early 1980s, but subsequently gradually increased again. The long-term effect of a series of dry years, with a low rainfall, on the flow of the Bani is evident. The Bani was a river with a fully natural flow until 2006 when the Talo dam became operational. Since 1922, the average annual rainfall in the Upper Niger catchment has varied between 1300 and 1600 mm. The long term trends of rainfall and flow in the Upper Niger resemble those shown for the Senegal and Bani Rivers, but where the Senegal and the Bani lost 80% of their flow during the Great Drought in the 1980s, the decrease in the Niger was slightly less than 50%.
Left: Annual rainfall in the Upper Bani sub-basin (based on 28 rainfall stations, left y-axis) and the river discharge in September at Douna (right y-axis). The trends show the 9-year running mean. See the map before for the location of Douna and the rainfall stations.
Right: Annual rainfall in the Upper Niger sub-basin (17 rainfall stations) and the river discharge in September at Koulikoro. The red bars show the effect of the Sélingué reservoir on the river discharge in September. See the map before for the location of Douna and the rainfall stations. Source Zwarts et al. (2009).
Discharge and water intake
Since 1982, the flow of the Upper Niger has not been fully natural due to the construction of the Selingue reservoir in the Sankarani. The Selingue reservoir covers 450 km2 when full (2.1 km3). The monthly inflow and outflow have been registered since its inception. The annual loss amounts to 0.83 km3, of which 0.57 km3 is lost to evaporation (source Zwarts et al. 2005); the rest becomes groundwater. More important than water loss is the change in seasonal water flow. The flow into the reservoir (representing natural flow) is reduced by, on average, 61% in August and by 36% in September due to the filling of the reservoir. In contrast, when water is released from the dam in the dry season, the outflow between February and April is about three times the amount of the natural flow. During the first years of its existence, the effect of the Sélingué dam (red bars in graph) was limited, because the lake was only partly emptied in the course of the year and not fully refilled.
Another dam in the Upper Niger, the Markala dam, is a low weir across the river, used to raise the water level sufficiently to irrigate the area managed by Office du Niger. The irrigation zone is located in the Delta mort, the dead Delta, an ancient branch of the Niger. The Markala dam has been operational since 1947, but it took many years before the irrigation scheme was developed.
The water intake, as registered by Office du Niger, amounts to 2.69 km3 per year, equivalent to 86 m3 /s (source Zwarts et al. 2005). Despite the gradual extension of the irrigated zone, water extraction remained at the same level between 1988 and 2006. Over the same period, the annual river discharge at Koulikoro varied between 624 and 1258 m3/s. As a consequence, water use by Office du Niger is not more than 6% of a high river flow (as in 1995), but this may increase to 16% when the flow is low (as in 1989). The monthly water use by Office du Niger is 60 m3/s in January, gradually increasing to 130 m3 /s in October, decreasing to 90 m3/s in November and 50 m3/s in December. The available water supply varies between 100 m3/s in March and 3200 m3/s in September. Hence 60% of the flow is tapped in March, as opposed to only a few percent in September.
The effect of irrigation on the river flow differs from the impact of dams, which is illustrated in the figure below. In the case of dams, water is withheld during the wet season to fill the reservoir and released in the dry season to generate electricity. Water is lost due to evaporation. In case of irrigation there is a continues water intake; the amount of water intake depends on the irrigated surface area. The impact on the river flow is relatively small during a high flood and large during a low flood.
Visualisation of hydrological effects of dams and water intake for irrigation during the flood cycle in years with a high and low flood. The flood cycle in the Western Sahel zone runs from June to December. Lowest flood levels occur from March to May. From Zwarts et al. (2005).
The impact of upstream interventions on the downstream discharge and flooding of the Inner Niger Delta have been subject to many studies. An overview of hydrological and hydrodynamic models for the Upper Niger Basin, which are able simulate the impact of water intake for irrigation and dams, is given in Section Hydrological modelling. Background information on the hydrology of the Upper Niger Basin and the impact of upstream interventions can be found, amongst others, in the work of the IRD (Orange et al. 2002) and Zwarts et al. (2005, 2009), and on the level of the entire Niger River Basin in the studies of BRLi & DHI (2007) for the ABN (Niger Basin Authority).
Recent studies, that include the latest information on planned water intake and dimensions of infrastructure, are carried out in the framework of impact assessment studies on the Fomi-Moussako dam (AECOM 2017, Tractebel et al. 2017) and on the future extension of the Office du Niger (PAHA-studies; BRLi & BETICO 2015, 2016).
The (near) future interventions in the hydrological regime of the Upper Niger Basin require a balanced management of its water resources. Within this framework the BAMGIRE- project (2016-2019) was initiated to investigate and promote the concrete options for an integrated water resource management in the Upper Niger Basin. BAMGIRE is funded by the Dutch ministry of International Cooperation and led by Wetland International Mali. The studies in this context are coordinated by Wetlands International and carried out in cooperation with the Potsdam Institute for Climate Adaptation (Aich et al. 2016, Liersch et al. 2017), the University of Ottawa (Haque et al. 2017), Altenburg & Wymenga (Zwarts in prep.) and the International Water Management Institute (IWMI, e-flow modelling).
Sources and more information
- AECOM 2017. Actualisation de l’étude d’impact environnemental et social du barrage à buts multiples de Fomi en Guinée. Phase 1. Scoping pour déterminer la configuration du barrage de Fomi. Ministère de l’Agriculture, Direction Nationale du Génie Rural. Rapport provisoire de la Phase 1 – 60543397 – Novembre 2017. AECOM.
- Aich V; Liersch S, Vetter, T, Fournet S, Andersson JCM, Calmanti S, van Weert FHA Hattermann FF & Paton EN 2016. Flood projections within the Niger River Basin under future land use and climate change. Science of The Total Environment 562: 666-677, ISSN 0048-9697,
- BRLi & BETICO 2015. Projet d’accroisement de la productivité agricole au Mali (PAPAM). Étude du Programme d’Aménagement Hydro-Agricole (PAHA) de la zone Office du Niger (ON). Rapport de Phase 1. Etat des lieux. Volume 2. Aménagements hydrauliques. Janvier 2015.
- BRLi & BETICO 2015. Projet d’accroisement de la productivité agricole au Mali (PAPAM). Étude du Programme d’Aménagement Hydro-Agricole (PAHA) de la zone Office du Niger (ON). Rapport de Phase 2. Elaboration de scenarios. Août 2015.
- BRLi & BETICO. 2016. Projet d’accroisement de la productivité agricole au Mali (PAPAM). Étude du Programme d’Aménagement Hydro-Agricole (PAHA) de la zone Office du Niger (ON). Rapport de Phase 3. Concertation et choix du scenario. Février 2016.
- BRLi & DHI. 2007. Establishment of a water management model for the Niger River Basin. Final Report (= document Rapport_final_Corps-rapport_anglais.pdf).
- Liersch, S, S. Fournet, H. Koch 2017. Assessment of Climate Change and Water Management Impacts on the Water Resources in the Upper Niger and Bani River basins. Potsdam Institute for Climate Impact Research, Potsdam. Draft, 9 June 2017.
- Orange D, Arfi R, Kuper M, Morand P & Poncet Y, editors 2002. Gestion intégrée des ressources naturelles en zones inonables tropicales. Paris: IRD.
- TRACTEBEL ENGINEERING s.a. 2017. FOMI dam project on the Upper Niger: update of the feasibility study, detailed design (APD) – Phase I. Hydrological report - Hydrological study. P.010186 RP 2-001-E-Vol. 1a & 1b. TRACTEBEL, Cedex.
- Wymenga, E, Kone, B, van der Kamp J & Zwarts L. 2002 – Delta intérieur du fleuve Niger. Écologie et gestion durable des ressources naturelles. Veenwouden: A&W/Wetlands International /Rijkswaterstaat.
- Zwarts, L, Bijlsma RJ, van der Kamp J & Wymenga, E 2009. Living on the Edge. Wetlands and birds in a changing Sahel. KNNV Publishing, Zeist. p. 1-564.
- Zwarts, L, Cissé N & Diallo M. 2005a. Hydrology of the Upper Niger. In: Zwarts L, Beukering van P, Kone B, Wymenga E, editors. The Niger, a lifeline. Lelystad: RIZA/Wetlands International/IVM/A&W. p. 15-40.
- Zwarts, L, P. van Beukering, B. Kone & E. Wymenga 2005b. The Niger, a lifeline. Lelystad: Rijkswaterstaat/IVM/Wetlands International/A&W. p .1-304.
- Zwarts, L. 2010. Le Delta Intérieur du Niger s’assèchera-t-il du fait du changement climatique et de l'utilisation de l'eau en amont ? Report A&W 1556. 33 p.
- Zwarts, L. 2010. Will the IND shrivel up due to climate change and water use upstream? A&W Report 1537. 33 p. http://www.altwym.nl/uploads/file/389_1294301369.pdf.
- Zwarts, L., Bijlsma, R., van der Kamp, J. & Wymenga, E. 2009. Living on the Edge: Wetlands and bird in a changing Sahel. KNNV Publishing. 564 p. http://www.altwym.nl/uploads/file/540_1433753005.pdf