ABSTRACT

The river Niger is the 3^rd longest river in Africa, with a stream length of 4200 km, a drainage basin of 2,170,500 km² of which 1,500,000 km² is an active basin, and an average discharge of about 6000 m³/s. The natural variability of its rainfall and discharge is analyzed for several major sub-basins, in the context of the West African drought which has lasted for nearly 40 years, showing two paradoxes: 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. Much information about the water resources available in the basin is collected and analysed by the NBA¹, which is summarized in this paper including surface water resources, rainfall over the basin, existing and projected dams. 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. All of these will have a large impact on the river Niger regime and the environment, especially the Fomi dam which will change significantly 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. It is very important before building these dams to take into account the past years variability of climate and river regime.

1. Introduction

The West African drought has now extended for nearly 40 years. It has had tragic consequences in the Sahel countries, such as desertification. This drought, which is notably characterized by a decrease of rainfall, decrease in surface water flows and with changes in the rainy season characteristics all contribute to reduce the water availability in the Niger River basin. This shift in climate characteristics must be born in mind if one wants to understand the present hydrology and water uses in the basin.

The river Niger rises in the Futa Jallon Highlands in Guinea with an average altitude of 1100 m. It flows north east and during the rainy season forms a vast flood plain in Mali known as the inland Delta. Away from the inland Delta the river meanders in Mali then flows south east to Nigeria where it is joined by the River Benue and empties into the Atlantic Ocean. Nine countries sharing the active basin are members of the Niger Basin Authority (NBA): Benin, Burkina Faso, Cameroon, Guinea, Ivory Coast, Mali, Nigeria, Chad and Niger. Algeria has an important part of the inactive desert basin.

From the stand point of water resources, the Niger basin can diagrammatically be divided into four zones with more or less homogenous physical and geographical characteristics (Figure 1):

1) The Upper Niger Basin is located in Mali, Guinea, and Ivory Coast. It has an area of 257,000 km², of which 140,000 km² are situated in Guinea, serving as the major watershed and is seen as the dominant portion which can be used to partially regulate water flow throughout the length of the river.

2) The inner Delta is entirely situated in Mali, covering a rectangular area facing south west and north east with a length of 420 km and a width of 125 km between Ke-Macina and San in the south and Timbuktu in the north. It has a surface area of 84,000 km² and comprises four agro-ecological zones: the living Delta, the middle

Bani-Niger, the dead Delta and the lacustrine zone between Gao and Timbuktu. It accounts for almost all of the rice cultivation which is the staple food in Mali. This is done thanks to irrigation from the Markala Dam.

3) The Middle Niger Basin lies within Mali, Niger, Benin and Ivory Coast. It stretches from Timbuktu to Benin, covering an area of 900,000 km², 230,000 km² of which are inactive. It is made up of a series of irrigated terraces. Water flow in this basin largely depends on additional influx from the inland Delta and navigation is hampered by waterfalls.

4) The lower Niger Basin lies between Cameroon, Nigeria and Chad. It is characterized by large dams for hydro-electric power production, irrigation and for industrial activities throughout the basin. Energy production is mainly derived from the Kainji, Lagdo and Jebba dams which supply 68% of Nigeria’s electricity needs and 22% of her total energy needs.

2. Water Resources in the Basin and Their Variability

Inventories of data are available by consulting the data base of the NBA and the SIEREM base from HydroSciences Montpellier Laboratory. Direct observation of surface water flow on the topographic slope of the Niger enables us to realize that some parts are not hydraulically linked to the river. These include the Algerian section of the basin (the Tassir Oua Ahaggar region) and those of Tamesna and Tahoua found in Mali and Niger. The large tributaries of the Niger which used to drain these regions at humid times, presently exist only as dry valleys covered by a great thickness of sand. Even the Continental Terminal aquifer found in the Iullemeden Sedimentary Basin is cut off from the hydrological system of the river Niger [1]. The same situation exists with the Gando and the Liptako regions at the boundary between Mali and Burkina Faso.

2.1. Rainfall and Climatology

The rainfall regime of the Niger River depends on the fluctuations of the Atlantic Monsoon which generally occurs between May and November. The intensity of the phenomenon is relatively homogenous on the east-west axis but experiences a serious gradient on the northsouth axis following the scale of the basin. There are 530 rainfall stations and 105 climatic stations with at least 20 years of observations. Data from Nigeria and Guinea are difficult to obtain. The average annual rainfall rises to 2000 mm in portions further south in the basin, while it decreases to less than 400 mm in the north under Sahelian and semi-desert climate (Figure 2).

2.2. Flows

2.2.1. Rainfall-Runoff Variability

The hydrologic times series for the Niger began in 1907 with the installations of stations in Koulikoro (Mali) and Jebba (Nigeria). The present hydrologic observation is estimated at 250 stations including the specific network meant to check the river flow within the framework of the Hydroniger Programme. The volumes discharged are lower in the upper basins, and increase markedly when entering Nigeria where rainfall is now heavier over the Niger basin (Figure 3). The hydrological regimes strongly change for the upper Niger when passing through the inland Delta, where the flood flow is delayed by two to three months, and is reduced from 24% to 48% during extremely dry years.

The river Niger basin has been submitted to a strong rainfall deficit since 1970, which has occurred over the whole basin. All the sub-basins experienced a reduction of runoff. The 80s are the driest decade since the beginning of the 1900s (Figure 4). The rainfall deficit is less in the southern part of the basin, mainly over the Benue river basin. However the Niger basin can be divided into three main areas: the upper basin of the river Niger in Guinea, Mali and Ivory Coast, where the runoff deficit is very large (Figure 5); the lower river Niger basin, including the Benue river, where the runoff deficit is limited; and the Sahelian tributaries, mainly in Mali, Burkina Faso and Niger, where the runoff has increased, due to changes in land-use (Figure 6).

In the Sahelian parts of the basin runoff coefficients have seriously increased, which has led to higher flood peaks, erosion, sediment transport and dam silting (Figure 6). This is linked partly to the climate change-related rainfall reduction, but mainly to the increase of the cultivated surfaces, and the related disappearance of the natural vegetation. In Soudano Guinean parts of the Niger River basin, the runoff decrease has been much more than that of the rainfall, due to the cumulative (memory) effect of the rainfall lasting shortage on the groundwater

levels [3].

2.2.2. Groundwater Resources

Discontinuous aquifers are mainly found on the right bank in the Niger (Guinea, Mali, Ivory Coast, Burkina Faso and Niger), in the Guineo-Sudanian zones and the Sudano Sahelian zone. Pipe borne water projects in villages here make use of such aquifers. Specific flows and the rates of failure in bore-hole realization are very unsteady (between 30% to 70%). Generalized aquifers can

be found in large sedimentary forms, especially on the right bank of the Niger River (Mali, Niger, Chad, Nigeria and Cameroon). The Iullemeden aquifer system, shared by Mali, Niger and Nigeria, designates a group of sedimentary deposits containing two major aquifers: the Intercalary Continental (IC) at the bottom and the Continental Terminal (CT) at the top [1]. On plateau surfaces, superficial aquifers are superimposed on the deeper aquifers. The outer aquifers can be considered to be semicontinuous thereby forming a hydraulic link with the deeper layer or it can be discontinuous.

Figure 7 shows the importance of the baseflow in the annual runoff. This is for the Bani at Douna, but this is representative of most of the River Niger tributaries of Guinea, Mali, Ivory Coast and Cameroon, under Soudano Guinean climate.