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The Niger River basin

The Niger River basin, located in western Africa, covers 7.5% of the continent and spreads over ten countries (Map 2 and Table 12).

Table 12: Niger River basin: areas and rainfall by country

Algeria and Chad together cover about 9% of the total Niger River basin, but there are almost no renewable water resources in these areas.

The area of the Niger River basin in Guinea is only 4% of the total area of the basin, but the sources of the Niger River are located in this country. The quantity of water entering Mali from Guinea (40 km³/yr) is greater than the quantity of water entering Nigeria from Niger (36 km³/yr), about 1800 hen further downstream. This is due among other reasons to the enormous reduction in runoff in the inner delta in Mali through seepage and evaporation combined with almost no runoff from the whole of the left bank in Mali and Niger.

The most important areas of the Niger basin are located in Mali, Niger and Nigeria (25 % in each of these three countries). Mali and Niger are almost entirely dependent on the Niger River for their water resources. In the case of Niger nearly 90% of its total water resources originates outside its borders (the Niger River and other tributaries from Burkina Faso and Benin).

Rivers and discharges

The Niger River, with a total length of about 4100 km, is the third-longest river in Africa, after the Nile and the Congo/Zaire Rivers, and the longest and largest river in West Africa.

The upper Niger River system

The source of the Niger River farthest away from the mouth is in the mountains of Guinea near the border with Sierra Leone. Together with several tributaries it traverses the interior plateau of Guinea flowing north-east towards the border with Mali. Just after the border it is joined by another tributary which also originates in Guinea. The total annual flow entering Mali from Guinea is estimated at 40 km³.

The river then proceeds north-east towards the inner delta in Mali, where it is joined at Mopti by an important tributary, the Bani River, which is about 1100 km long and has its sources in Côte d'Ivoire and Burkina Faso.

The inner delta

The total area covered by the inner delta, which is a network of tributaries, channels, swamps and lakes, can reach about 30000 km² in flood season. The delta area is swampy and the soil sandy. Consequently, the river 'loses' nearly two-thirds of its potential flow between Ségou (at 900 km from its source) and Timbuktu (at 1500 km) due to seepage and evaporation, the latter being aggravated by the fact that the river here touches the southern flanks of the Sahara desert. All the water from the Bani tributary, which flows into the Niger River at Mopti (at 1150 km), does not compensate for the 'losses' in the inner delta, as the total flow further downstream still decreases rather than increases (Figure 13). The average 'loss' is estimated at 31 km³/year, but varies considerably according to the years: it was 46 km³ during the wet year of 1969 and about 17 km³ during the dry year of 1973 [29].

Figure 12: Average discharges of the Niger River and its main tributaries

The middle Niger River system

From the inner delta the river continues to flow north-eastwards before turning south-east to form a great bend, the Niger Loop. After meandering through arid areas it enters Niger. In the Niger Loop another 4 hen /year of water disappear between Dire and Ansongo. Like in the inner delta, these losses are mainly caused by evaporation, but they are much less because of the smaller area inundated during and after the floods. 'Losses' by infiltration are limited.

Within Niger the river receives water from six tributaries originating in Burkina Faso (Gouroual, Dargol, Sirba, Gouroubi, Diamangou, Tapoa). The total annual discharge leaving Burkina Faso is estimated at about 1.4 km³.

Further downstream the river becomes the border between Niger and Benin, from where three main tributaries enter the river (Mekrou, Alibori, Sota) with a total annual discharge of about 3 km³.

At Gaya in Niger or Malanville in Benin, just upstream of the border with Nigeria, the average annual discharge has been estimated at about 36 km³ [35], but only about 18 km was measured in 1986 [29].

The lower Niger River system

Leaving the border between Niger and Benin the river enters Nigeria, where it is joined by numerous tributaries. The most important tributary of the Niger is the Benue which merges with the river at Lokoja in Nigeria. The Benue itself rises in Chad although there are almost no surface water resources in its uppermost part. In Cameroon it receives water from several tributaries. The slope in Cameroon is considerable and the discharge there has important seasonal variations. The quantity of water entering Nigeria was estimated at 25 km³/year before the 1980s [25] and at 13.5 km³/year during the 1980s [172]. In Nigeria itself the Benue is joined by several tributaries, of which the ones at the left side originate mainly in Cameroon. The Benue reaches its flood level in September. It begins to fall in October and falls rapidly in November, continuing slowly over the next three months to reach its lowest level in March and April.

From the confluence with the Benue, the Niger heads southwards and empties in the Gulf of Guinea through a network of outlets that constitute its maritime delta.

Table 13 shows the difference between the long term annual flows in Nigeria before the 1980s [30] and the annual flows during the 1980s [172], which was a much drier period.

Table 13: Average annual discharges of the Niger River and its main tributaries in Nigeria over different periods

Irrigation potential and water requirements

The rainfall and hydrological conditions in Guinea make it possible to exploit, with good chances of success for an annual rainfed crop, the alluvial plains of the Niger River and its tributaries. However, to be able to cultivate all year round, irrigation is necessary. The irrigation potential in this region is estimated at 185000 ha, of which 100000 ha are relatively easy to develop, though the construction of dams is necessary for the storage of the water [116]. To date only about 6000 ha of rice are irrigated.

The irrigation potential for the whole of Côte d'Ivoire has been evaluated at 475000 ha, without giving details of location [21a]. It is estimated that 50000 ha are located in the Niger basin [*]

In Mali there are four climate zones in the basin area and rainfall ranges from 1500 mm in the south to less than 50 mm in the north.

The water in the Niger River is partially regulated through dams. The Sélingué dam on the Sankarani River is mainly used for hydropower, but also permits the irrigation of about 60000 ha under double cropping [14]. Two diversion dams, one at Sotuba just downstream of Bamako, and one at Markala, just downstream of Ségou, are used to irrigate the area of the Office du Niger (equipped area of about 54000 ha). However, double cropping in this area would only be possible if the Fomi Dam, planned on the Niandan river in Guinea, were constructed to provide a supplementary and regular amount of water. However, the negative effects on the environment that would be caused by the construction of this dam seem to be important.

Several irrigation projects have been identified, especially related to the construction of the Tala and Djenné Dams on the Bani River and the Dam at Tossaye on the Niger River. However, the drying up of several watercourses during the low-flow period in the dry years 1983-85 requires a careful re-examination of the projects identified, with the recent hydrological figures being taken into consideration [140].

The irrigation potential has been estimated at 556000 ha, of which about 200000 ha fully controlled and the rest for partially controlled schemes [138]. At present about 187000 ha are equipped in the Niger basin, but of this 57000 ha are already abandoned and of the remaining 130000 ha actually irrigated more than 60% need to be rehabilitated. Irrigation water requirements for double rice cropping in the Niger River valley range from over 30000 m³/ha per year in the southwest to nearly 50000 m³/ha per year in the northern part according to this study.

In Burkina Faso most of the irrigation is located outside the Niger basin. About 850 ha are irrigated in the Niger basin and the potential is estimated at about 5000 ha [67].

In Benin the irrigation potential has been evaluated at 300000 ha for the whole country, but no details about location are given [57]. In the present study it has been estimated at 100000 ha in the Niger basin [*]. The actual equipped area here is 1090 ha, of which 740 ha are cultivated.

The Niger River crosses the south-western part of Niger over a distance of about 550 hen with the final 150 km forming the border between Niger and Benin. There are no important tributaries in Niger, but there are two fossil valleys, the Dallols, where there is no permanent flow but where the water resources are quite important. Three other zones are considered as being part of the Niger basin, although in tact they are rather valleys or depressions at a considerable distance from the Niger River with no streams reaching the Niger River: the Ader-Doutchi-Maggia (ADM) valley, the Goulbis valley and the Agadez region.

The total irrigation potential of Niger has been estimated at 222000 ha, of which 140000 ha in the Niger River valley and the remaining 82000 ha spread over the other zones [166]. At present about 54000 ha benefit from irrigation, of which 16000 ha are in the Niger River valley.

Irrigation of the 140000 ha in the Niger River valley and its tributaries on the right side would only be possible through the construction of the Kandadji Dam in the north, just downstream of the border with Mali. Without this dam it would be possible to irrigate only 15000 ha. However, construction of this multi-purpose dam has so far not been possible due to financial and economic constraints. Reports also indicate that the dam would have a negative impact on the environment [15]. Several other storage works on the tributaries are under consideration.

The irrigation potential in the Niger basin for Cameroon has been estimated at 20000 ha [*]. The Lagdo dam on the Benue River, built primarily for hydroelectricity, regulates the flow of the river. It could also be used for irrigation.

The irrigation sector in Nigeria can be divided into three categories [172]:

• public irrigation schemes, which are government-executed schemes;
• farmer-owned and operated irrigation projects (improved fadamas);
• residual fadamas or floodplains.

About 275000 ha of public schemes are planned under the existing water infrastructure, but only 40540 ha have been completed and irrigated. As far as the fadamas¹ crop production has depended traditionally on rainfall in the wet season and on residual moisture after flood recession in the dry season. In areas with easily accessible shallow groundwater or surface water, water lifting devices are used to lift water into the land. The existing formal fadama area has been evaluated at 79000 ha and in addition there are about 550000 ha of residual fadama cultivation in the Niger basin.

Estimating irrigation potential is rather difficult, despite the considerable data available on surface water resources, because of the potential of large areas to be irrigated either by surface water or shallow fadama aquifers, two sources that are hydraulically connected. Table 14 presents irrigation potential as identified in the national water resources master plan (NWRMP) [172]

Table 14: Irrigation potential in the Niger River basin in Nigeria according to the NWRMP [172]

In this NWRMP there are also two proposals for water transfer schemes from the Niger to the Lake Chad basin and two for water transfer between different tributaries within the Niger basin.

Table 15 summarizes the irrigation potential of the Niger basin, per country and for the basin as a whole.

Table 15: Niger River basin: irrigation potential, water requirements, water availability and areas under irrigation

NOTES:

For the sake of simplicity it was supposed that if a certain quantity of water is abstracted upstream, this same quantity is subtracted from the resource downstream, except in cases where more information was available.

Mali:

Niger:

Nigeria:

The countries with the largest water requirements are Mali, Niger and Nigeria. Water problems may arise in the Niger basin if the whole potential is developed. The effect of water abstraction upstream of the inner delta on the quantities that disappear within this delta has not been studied. Probably, as is the case with the Sudd swamps in the Nile basin (see section The Nile basin), the lower the quantity of water entering the swamp area the lower the quantity of water disappearing in absolute as well as relative terms.

In Nigeria, the most downstream country, of the 177 km³/year flowing to the sea, only 36 km³/year enter from Niger and 25 km³/year from Cameroon. The rest is produced internally. More than 1 million ha of its potential of nearly 1.7 million ha is located in the tributary Benue basin.

In all cases, important storage works for the development of irrigation are necessary throughout the whole basin. Probable navigation and hydropower problems may arise if more water is abstracted for agricultural purposes.

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