Le delta intérieur du Niger est une région naturelle du Mali s'étendant sur 64 000 km 2 , entre les 4 ème et 6 ème degrés Ouest et les 13 ème et 16 ème degrés Nord, entre les villes de Djenné et Tombouctou .
C'est une partie intermédiaire du cours du Niger, où le fleuve se divise en de nombreux bras avant de reprendre un cours normal. Le Delta Intérieur du Niger est reconnu comme site Ramsar depuis le 1er février 2004
| Date de sortie | 20/04/2015 |
|---|---|
| Donateur | SIDI DIALLO |
| Couverture géographique | Mali |
| Mots clés | delta, Mali, Niger, zone humide, |
Géographie
Le delta est formé essentiellement par le fleuve Niger et son affluent le Bani et est également traversé par le canal du Sahel . Elle s'étend sur 390 kilomètres entre Djenné et Tombouctou avec le lac Débo en son centre. Le delta est en fait un immense réseau de canaux, de marécages et de lacs. Il est situé dans un bassin plat et sablonneux situé à une altitude d'environ 250-270 mètres. Autrefois en période plus humide, c'était un vaste lac alimenté par le haut Niger.
Many lakes are present in the region: Lake Korientzé , Lake Débo (and Oualado-Débo ), Lake Niangay , Lake Do , Lake Garou , Lake Haribongo , Lake Kabara , Lake Tanda , Lake Fati , Lake Horo , Lake Télé , Lake Kamango , and Lake Figuibine .
Each year, the simultaneous risings of the two great rivers flood vast surfaces of this enormous delta; the water level then increases, depending on the intensity of the floods, from 4.5 to 7 meters above the low water level . The annual maximum of flooded surfaces varies between 35,000 km 2 in a wet year and 7,000 km 2 in a dry year.
Hydrology
When passing through the delta, due to the very low slope of the terrain, the Niger River and the Bani lose nearly half of their water through infiltration and evaporation. The latter is intense in the region and is estimated between 2,000 and 2,500 millimeters per year.
Flows upstream of the delta
The Niger at Kirango downstream [ edit | modify the code ]
The flow of the river was observed for 67 years (1925-1992) at Kirango aval, a station located just upstream of the delta 4 . The average annual flow or module observed at this location during this period was 1,290 m 3 /s for a catchment area of more or less 137,000 km 2 , or almost the entire part of the basin feeding the delta.
The sheet of water flowing into this part of the basin thus reaches the figure of 297 millimeters per year.
Average monthly flow (in m 3 /s)
Hydrological station: at Kirango downstream, located just upstream of the delta
(Data calculated over 67 years)
The Bani in Douna
The flow of the Bani River was observed for 63 years (1922-1984) in Douna , a locality located near the town of Zinzana , shortly before the eruption of the Bani in the Inner Niger Delta, located more or less 150 kilometers from the confluence with the Niger to Mopti 5 . The average annual flow or module observed in Douna over this period was 513 m 3 /s for a catchment area of more or less 101,600 km 2 , that is to say almost the entire basin of the river feeding the delta.
The sheet of water flowing into this basin thus reaches the figure of 159 millimeters per year.
Average monthly flow (in m 3 /s)
Hydrological station: Douna
(Data calculated over 63 years)
Cumulative flows of the Bani and the Niger upstream of the Delta
The cumulative average annual flows of the two rivers thus observed from 1922 to 1992 was 1,803 m 3 /s for a catchment area of more or less 238,600 km 2 . This flow constitutes more or less 90% of the average water supply of the interior delta of the Niger, the balance being due to a few rare secondary tributaries of the Niger in the delta area as well as to local rainfall.
Average monthly flow (in m 3 /s)
Hydrological station: Cumulation of the Bani and the Niger upstream of the Delta
(Data observed from 1922 to 1992)
The flows of the Niger at Diré downstream of the delta
The flow of the river was observed for 67 years (1924-1992) in Diré , a town located in the downstream region of the delta, some 80 kilometers southwest of Timbuktu. The average annual flow or module observed at this location during this period was 1,003 m 3 /s for a catchment area of more or less 340,000 km 2 .
Average monthly flow (in m3/s)
Hydrological station: Diré, downstream of the delta (Data calculated over 67 years)
The period of maximum flood downstream of the delta occurs about two months later than upstream (September-October upstream and November-December downstream), the time needed to flood and then cross the delta. The same is true for the period of low water or low water. The period of maximum rain (rainy season) takes place in July and August throughout the Sahel, therefore in the Bani basin as in that of the upper Niger. Downstream of the delta, the flood corresponding to this maximum rainfall therefore occurs four months later. Downstream of the delta, the floods are largely clipped, the delta acting as a vast moderating buffer. As a result, the flood period is much longer there. The decline is slow and spread over many months: in March, at the worst of the dry season, 663 m 3 /s, while upstream the low water level is almost maximum ( 118 m 3 /s in March and 75.3 in April).
Finally, about half of the water is lost during the passage through the delta. The flow observed upstream (only the inflows from the Niger and the Bani) goes from 1,803 m 3 /s to 1,003 m 3 /s. In fact, the city of Diré not being located completely downstream of the delta, the flow still drops significantly between this city and the region of Timbuktu, still by infiltration and evaporation. A significant loss of flow occurs in particular following the exit from the bed of the Niger of the effluents feeding the system of Lake Figuibine (backwaters of Kondi, Tassakane and Goundam ).
Main articles: Lake Figuibine and Marigot de Goundam .
Evaporation in the delta zone
Evaporation from open water in the delta area was estimated at the hydrological stations of Mopti and Timbuktu . The difference between wet (1962-1966) and dry (1982-1986) years is however small, calculated annually: the average values would be 2,260 mm for wet years and 2,360 mm for dry years.
Evaporation over open water in the Inner Niger Delta (in millimeters per year)
Data calculated over the wet (1962-1966) and dry (1982-1986) periods
Rainfall
The amount of rainfall decreases from south to north in the inner Niger delta. The annual cumulative rainfall increases from more or less 550 millimeters in Mopti (confluence of the Niger and the Bani) to 320 millimeters in Niafunké . The rainy season lasts from June to September and the inter-annual variability of rainfall volume is high. The average monthly precipitation heights calculated from observations of nine stations in the delta area ( Ké-Macina , San , Ténenkou , Sofara , Mopti , Sah, Saraféré , Niafunké and Diré) donnent les résultats suivants : 490 mm pour la période humide 1962-66, et 330 mm pour la période sèche 1982-86, soit une moyenne générale, toutes années confondues, de plus ou moins 410 mm par an 2 répartis mensuellement comme suit :
Niveau mensuel moyen des précipitations dans la zone du delta intérieur du Niger (en millimètres par an)
Données calculées pour les périodes humides (1962-66) et sèches (1982-86)
Moyenne annuelle de 410 millimètres
Src : fr.wikipedia.org