Flood level and flood extent
A high inflow of the Bani and Niger Rivers not only produce a high flood level, but also floods a more extensive area. In a dry year (as 1984), just one third of the Delta became inundated; the northern Delta was not even reached by the flood. In a wet year (such as 1999) though, the southern Delta was fully flooded, as well as a large part of the northern Delta including several of the lakes just north of the Delta.
The topographical maps of the Institut Géographique National (IGN) reveal that the inundation area of the Inner Niger Delta measures 36 470 km2, including 5340 km2 of levees, dunes and other islands within that area. Hence the total surface of the floodplains 31 130 km2 including the areas being permanently covered by water, in total 3840 km2. The entire floodplain area is included in the 41 195 km2 designated as a Ramsar Wetland Site of International Importance in January 2004. Topographical maps show the floodplain as if it were flooded at a maximum level (viewer II). The area actually inundated, however, varies considerably between years, depending on the inflow of the Bani and Niger.
Satellite images can be used to produce water maps of the Inner Niger Delta covering the full range of water levels. That is why we know that in 1984 (at a maximal flood level of 336 cm in Akka) a mere 7800 km2 was flooded. In contrast, 18 000 km2 was flooded in 1999, when the water level at Akka reached the high level of about 511 cm.
Zwarts & Grigoras (2005) used satellite images to produce a continuum of 24 water maps of the Inner Niger Delta covering the range of water levels between -2 and +511 cm, as measured on the gauge at Akka. The satellite-derived water maps were combined by Zwarts & Grigoras (2005) to construct a digital flooding model showing the water coverage in steps of 10 cm, separately for incoming and receding water. Their model for incoming water was used to show the water coverage when the water level in Akka varies between 100 cm and 520 cm (Zwarts & Hoekema 2013).
Zwarts & Grigoras (2005) used a polynomial to describe the relationship between the peak flood level in Akka and the corresponding flood extent:
maximal flood extent (km2) = 0.099x2 - 35.521x + 8727, where x = water level in Akka (cm).
The function is based on water maps for which the water level in Akka was up to 511 cm, but in the past, peak flood level in Akka has been as high as 625 cm. Extrapolating this function for these higher water levels, the flood extent would arrive at 25 000 km2 when the water level in Akka would be 625 cm in Akka (as was the case in 1956). This is still substantially less than the total surface of the floodplains as shown on the IGN maps (31 000 km2) (viewer II). This apparent discrepancy is caused by the shallow northward slope of the floodplain that delays flooding in the north with two-three months; by that time the southern floodplain has already been drained of water.
Using the function given above, the variation in flood extent can be shown for all years since 1907:
The flood extent of the Inner Niger Delta derived from the annual peak flood level in Akka.
Combining the data given above, it is also possible to show the relation between the total inflow of Bani and Niger Rivers and the flood extent in the same year:
The maximal flood extent of the Inner Niger Delta as a function of the annual inflow of Niger and Bani during incoming water.
The water maps given by Zwarts & Hoekema (2013) are intended to show the inundation at different water levels during the peak flood, but may also be used to show the change in the water cover during incoming water. The maps cannot be used, however, to show the water cover as a function of the water level during the deflooding when a large part of the inundated area lost its connection to the river system. The higher the flood, the more isolated lakes come into existence and the larger the fraction of inundated area being unconnected to the river. Hence, during the deflooding it is not the water level in the river, but the maximal water level, as well as the time passed since the water level has reached its peak, which determines where isolated and temporary lakes with water can be found. This is true for small depressions of some ha of shallow water, but also for Lac Faquibine and the other, large lakes west and east of the Inner Niger Delta.
Seasonal variation in flooding
Also the annual variation in flood extent is large as a result of the varying discharge and rainfall. The figure below illustrates the large difference in flooding during the course of a dry and a wet year.
The graphs show daily measurements of the water level at Akka (left axis in both graphs) between June and May in 1984/85 (left graph; the lowest flood ever measured) and in 1999/2000 (right graph; one of the highest floods since 1970, but of normal height when compared with pre-1973 floods). The surface areas flooded (right axis in both graphs) in 1984/85 and 1999/2000, shaded in the same graph, are derived from the water level data, using the digital elevation models of Zwarts & Grigoras (2005). The maps show the flooded area when the water level at Akka reached its peak, and for 1 March. Note that the peak flood level in 1999 (511 cm) was 1.5 times higher than in 1984 (336 cm), but that the maximal flood extent was nearly 4 times larger. The difference is even larger some months later. The flood extent on 1 March (shown as dark blue on the map) was in 2000 13 times more than in 1985 (right map). From: Zwarts et al. (2009).
The model of Zwarts & Grigoras (2005) is a statistical regression model and specifically developed to capture the spatial flooding of the Inner Niger Delta. The resulting spatial flooding patterns closely reflect the real inundation at a certain water level. The flooding model relies on the input of discharge or water levels. In 2016-2018 a new flooding model is build, covering the entire delta and based on 273 satellite images during upcoming water. The model separates water on the floodplain which are connected to the river and which are not connected (most depressions with rainfall, not (yet) flooded). With this model for each water level the spatial flooding pattern during incoming water can be established.
The annual flood extent is the driver of many socio-economic and ecological functions in the delta, such as the production of fish, rice and bourgou. Information on the expected timing and extent of flooding is of interest to many stakeholders. At present, monitoring of flooding can be done through recent satellite images, but these still need processing to translate them into actual water maps, but it might be a promising tool in the near future. Flooding can also be monitored indirectly through water levels. The flood in the Inner Niger Delta is forecasted each flooding season using the tool OPIDIN.
OPIDIN stands for ‘Outil de Prédiction des Inondations dans le Delta Intérieur du Niger’ and is based on a model in which flooding characteristics are related to water levels in the delta. With the (automatic) input of rainfall data and actual water levels in Mopti, the tool forecasts the timing and maximal spatial flooding in the delta at the peak of the flood. The forecast is done annually during the flooding season, and is updated every week on the OPIDIN website (www.opidin.org). On this website also all background information on OPIDIN can be found as well as the actual predictions on flooding.
The following information is given on the OPIDIN site:
Maximum inundation level
Starting each year on August 1st the maximum flood level is predicted on a weekly basis for three stations in the delta: Mopti, Akka and Diré. The expected level is given and the confidence range, expressed as a lower and upper limit. The forecast is stopped when the maximum flood level in Mopti has been reached in October.
Timing of flooding
Next to the height of the maximum flood also the timing is predicted. For each of the three stations the expected date (and lower and upper range) is given at which the peak flood level is reached.
Spatial extent of flooding at time of flood peak
The spatial extent of inundation during the expected flood peak is presented on an interactive map. This is based on the flooding model explained elsewhere in this Section.
As soon as the peak flood is reached, the timing of deflooding is important to stakeholders. This is predicted for the level of 200 cm at the Akka gauge (below this level the first sand banks in the central lakes become exposed), and the level of 100 cm at the Akka gauge (most mud banks exposed).
Sources and more information
- Brunet-Moret Y., Chaperon P., Lamagat JP. & Molinier M. 1986/ Monographie hydrologique du Fleuve Niger. Tome 1. Niger supérieur. Editions de l’ORSTOM ; Monographies Hydrologiques 8. 2605 pp.
- Orange D, Arfi R, Kuper M, Morand P, Poncet Y, editors 2002. Gestion intégrée des ressources naturelles en zones inonables tropicales. Paris: IRD.
- Zwarts L. & Diallo M. 2002. Eco-hydrologie du Delta. In : Wymenga E., Kone B., van der Kamp J. & Zwarts L. editors. Delta intérieur du Fleuve Niger. Ecologie et gestion durable des ressources naturelles. Wetlands International,Altenburg & Wymenga, RIZA.
- 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. Mali-PIN-publication 2002-01. http://www.altwym.nl/uploads/file/392_1294302275.pdf
- Zwarts L, van Beukering P., Kone B. & Wymenga E. 2005b. The Niger, a lifeline. Rijkswaterstaat/IVM/Wetlands International/A&W. p .1-304.
- http://www.altwym.nl/uploads/file/361_1289481552.pdf (English), http://www.altwym.nl/uploads/file/388_1294300622.pdf (French).
- Zwarts L. & Frerotte, J-L. 2012. Water crisis in the Inner Niger Delta (Mali). Causes, consequences, solutions. http://www.altwym.nl/uploads/file/490_1369389376.pdf.
- Zwarts, L & Hoekema, F.S. 2013. Atlas. Les plaines inondables du Delta Intérieur du Niger. Altenburg & Wymenga, ecological consultants, Feanwâlden. http://www.altwym.nl/uploads/file/494_1372419095.pdf (13.7 MB) or http://www.altwym.nl/uploads/file/495_1372420527.pdf (24.0 MB).
- Zwarts L. & van der Kamp J. 2013. Does the Inner Niger Delta suffer from a reduced river flow in the dry season? A&W-rapport 1938. Altenburg & Wymenga ecologisch onderzoek, Feanwâlden. http://www.altwym.nl/uploads/file/514_1405344577.pdf
- Zwarts L. 2012. L’impact d’un faible débit du fleuve sur l’inondation, la végétation et l’utilisation des terres dans le Delta Intérieur du Niger. A&W-report 1868 / 1877. Altenburg & Wymenga ecological consultants, Feanwâlden. http://www.altwym.nl/uploads/file/489_1369388731.pdf (English), http://www.altwym.nl/uploads/file/488_1369388423.pdf (French)