Analysis of the model for a 6.21 m flood The NIV_621 model is compared with the 2001 flood, the characteristics of which are as follows: * The model by Jean Pierre Lamagat, a Hydrologist at IRD, dates from 1983. It established correspondences between the heights of the floods between the main gauges along the Niger River. It was used in particular for the development of the Mathematical Model of the Niger River . Once again the 2001 flood is quite particular: the flood level at Ké Macina matches a 6.60 m flood at Mopti and not a 6.21 m flood. The Bani flood at Sofara is slightly lower than expected, given the flood height in Mopti (about - 6 cm). The flood height is a little lower than expected in Akka at the exit of the Débo lake. Moreover, the observed duration of the flood propagation strongly differs from the model predictions for the Sofara to Mopti and Mopti to Akka stretches. These peculiarities partly explain the differences between the analysis of model outputs and Landsat images for: October 7, 198_050 (west of the Delta), October 16, 197_051 (south of the Delta) and November 17 for 197_051, 050 and 049 (entire Delta). Flood NIV_621 takes the form of a massive block in the center and north of the Delta, with only a notch by Peru Dialloubé. The flood extent is more complex in the south with large flooded basins (south of Mopti, Yongari, Mangari, Pondori) soon tapering between Diafarabé and Ké Macina, upstream of the Delta. The total flooded area calculated by the model is 1,417,331 ha. After correcting the errors related to the smoothing of the matrix (both in terms of additions and substractions), the potentially flooded area of ​​NIV_621_Veg7 is 1,367,484 ha. The relatively large gap due to correction is explained by the deletion of vegetation unit fragments whose depth range is between -0.05 cm and -0.35 cm. The flood depths between -0.35 m and -0.45 m are dubious for a 6.21m flood at the Mopti gauge (-0.39 m below the reference height), as they are intergraded mosaics between levels -0.10 and - 0.60 (-0.35 m) and between -0.30 and -0.60 (-0.45 m). Parts of these mosaics were therefore likely to be flooded. The floodable area of ​​1,367,484 ha should therefore be taken as minimum area, the areas at level -0.35 in the niv_621 model extending over 1672 ha while the areas at -0.45 m were kept within flooded areas. The total flooded area on Landsat (shape VEG7_2001) reaches 1,426,684 ha, a greater value than the flooded area calculated by the model (NIV_621_Veg7) with, in particular, flooded areas in the southwest of the Delta (Diafarabé region) sharply marked by flood on images of October 7 and 16 , which no longer appear flooded on the image of November 17. The fully flooded or vegetated areas cover 1,219,477 ha, to which 207,207 partially flooded ha must be added. Out of this total, 11,611 ha (4,597ha totally flooded and 7,014 ha partially flooded) were flooded on October 16 but did not bear a marked green vegetation cover in November 17. These formations, all located in the Diafarabé region, have a flood depth equal to, or lower than -0.30 m. This complex reality is reflected in the flooded area common to the model shape (NIV_621_Veg7) and to Landsat 2001 images -shape (VEG7_2001)-. The shape file (Commun_621_2001) resulting from their overlay shows a flooded area of ​​ 1,276,476 ha , of which 1,154,624 ha are totally flooded / or vegetated; and 121,852 ha are partially flooded. 1 – If partially flooded areas are included, the confidence ratio of the model – the ratio between the total surface of (Com_621_2001) and that of (Niv_621_Veg7) – reaches 93.3% . If only the totally flooded/vegetated areas are taken into account, the ratio is 84.4% . The actual value must be between these two rates. 2 - The comparison between (Niv_621_Veg7) and (VEG7_2001) shows flooded areas on Landsat which are not flooded in the model shape (Inon_plus_2001); and conversely, areas calculated as flooded by the model which are not so on Landsat shape (Inon_ moins_2001) The shape (synthèse_621_2001) includes (Commun_621_2001, Inon_plus_2001, Inon_ moins_2001) On the combined map, the areas flooded on Landsat and which are not flooded by the model all have depths between -0.05m and -0.35 m and the majority are only partially flooded. They are mainly distributed in the Diafarabé region, at the edges of Yongari- Mangari, at the edges of the main flooded area which extends from the west of the Delta to the Peru Dialloubé. They clearly mark the influence of the strong flood observed at Ké Macina (+63 cm), compared to the model prediction. This strong flood affects the Diaka and the Niger up to the defluent feeding the Yongari, at Kouakourou. Beyond Kouakourou, this influence is offset by the deficit contributions from the Bani River and becomes negligible. A significant part of these areas (shown in black on the map), all located in the Diafarabé region, were flooded on October 16, but do not bear a marked green vegetation cover marked on November 17. All these vegetation units have a maximum water depth of -0.30 m. The cause is probably linked to a flood peak (between 6 m and 6.62 m) which lasts only 25 days, too short to sustainably affect vegetation growth. The non-flooded areas illustrate two concomitant phenomena: the poor water supply of the Pondori basin where flood is very early and is reinforced, in 2001, by the Bani flood, which is slightly in deficit and late. The areas flooded on Landsat images and which are outside the areas calculated by the model illustrate the role played by an imbalance in the inflows between Niger and Bani. In the case of the 6.60 m flood, the deficit from the Niger at Ké Macina resulted in smaller flooded areas in the Diafarabé region and the Moura plain. In the case of 2001, the higher flood in Ké Macina resulted in a larger flooding in the same region, although transient in part. This imbalance of contributions partly affects the logic by depth levels developed in Delmasig.

The areas calculated as flooded by the model which are not flooded on Landsat represent 89,930 ha (inon_ moins_2001) of which 31,766 ha extend over vegetation mosaics which include vegetation associations that are first flooded by run- off, and 58,164 ha extend on vegetation which is normally floodable.The distribution by flood depth level is given in the following table: Floodable vegetation not flooded on Landsat 2001 images (on 58,164 ha) Among the areas not flooded, those first flooded by run-off are mainly distributed on the periphery of the Delta, in Dialloubé Peru and on the Chibon, at the far east of the Pondori basin. The flood-prone formations (in purple) are found mainly in the south of Pondori, and along the right bank of the Bani, south of the erg of Femaye, and in the high plains of the Sébéra, whose flood is provided by the Bani, as well as in Peru Dialloubé where flood is provided by the Niger, downstream from the confluence with the Bani. However, the Landsat images also reveal flooded and/or vegetated areas which are not included in the flooded areas calculated by the model for a total of 136,841 ha (Inon_plus_2001), which are distributed according to the level of flooding:

621_2001.rar

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