January 20, 2022 4:34 pm

Seasonal emerald greening in the Sahel

Francisco Martin Leon 5 min
The progression of this greening is shown in the natural color images of May, August and November 2021 in an area of ​​the Sahel

The area shown is one of the most productive wetlands in the world, although it is mostly dry for almost half of each year. According to the abundance and timing of the rains upstream, the inner delta of the Niger River in Mali is often flooded of water approximately from August to December. The result is a seasonal explosion of green vegetation. at the intersection of the Sahara desert and the Sahel.

Inland deltas generally occur where rivers divide and branch through depressions, valleys, or former lake beds, often in arid areas. According to NASA Earth scientist and geomorphologist Justin Wilkinson, there are at least 86 interior deltas, sometimes referred to as megafans, scattered across Africa. The Niger Inland Delta is the largest in West Africa.

The waters that wash this delta originate in the Highlands of Guinea, where the rainy season rains usually begin to fall in July and then meander northeast towards Mali on the Niger River. Upon reaching the southern tip of the Niger Inland Delta, the waters spread over floodplains and swamps filled with reeds and wetland pastures (particularly bourgou). The northern part of the delta is lined with branching sand ridges and dry stream channels that emerge from the wetlands as the season progresses.

The progression of this greening is shown in the May natural color images., August and November 2021. Each image is a monthly composite view constructed from data acquired by the MODIS sensor on NASA’s Aqua satellite. (Compound approach allows for a cloudless view in a region that can often be cloudy.) The following image, acquired on November 16, 2021 by Operational Land Imager (OLI) on Landsat 8, shows a close-up of the sand ridges. in the northern half of the delta.

Enlarged image of the area taken by Landsat 8

The Niger River runs for more than 500 kilometers through the plains and channels of the inner delta before merging again and continuing with Niger, Benin and Nigeria. When seasonal pulses of river water reach the northeastern part of the delta, water levels are generally dropping in the southwestern part. Wilkinson added that greening occurs in the “active lobe“of the delta, which is actually twice as large as the area shown above.

The waters in parts of the river and delta can rise and fall as much as 6 meters from the dry season to the wet season and vice versa. Wilkinson noted that nearly 60 percent of the river’s discharge is lost en route to evaporation in the delta and uptake by soils from wetlands and plants that thrive here seasonally.

Birds, fish, and other wildlife flock to this area during flood season, including manatees from West Africa. At least one million people make their living in the inland delta by fishing, growing rice, and grazing and raising livestock.

Images of NASA Earth Observatory by Joshua Stevens, using data from the Level 1 Atmosphere Distribution and Archiving System (LAADS) and Near Real-Time Capacity of the Earth’s Atmosphere for EOS (LANCE), Landsat data from the US Geological Survey. USA Michael Carlowicz Story.

NASA Earth Observatory


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