On the Seno Plain of Mali, the population has more than doubled since 1972. In this wide view of the region, the Seno Plain sweeps from lower left to upper right. The feature to the left is the Bandiagara escarpment, with a rocky plateau to the west. The rocky surface on this plateau is not suitable for agriculture. So the plain, which is 200 meters lower than the plateau, is where there is increasing demand for agricultural land as the population grows.
Growing vegetation is indicated by red tones in these Landsat images, which are visible and near-infrared composites. These images are from the dry season, so cropland is indicated by bright tones.
Villages are scattered throughout the Seno Plain. They increase in size throughout the series of images. In the 1972 and 1986 images, the villages are the dark spots surrounded by a light color. Those light areas are cropland surrounding each village. In the 2016 image, the light-colored areas merge, and the dark spots stand out as the location of the villages. The population of these villages ranges from a few hundred to several thousand.
These close-up images from Landsat show the Seno Plain in a bit more detail. The images were acquired during the dry season, so crops were not growing at the time. However, the images do show evidence of how intensely the land was being used for agriculture, indicated by the lighter toned patterns. As in the wider views, the Bandiagara escarpment is the feature on the left of these images.
The 1972 image shows that farmers practiced crop rotation, leaving some land fallow. The fallow fields are represented by dark patches. In 1986, however, much of the fallow is gone, and there is even less of it by 2016. Bright areas dominate the 2016 image, indicating wall-to-wall cropland over the plain. The darkest patches are grassy fallow areas and are either protected lands or used for cattle grazing.
On the edge of the Seno Plain, up against the Bandiagara escarpment, a brighter red appears in a few areas. This is denser tree cover and vegetation near a seasonal watercourse. But it’s not all tree cover.
There is farmland under those trees in a farming strategy called recessive farming, that is, farming when flood waters recede after the rainy season. So the red seen in the Landsat imagery is not only from the trees but also from crops. The trees are darker red tones, and crops are light red.
An image from the WorldView-2 satellite shows much more detail, 1.8-meter resolution. Plots of farm fields occupy most of this close-up image. Individual trees are the bright red spots. Villages are the bluish-gray patches.
While WorldView-2 does not offer the frequent repeat coverage of Landsat, it does help show enough detail to see whether tree coverage is changing over time. Click this image to see an enlarged view.
Some of the dark lines in the images might look like roads. Those lines are a transportation network of sorts, just not transportation for people. They are cattle corridors that connect grazing lands (dark patches) and sometimes connect to villages. They are lined with hedges to keep the cattle out of the crops.
One line that goes all the way across these images is indeed a road. It passes through the city of Bankass. But the other lines are cattle corridors.
To get an even more detailed view of this region, we can actually go back to satellite imagery from 1967. This pre-Landsat imagery, while black-and-white, helps to extend the record of land change on the Earth. These images are declassified reconnaissance satellite images and show detailed and informative views of the Seno Plain.
Comparing WorldView-2 with Corona shows the pattern of change that we see in Landsat, only in more detail. In the grayscale Corona imagery, bright areas are cultivated cropland. Dark gray is grass with shrubs and trees. In the natural color WorldView-2 images in 2011, much of that fallow or grassy land has converted to agriculture, indicated by light tan.
The resolution of the Corona image is 6 feet. For Landsat, it’s 30 meters. So in Corona, even single trees can be distinguished.
The more up close images show a few villages in higher detail and demonstrate that they have expanded with the growing population. The darker gray at the bottom of the 1967 image gives way to tan in 2011, which is intensive cropland. Those cattle corridors are also clearly visible in this WorldView-2 image.
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