The Great Green Wall, Senegal – to see if it is affecting temperature and soil moisture levels
The judges were intrigued by Stephen’s entry because none of them had heard of this project. Stephen wrote that Senegal, Mauritania and other countries immediately south of the Sahara desert are “part of a 3.5 million square kilometre area of very arid land – the Sahel. [They] are trying to combat desertification by building a great green wall of trees.” He went on to explain that the trees will retain moisture and their roots will help hold soil together, preventing it from being blown around or washed away in flash floods. “Having good soil is very important in Senegal, given the large agricultural industry, however, it is something they do not have in abundance, so if the wall was helping it could have a major impact on Senegal’s economy.”
He asked for images covering a large area so that he could compare conditions in the Sahel with those in the desert. Since the ISS is in a relatively low orbit – lower than that of most earth observation satellites – photographs of large areas are often oblique, that is, taken across the surface of the Earth rather than straight down. In November 2015, just before Tim Peake arrived on the space station, the crew captured a moonlit view of the area Stephen is interested in as the sun set over the Atlantic.
Stephen said that “I expect that nearer the wall, the temperatures will be lower and there will be a greater level of soil moisture, whilst as you head North to the Sahara, temperatures will rise and soil moisture levels will drop, creating more difficult conditions for farming.” He therefore asked for a false colour image that assigned thermal infra-red to the red channel and short-wave infra-red, which is absorbed by water, to the blue channel. Although there are complex interactions between types of radiation and surfaces, this roughly means that hotter places would appear more red, drier places more blue and hot, dry places would be a shade of magenta and there are certainly plenty of these in his image which was produced using data from the Thermal Infra-Red (TIR) and Operational Land Imager (OLI) instruments on Landsat 8. The data used was gathered on two separate days a week apart in April 2016, again, because of the large area that Stephen was interested in. The different lighting and atmospheric conditions on the different days have led to some discontinuities which demonstrate why scientists carry out calculations to correct for these effects before using the data to generate values of variables like temperature, vegetation cover or soil moisture.
Note: the green line snaking across the picture is actually the path of a river – the wall is being planted further south and is much more diffuse.