Potential of medium-resolution satellite imagery for monitoring sand transport in pre-Saharan urban areas

Abstract

One of the most spectacular manifestations of desertification is silting up. On a sandy windy day in the Saharan environment, the atmosphere can be so loaded with solid matter that it feels like twilight. It thus presents a danger and a health risk for the population at two levels: i) at the respiratory level, firstly because of the inhalation of solid matter and ii) at the level of sight; solids entering the eye can cause irritation and over time serious infections. One of the most spectacular manifestations of desertification is silting up. On a sandy windy day in the Saharan environment, the atmosphere can be so loaded with solid matter that it feels like twilight. Wind erosion is the main cause of all silting up, the wind playing the double role of erosive agent and transport agent. However, in pre-Saharan environments, sand can also be transported by bedload in wadis; in some cases, the sand can also be of very local origin (erosion of soft sandstone for example); some popular cultures then speak of a "source of sand" which they describe as "inexhaustible". The silting up of a territory can be accelerated (climatic conditions, existence of favorable transport corridors) or slowed down by natural screens (vegetation or relief, for example). Studying the silting up and trying to understand its causes and evolution requires having a multi-date synoptic view of the territory; satellite imagery then appears to be the most appropriate means to provide adequate basic information. The present study has three main objectives: i) delimit areas covered with sand or likely to be; ii) delimit the potential foci (or sources) of supply for the silting up phenomenon and finally iii) hypothesize on the progression of the silting up phenomenon for a pre-Saharan area where we are witnessing a significant and rapid extension to the both urban space and cultures on the outskirts. It mainly concerns the city of Laghouat, a former oasis in the southern foothills of the Saharan Atlas, and its periphery. The population of Laghouat increased from 42,800 inhabitants in 1977 to 119,043 inhabitants in the year 2003 (estimate), i.e. a variation of 178% (i.e. a multiplication by a factor of 2.8) in 26 years resulting in increasingly important consumption needs of all kinds and a profound change in space. It is estimated at 170,693 in 2021. Data from the National Statistics Office are only available until 2019 for overall statistics. For the present work we used several satellite images at different spatial resolutions: a Landsat MSS image from April 1972 at 80 meters resolution, two Landsat TM images at 30 meters from April 1987 and April 2000 and finally an image extract. ASTER from 2004 to 15 meters. We applied a specific index combining some of the channels to accentuate the perception and the distinction of the zones of transfer and accumulation of sand. The comparative analysis of the images makes it possible to specify, firstly, the preferred directions of sand transport and, secondly, the directions of transport and then the areas likely to be most affected.