Remotely sensed dune sand flux measurements in the dustiest place on Earth (Bodélé, Chad)

The Bodélé Depression of northern Chad is the dustiest place on Earth, thanks to a unique combination of two factors. First, the Bodélé is located downwind of a narrow gap between the Tibesti and Ennedi mountain ranges, which act as a funnel creating the strong winds of the Bodélé Low Level Jet. Second, the depression forms the deepest part of palaeo-lake Megachad, which once rivaled the Caspian Sea in size. When the lake dried out during the Holocene, it left behind thick deposits of diatomite, which currently provide an abundant and readily available source of dust. Previous work has shown that dust is primarily produced by eddies near the horns of barchan dunes. Using a new remote sensing technique called COSI-Corr, these barchans were shown to be not only the largest, but also the fastest on Earth, featuring the highest sand fluxes. Quantifying sand flux with field measurements is an expensive and time-consuming process. COSI-Corr -which stands for 'Co-registration of Optically Sensed Images and Correlation' and was originally developed for the purpose of detecting surface deformation caused by earthquakes- offers an alternative approach to measuring sand flux, using remote sensing. From pairs of ASTER imagery, dune migration in the Bodélé depression was successfully measured over time intervals of one month to 6.5 years. The displacement maps produced by COSI-Corr can be used to automatically distinguish dunes from interdunes, which is a crucial step towards calculating sand flux. Dune heights and volumes were obtained by interpolating a surface between the interdune areas and subtracting it from a digital elevation model. Multiplying height with celerity yields a pixel-by-pixel estimate of the sand flux. Applying this method to large diatomite dunes in the Bodélé confirms that these are some of the world’s fastest moving barchans. Plotting dune height against inverse celerity reveals sand flux at the dune crest of >200 m3/m/yr. Average dune sand flux values for the eastern and western Bodélé are 76 and 99 m3/m/yr, respectively. The contribution of the dunes to the total area-averaged sand flux is 24-29 m3/m/yr, which is ~10% of the saltation flux determined by previously published field measurements. The displacement field of quartz dunes in the northeastern Bodélé displayed as a vector field.