Saltation vs. soil stabilization: Two processes determining the character of surfaces in arid regions

Abstract

In satellite images of sandy areas in arid regions disturbed (crumbled) surfaces appear much brighter than stabilized soil. From observations of Landsat satellite images that show directional surface features along the dominant wind direction, two theses are inferred concerning eolian erosion and transport: (1) transport is dominantly a near-surface phenomenon of saltation rather than of aerodynamic entrainment and later deposition; (2) both the disturbed surface characteristics in the saltating regions (saltation formations in our terminology) and the stability of the crusted soil in the quiescent regions are locally self-perpetuating. Thesis (1) is suggested by the appearance of the zones in the lee of protrusions from the surface, or even a river, that constitute obstacles (across the dominant wind direction) to particle flow. In the lee of a ridge between two “streaks” or in the lee of a protruding outcropping within a “sheet”, one finds darker zones, in contrast to the adjacent bright saltation formation. These darker zones are laterally uniform, and have to be interpreted as stable, quiescent areas of non-deposition and non-erosion. Thesis (2) is inferred from (i) the sharpness of the transition in the reflectivity at the boundaries (parallel to the dominant wind direction) between the bright “streaks” or “sheets” and the darker stable surface, and (ii) the constant width of some long quiescent areas in the wind-shadow of an isolated obstacle. Variability of wind direction, in any season of any strength, should have produced a gradual boundary. The disturbed and the stable surfaces are therefore interpreted as locally a bi-stable situation: an occasional wind will produce no effect over a stable surface, which tends to be “immune” to the eolian influences, whereas the surface in a saltation formation is prone to be disturbed anew by a comparable wind. The bistable situation is linked to positive feedback mechanisms; the most important mechanism appears to be the cohesion of the crusted surface. Such a surface can be broken only by high energy impact. The feedback mechanisms have implications for designing measures to reduce the danger of dust storms.