The varying fetch effect of aeolian sand transport above a gobi surface and its implication for gobi development process

The Gobi deserts are important landscapes and major sandstorm source areas in arid northwestern China. Unsaturated sand flow and decreasing sand supply capacity is well known as the basic characteristics of gobi surface, but relatively little attention has been paid to the fetch effect of sand transport which is closely related to sand supply and indicative of wind erosion process in gobi. Using a field experiment, we investigated the spatial and temporal variations on a manually disturbed gobi surface downwind a sand-blocking belt and a sand-fixing belt by measuring the sand transport rate and the height profile of flux density at different fetch lengths during a sequence of wind erosion events. Results showed that the sand supply capacity determined the critical fetch length (Lc) for the sand transport rate so that the fetch effect varied with wind erosion proceeding due to depletion of erodible material. The height profile of flux density above the surface followed two distributions: an exponential decrease with increasing height, which commonly occurred above the newly treated gobi surface during the early wind erosion events; or a Lorentzian distribution with a peak flux at a certain height, which often happened in the later wind erosion events. The varying fetch effect, decreasing sand transport rates, and the non-erodible area expanded downwind are an epitome of the gobi development and expansion process from the perspective of wind erosion.