Comparison of the sand flow over mobile sand, farmland, and gobi surfaces and the conditions required for saturated sand flow to develop

The soil surface’s content of particles that are erodible at different wind velocities determines whether the sand flow can become saturated. However, the quantitative relationship between the content of erodible particles and the saturation degree of the sand flow is not yet known. Using field observations of the sand flow above surfaces with greatly different erodibility (mobile sand, farmland, and gobi), we calculated the degree of saturation of the sand flow (δ) at each site, which is defined as the ratio of the measured sand transport rate to the potential saturated transport rate. The sediment’s grain-size composition determined the content of erodible particles, which increased with increasing wind velocity. We found that δ of each surface increased following a power function with increasing shear velocity (u_*) and exponentially with increasing content of erodible particles. When u_* increased to 0.5 m s⁻¹, all particles from the mobile sand were erodible, enabling the sand flow to become almost saturated, whereas the sand flow over farmlands and gobis was far from saturated because fewer particles were erodible at a given wind velocity. Based on the relationship between δ and u_*, we identified the conditions for sand flow to achieve saturation at each site; that is, the shear velocity (u_*sat) that can entrain most particles from the surface. Under normal weather conditions, the near-surface shear velocities of bare farmland and gobi surfaces rarely reaches u_*sat, resulting in a generally unsaturated sand flow. Thus, both land types were always exposed to a wind-erosion-dominated environment.