The fetch effect on sand transport above a farmland surface and conditions for saturation

The existing aeolian transport models have been developed for saturated sand flow. For unsaturated sand flow, these models produce larger errors. To mitigate this problem, this paper attempts to clarify the fetch effect on the sand transport rate above a farmland surface so as to improve sand transport model. We first defined the C value in the 1978 Lettau and Lettau model as a sand transport coefficient. For saturated sand flow, the C value equals the saturated sand transport coefficient (i.e., C=Csat=6.7); but for unsaturated flow, the C values would be less than 6.7. We found that C increased with increasing fetch length (L) and shear velocity (u*) using data from a field investigation above a farmland surface. The measured C was far less than Csat even at the maximum fetch length (100 m) and the largest shear velocity (0.809 m s−1) during the sand collection in field observations. Based on the relationships among C, L, and u*, we developed an aeolian transport model that accounted for the shear velocity and fetch length. We confirmed that the essential condition for the sand flow to reach saturation was wind speed is strong enough to transport most of the surface soil particles. Thus, the critical shear velocity at which the sand flow can reach saturation (u*c-sat) is close to the threshold shear velocity for entrainment of the largest soil particles. When u* > u*c-sat, the sand transport saturation fetch length decreased rapidly with increasing wind speed. However, due to the high wind speed requirement, it is difficult for the sand flow to reach saturation, which leads to the farmland surface in an environment dominated by wind erosion under normal wind conditions.