Distinct Thresholds for the Initiation and Cessation of Aeolian Saltation From Field Measurements

Wind-blown sand and dust models depend sensitively on the threshold wind stress. However, laboratory and numerical experiments suggest the coexistence of distinct fluid and impact thresholds for the initiation and cessation of aeolian saltation, respectively. Because aeolian transport models typically use only a single threshold, existence of separate higher fluid and lower impact thresholds complicates the prediction of wind-driven transport. Here we extend the statistical Time Frequency Equivalence Method to derive the first field-based estimates of distinct fluid and impact thresholds from high-frequency wind and saltation measurements at three field sites. Our measurements show that when saltation is mostly inactive, its instantaneous occurrence is governed primarily by wind exceedance of the fluid threshold. As saltation activity increases, so too does the relative importance of the impact threshold, until it dominates under near-continuous transport conditions. Although both thresholds are thus important for high-frequency saltation prediction, our results suggest that the time-averaged saltation flux is primarily governed by the impact threshold. Plain Language Summary In our article, we address a core issue in studies of Earth and planetary surface processes: What is the minimum or threshold wind speed for moving sediment? Predictions for sediment transport movement by air and water depend sensitively on the choice of thresholds in models. In the case of wind-blown sand particles, the story is complicated by the presence of separate fluid and impact thresholds for the respective initiation and cessation of transport. Using a comprehensive field data set, we provide the first field-based determination of distinct fluid and impact thresholds, and we demonstrate how both thresholds together are needed to predict the instantaneous occurrence of wind-blown sand transport. We also find that models for large-scale processes driven by wind-blown sandatmospheric dust aerosol generation, transport of wind-borne nutrients, erosion of agricultural soils, and formation of protective coastal dunesshould primarily use the impact threshold rather than the fluid threshold.