Field evidence for fluid and impact thresholds in aeolian saltation
Raleigh L. Martin, Jasper F. Kok
University of California, Los Angeles, Los Angeles, CA, USA
Predictions for wind-blown sand and dust transport depend sensitively on selection of threshold wind shear velocities. Theory and experiments demonstrate distinctive "fluid" and "impact" thresholds for initiating and ceasing saltation, respectively, but field evidence for these separate thresholds is lacking. Here, we present direct field evidence that impact threshold shear velocity equals about 80% of fluid threshold, in agreement with theory and numerical predictions for Earth conditions. Determining threshold by the Time Frequency Equivalence Method (TFEM), we find that threshold decreases linearly with transport frequency, from fluid threshold in the zero transport limit to impact threshold at continuous transport. When instantaneous streamwise wind velocity is within an "inter-threshold" state between corresponding impact and fluid threshold shear velocities, we expect path-dependent occurrence of saltation. Our data show that a decreasing proportion of these inter-threshold events are approached from below impact threshold as transport frequency increases, thereby explaining the observed decline in TFEM threshold by transport hysteresis. Furthermore, we find that the impact threshold shear velocity corresponds to the zero-intercept in comparison of saltation flux to wind shear velocity. Therefore, the impact threshold appears the appropriate choice for long-term saltation flux predictions, though the fluid threshold may be an important factor in short-term predictions of aeolian sand and dust transport.