  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 the fluid threshold, existence of a separate lower impact threshold
complicates the prediction of wind-driven transport. Here, we derive the first
field-based estimates of distinct fluid and impact thresholds from
high-frequency 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, we find that the time-averaged saltation flux is primarily governed
by impact threshold.
