Establishing the threshold for intermittent aeolian sediment transport.

A key variable to be determined in a wind-erosion scheme is the threshold wind speed - the wind speed at which soil movement is initiated. Under typical field conditions with gusty turbulent winds and mixed sediment soils, the correlation between wind speed and soil movement is weak. To accurately establish a single representative value of threshold, the time fraction equivalence (TFE) method is compared with a wind speed fractionating method for a single event with constant sediment conditions but with varying wind parameters and for synthetic time series modeled with an autoregressive Markov process. The TFE method provides a varying threshold with varying wind conditions. The discrepancy between constant sediment conditions and varying threshold is caused by the dependence of the threshold on varying wind statistics and the absence of only one threshold. Periods with wind accelerations and periods of wind delay provide two threshold conditions: the fluid threshold, which is defined as the wind speed at which particles start moving due to the forces of wind only, and the impact threshold, which is the wind speed at which the combined action of wind forces and saltation impacts can just sustain movement. These two thresholds were determined by analysis of gust intervals where the wind speed rises and the saltation begins and lull intervals with falling winds and stopping saltation.