EFFECT OF AVERAGING TIME ON THE APPARENT THRESHOLD FOR AEOLIAN TRANSPORT

Authors: Stout, John

Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/11/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: During wind erosion events, soil movement and dust emissions are restricted to periods when winds exceed the threshold wind speed or the minimum wind speed necessary to set surface grains in motion. Values of threshold wind speed have been reported widely for soil surfaces in different parts of the world under different conditions. Few realize that a measured value of threshold is affected significantly by the averaging time used to measure wind speed. Thus, for example, hourly threshold values cannot be compared directly to values obtained from one-minute wind speed records. Here we report results from a field experiment in which we used the intermittancy of the wind erosion process and recent advances in instrumentation to establish the threshold wind speed under natural field conditions. Threshold values were then recalculated using wind speed records based upon different averaging times. It was found that large averaging times can produce an apparent threshold which is significanlty lower than the actual threshold.

Technical Abstract: A fundamental feature of any wind-eroding surface is its threshold - the lowest wind speed (or friction velocity) at which soil movement is initiated. Many theoretical equations and numerical models of the saltation process include threshold as an important basic parameter. The question addressed here is whether or not the averaging time of a wind speed measurement affects the observed wind speed threshold. Using wind speed data taken at 1 Hz and simultaneously measuring saltation activity with a piezoelectric saltation sensor it was possible to calculate threshold. Threshold was then recalculated using the same data set averaged over periods of 2, 5, 10, 20, 30, and 60 s. The results reveal that, under typical field conditions with gusty turbulent winds, long averaging times produce an apparent threshold that is considerably lower than the true wind speed at which saltation is inititated. This result suggests that high frequency sampling of wind speed and saltation activity is critical to the accurate determination of the true threshold of a wind-eroding surface.