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United States Department of Agriculture

Agricultural Research Service

Title: Establishing the Threshold Condition for Soil Movement in Wind-Eroding Fields

Authors
item Stout, John
item Zobeck, Teddy

Submitted to: Proc First Inter Conf on Air Pollution from Agricultural Operations
Publication Type: Proceedings
Publication Acceptance Date: February 7, 1996
Publication Date: N/A

Interpretive Summary: During the wind erosion of sandy soils, dust is generated by the action of larger sand grains bouncing across the surface releasing clouds of fine particles. Once detached from the surface, these fine dust particles are blown away by the turbulent wind. Unlike sand grains, which often deposit in dunes at the edge of the eroding field, dust is sometimes transported vast distances across the continent, representing a true soil loss and a major source of air pollution. Ideally one would like to able to predict periods of dust generation from a measured wind velocity record. To do so requires knowledge of the critical threshold condition for soil movement. Here we report results from a field experiment in which we used the intermittency of the wind erosion process and recent advances in instrumentation to establish the threshold wind speed under natural field conditions. This was accomplished by simultaneously monitoring soil movement and wind velocity at a single location within the field.

Technical Abstract: One of the most basic features of a wind eroding surface is its threshold velocity - the wind velocity at which soil movement is initiated and just is generated. Many theoretical equations and numerical models of the wind erosion process include threshold as an important basic parameter. SENSIT, an instrument that uses a piezoelectric crystal to count particle impacts, provides a means of indirectly measuring threshold in the field. If we simultaneously observe the number of particle impacts and the wind speed them, in principle, it is possible to detect the lowest wind velocity where particle impacts are first recorded. Unfortunately, under field conditions with gusty turbulent winds and mixed sediment soils the correlation between wind velocity and soil movement is weakened due to many factors. As a result it is hard to accurately pinpoint a single representative value of threshold by direct observation. We have developed a new method, called the e"time fraction equivalence method", which eliminates this problem. This method is based upon the principle that the fraction of time that erosion occurs should be equivalent to the fraction of time that winds exceed threshold. We simply have to determine the value of threshold that yields this equivalence. Example threshold calculations using the "time fraction equivalence method" are presented from data taken during a study in an agricultural field in New Deal, Texas.

Last Modified: 9/2/2014
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