GRAIN-SIZE DISTRIBUTIONS OF WIND-ERODED MATERIAL ABOVE A FLAT BARE SOIL

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Abstract
Grain-size distributions and fluxes of each size fraction of aeolian materials in the surface layer are basic to understanding and predicting soil loss, air quality, and visibility, both in time and in space. Vertical distributions of eroded soil materials in a 612-cm-deep flow layer above an Amarillo fine sandy loam soil in Big Spring, Texas were measured during sand-dust storms in the 1995 wind-erosion season. All 942 samples in 33 groups of measurements show that the airborne particles in the flow layer below 40 cm are bimodal in distribution, which is similar to the size distribution of the original soil. In the flow layer above 80 cm, the airborne material is distributed unimodally and consists of silt and very fine sand. The distribution of the mean diameter is a discontinuous function of height. Mean diameters vary by an asymmetric double sigmoid function of height. Size distribution of airborne particles in the 0- to 35-cm flow layer is controlled by saltation mechanisms, becoming larger with an increase in height, and moderately sorted, positively skewed, and platykurtic. In the 35- to 50-cm flow layer, particle size decreases very rapidly with increased height, becoming poorly sorted, symmetrical, or platykurtic, and thus uniformly distributed. This represents a transition zone from saltation to suspension. Grain-size parameters in the flow layer above 50 to 80 cm demonstrate a wavelike pattern with a moderately well to well-sorted, negatively skewed, and leptokurtic distribution. The weight percent of the silt content and phi95 diameter increase by a natural logarithmic polynomial function of height. Inclusive standard deviations vary with height by an asymmetric logistic function. Total distributions of the skewness and kurtosis are wavelike. The fluctuation of grain-size parameters of eroded soil materials in the surface layer is related to saltation mechanisms and the turbulence of air flow.