THE WOLFFORTH FIELD EXPERIMENT: A WIND EROSION STUDY

The goal of this field study was to obtain a detailed account of wind erosion processes within a single agricultural field during a regional dust storm in the Southern High Plains of West Texas. The field, located in Wolfforth, Texas, was observed as the wind grew in strength, peaked, and later weakened. Sediment transport was monitored by an array of samplers spaced across the field, and meteorological information was obtained from a 10-m tower erected within the field. Erosion activity was monitored by a piezoelectric sensor that responded to the impact of saltating grains and provided a means for detecting the threshold of soil movement. Attempts were made to relate the observed temporal and spatial variations of sediment transport to meteorological factors and surface conditions. The results indicate that at the beginning of the storm, threshold was around 7 to 8 m/s. As the storm progressed, threshold appeared to slowly shift downward with time, suggesting a surface that was becoming increasingly erodible. Mass flux measurements showed substantial temporal variations that reflected changes of wind strength and changes in surface erodibility. The pattern of mass flux variation across the field was dependent on the height of measurement. The near surface flow of saltating grains (z < 0.25 m) was found to vary according to surface conditions, especially surface roughness. At greater heights, the flow of fine dust was less affected by surface conditions immediately beneath the point of measurement. Within the fully developed surface layer (z < 0.25 m), the mass flux profile was found to follow a modified powerlaw function. The near surface mass flux consisted of a broad range of particle sizes ranging from 50 μm to 300 μm, whereas farther from the surface the mode shifted distinctly toward smaller particle sizes with few particles larger than 100 μm. We show that it is possible to display graphically the relative contribution of the various grain sizes to the mass flux at each height in a way that makes it possible to visualize the zones of saltation and suspension, and the region of transition between these regimes.