Jeffrey A. Lee

Indirect evidence of wind erosion trends on the Southern High Plains of North America

Abstract Detecting and quantifying changing regional patterns of wind erosion activity is complicated by a lack of long-term records of direct wind erosion measurements. Here, we attempt to piece together indirect evidence of changing wind erosion activity on the Southern High Plains. Sources of indirect evidence include visibility-based observations of blowing dust as well as past measurements of ambient particulate matter concentration. Both the visibility record and particulate matter record suggest independently that there have been significant declines in blowing dust during the last 40 years. There are three key factors that may have contributed to the observed reduction of blowing dust—natural climatic variations, changing land use, and improved agricultural practices. Historical climatic records suggest that there have been no appreciable climate shifts that could account for the observed decline in blowing dust. Although it is not possible to rule out land use as a factor in the reduction of blowing dust, one can point to periods when land use changed very little while annual dust levels decreased significantly. Overall, the relatively minor changes in land use, including the removal of land from production, cannot fully account for the magnitude of the observed reduction of ambient dust levels on the Southern High Plains. We are left to conclude that the adoption of improved agricultural practices has played a crucial role in reducing wind erosion activity and dust emissions on the Southern High Plains.

DROUGHT, WIND, AND BLOWING DUST ON THE SOUTHERN HIGH PLAINS OF THE UNITED STATES

Blowing dust is commonly studied in terms of climatic conditions that affect the amount of erosion. A new measure of blowing dust amounts on a regional scale is derived from meteorological data for Lubbock, Texas, based on visibility records and wind speeds during blowing dust events. Regression analysis suggests that the fraction of time of potential erosion (based on wind speed) and the Palmer Drought Severity Index (an index of regional soil moisture) are poor predictors of blowing dust amounts. Changing agricultural management techniques may account for much of the variability of blowing dust in the region. [Key words: blowing dust, drought, wind erosion, Southern High Plains.]

THE ROLE OF DESERT SHRUB SIZE AND SPACING ON WIND PROFILE PARAMETERS

Wind flow above desert surfaces is modified by shrubs, which slow the wind by friction. This study examines the effect of shrub size and spacing on the shape of wind profiles measured above shrubs at sites in the Mojave and Sonoran Deserts. The profile data were reduced to determine the values of the profile parameters, which are the zero plane displacement, d, and aerodynamic roughness parameter, z0. The results show that d is above the tops of the shrubs and z0 is small, indicating that the wind is effectively skimming over the shrubs and encountering little friction from the ground surface and the shrubs. No discernible pattern in the variation of d with shrub spacing is found, though z0 decreases with shrub spacing in a manner consistent with previous studies.

Aeolian saltation transport rate: an example of the effect of sediment supply

The channel of the Whitewater River near Palm Springs, California, is an excellent location to assess the effect of sand supply and mobility on the quantity of material moved by aeolian saltation. Saltation fluxes at the site taken by Sharp (1964) were compared to the fluxes that would have been produced under ideal conditions for the spectrum of winds inferred for the site. The discrepancy between the two fluxes is, in general, consistent with whose observed elsewhere for similar surfaces. Variations are linked to runoff in the Whitewater River and, hence, the supply of fresh sand.

NEAR-SURFACE WIND FLOW AROUND DESERT SHRUBS

Desert shrubs modify the pattern and magnitude of shear stress exerted on the ground surface by the wind. Wind profiles near the surface were measured at different locations around shrubs at six field sites. The profiles show that the pattern of wind flow around shrubs is complex and variable between sites. Estimates of the magnitude of surface shear stress around shrubs indicate that the presence of shrubs decreases shear stress on the surface and that the degree of protection is partly related to the density of shrubs on the surface. [Key words: wind, wind erosion, aeolian geomorphology, deserts.]

Modeling Seasonal Patterns of Blowing Dust on the Southern High Plains

The Southern High Plains of the United States are notorious for blowing dust, especially during the “dust bowl” of the 1930s. Dust is an environmental and health hazard that can be mitigated with understanding and management. To aid in the understanding of wind erosion, a process-based mathematical simulation model has been developed at Texas Tech University. This paper overviews the components of the model, and illustrates its use with a long-term simulation of expected dust hours for each month of the year. Predicted values were compared to average monthly dust hours reported for Lubbock, Texas, from 1947 to 1989. Predictions matched measured values well, especially considering that the measured data were not used to calibrate or obtain regression coefficients. It is concluded that the Texas Tech model is a reasonable simulator of soil movement, dust concentration, and visibility.