Factors controlling threshold friction velocity in semiarid and arid areas of the United States

A physical model was developed to explain threshold friction velocities u., for particles of the size 60-120 gm lying on a rough surface in loose soils for semiarid and arid parts of the United States. The model corrected for the effect of momentum absorption by the noncredible roughness. For loose or disturbed soils the most important parameter that controls u., is the aerodynamic roughness height z0. For physical crusts damaged by wind the size of credible crust pieces is important along with the roughness. The presence of cyanobacteriallichen soil crusts roughens the surface, and the biological fibrous growth aggregates soil particles. Only undisturbed sandy soils and disturbed soils of all types would be expected to be credible in normal wind storms. Therefore disturbance of soils by both cattle and humans is very important in predicting wind erosion as confumed by our measurements.