FACTORS CONTROLLING THRESHOLD FRICTION VELOCITY IN SEMIARID AND ARID AREAS OF THE UNITED-STATES

A physical model was developed to explain threshold friction velocitie s u(t) for particles of the size 60-120 mu m 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 nonerodib le roughness. For loose or disturbed soils the most important paramete r that controls u(t) is the aerodynamic roughness height z(0). For ph ysical crusts damaged by wind the size of erodible crust pieces is imp ortant along with the roughness. The presence of cyanobacterial-lichen soil crusts roughens the surface, and the biological fibrous growth a ggregates soil particles. Only undisturbed sandy soils and disturbed s oils of all types would be expected to be erodible in normal wind stor ms. Therefore disturbance of soils by both cattle and humans is very i mportant in predicting wind erosion as confirmed by our measurements.