Field determination of drag forces and shear stress partitioning effects for a desert shrub (Sarcobatus vermiculatus, greasewood)

Drag coefficients (C-d) for the desert shrub greasewood (Sarcobatus vermiculatus) were developed from force versus wind speed data collected with an omnidirectional force balance. The average C-d for a small (0.6 m high, 0.5 m wide) shrub and a larger (1.6 m high, 1.3 m wide) shrub were 1.425 (+/-0,103) and 0.435 (+/-0.200), respectively. These values are much larger than similarly shaped solid elements and previously reported values for creosote bush (Larrea tridentata, C-d = 0.485) and an artificial tree (0.4). The greater C-d value for greasewood probably results from factors related to porosity and vegetation structure that gives this shrub-type greater momentum extracting potential. The drag coefficients for the greasewood shrubs were found to show dependence upon flow Reynolds numbers >6 x 10(5), corresponding to wind speeds greater than 18 m s(-1) at 10 m. The developed greasewood C-d values were used in a shear stress partitioning model that indicated they would be extremely effective at reducing wind-generated sediment transport at low-percent coverage.