DRAG AND SHEAR-STRESS PARTITIONING IN SPARSE DESERT CREOSOTE COMMUNITIES

Most research to characterize the wind-erosion susceptibility and the degree of surface roughness required to suppress erosion on erodible s urfaces has been empirical. However, a recently proposed shear velocit y ratio model attempts to place shear stress partitioning in an entire ly theoretical framework. The purpose of this study was to directly me asure components of shear stress in a sparsely vegetated environment i n order to evaluate the model. Far the field study, new instrumentatio n was developed to measure drag on a creosote shrub, and Irwin sensors were modified to measure surface shear stress in the field. Simultane ous measurements of total shear stress and surface shear stress were t aken at four sites of different roughness densities, in the Eldorado V alley, Nevada. Results indicate that porous shrubs had greater drag co efficients (C-d = 0.485) than did solid elements (sphere C-d = 0.3) an d are more effective at protecting a surface. Values of beta, the rati o of element to surface drag coefficients, were therefore higher than previously published values. Surface and total shear stress scaled con sistently with each other at a range of wind speeds, and varied accord ing to the roughness density of the surface, Shear stress partitioning values agreed well with previously published field data and some wind -tunnel data. The theoretical model predicted the results successfully when m = 0.16, where m is an empirical constant that accounts Cor the difference in average stress and the maximum surface stress in initia ting erosion. The wide applicability of the model is likely due to the inclusion of the adjustable m, which accommodates all values of beta and sigma (ratio of roughness element basal area to frontal area).