SHEAR STRESS PARTITIONING IN SPARSELY VEGETATED DESERT CANOPIES

Shear velocity ratios were derived for six neighbouring localities representing rangeland, abandoned farmland and a playa, in the semi-arid region of the Sonoran Desert. Near-surface and above-canopy wind speeds, mostly below saltation threshold, were used to evaluate the partitioning of shear velocities and shear stresses between the plant canopy and the surface. The technique utilized represents an alternative to the threshold shear velocity ratio (Rt) approach of Musick and Gillette (Land Degradation and Rehabilitation, 1990, 2, 87–94) and results demonstrate the need for partitioning over a range of wind speeds. Shear velocity ratios were variable over the playa and abandoned farmland, while variance in near-surface wind speeds was highest over shrub-dominated sites. The relationship between lateral cover (Lc) and shear velocity (R) is comparable to previous findings utilizing Rt, although methodological differences result in mean shear velocity ratios less than shear velocity ratios derived for threshold conditions. With respect to the model of Raupach et al. (Journal of Geophysical Research,1993, 98-D2, 3023–3029), this may be attributed to the difference between measured shear stresses and the average shear stress on the exposed surface. At high lateral covers, shear velocity ratios are lower than threshold shear velocity ratios due to increased wake interactions or decreased drag on the shrub elements at higher wind speeds.