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University of Guelph (1993)

Sparse vegetation as a surface control on wind erosion

Wolfe, Stephen Andrew

Titre : Sparse vegetation as a surface control on wind erosion

Auteur : Wolfe, Stephen Andrew

Université de soutenance : University of Guelph

Grade : Doctor of Philosophy (Ph.D.) 1993.

The amount of vegetation cover required to stabilize natural and uncropped arid lands from wind erosion remains uncertain, despite recent theoretical and empirical studies. This study investigates the role of sparse vegetation as a surface control on wind erosion. The study addresses the problem of relating measured wind profiles parameters to site characteristics and of evaluating the potential for wind erosion for various surface types, vegetation covers and land uses. Wind profiles over sparse vegetation are well described by the Prandtl-von Karman equation, provided that wind speed measurements are obtained sufficiently above the vegetation canopy. The incorporation of a displacement height into the logarithmic model was found to be inappropriate for sparsely vegetated, shrub dominated surfaces. Air flow measurements around isolated shrubs revealed an absence of flow separation in the lee of the elements. Creosote (Larrea tridentata) and mesquite (Prosopis juliflora) with optical porosities ranging from 28% to 48.5% show considerable air flow through the elements. Wake areas, with flow reductions of 10% or more at one-half the element height, are on the order of 3 to 4 times the element width. Consequently, flow regimes within desert shrub communities are typically ones of partial wake interference, ranging from isolated roughness flow to full wake interference, but are not characterized by skimming flow. These observations may account for the absence of a displacement height in these canopies. Shear velocity ratios and shear stress partitioning are derived from simultaneous measurements of wind speeds above the vegetation and near the surface. Shear velocity ratios derived in this study add support to the model of Raupach et al. (1993), but suggest that the model may underestimate the protection provided by vegetation under conditions of mixed vegetation canopy coverage. Despite the sparse vegetation cover of many desert surfaces, the cover appears sufficient to inhibit wind erosion of most surfaces, with the notable exception of abandoned farmlands. In addition, observations during dust storm conditions indicate that the spatial variability of desert shrub vegetation may result in sporadic erosion of the surface.

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