A field‐based parameterization of wind flow recovery in the lee of dryland plants

Abstract

Wind erosion is a key component of land degradation in vulnerable dryland regions. Despite a wealth of studies investigating the impact of vegetation and windbreaks on windflow in controlled wind-tunnel and modelling environments, there is still a paucity of empirical field data for accurately parameterizing the effect of vegetation in wind and sediment transport models. The aim of this study is to present a general parameterization of wind flow recovery in the lee of typical dryland vegetation elements (grass clumps and shrubs), based on their height (h) and optical porosity (θ). Spatial variations in mean wind velocity around eight isolated vegetation elements in Namibia (three grass clumps and five shrubs) were recorded at 0.30 m height, using a combination of sonic and cup anemometry sampled at a temporal frequency of 10 seconds. Wind flow recovery in the lee of the elements was parameterized in an exponential form,The best-fit parameters derived from the field data were u 0 = u ref (0.0146θ À 0.4076) and b = 0.0105θ + 0.1627. By comparing this parameterization to existing models, it is shown that wind recovery curves derived from two-dimensional wind fence experiments may not be suitable analogues for describing airflow around more complex, three-dimensional forms. Field-derived parameterizations such as the one presented here are a crucial step for connecting plant-scale windflow behaviour to dryland bedform development at landscape scales.

Potential differences in seed dispersals of low‐height vegetation between single element and windbreak‐like clumps

“…Comparison studies (Figure b) suggested that under identical physical settings, (a) the lowest wind speed in the leeside for windbreak‐like clumps is smaller than that for single element, and (b) the wind speed in leeward wind reduction region for single element recovers much faster than that for windbreak‐like clumps (Mayaud et al, ). Therefore, the responses of r 0 to involved factors (release height, vegetation porosity, and vegetation height) for single element are more sensitive to those for windbreak‐like clumps.…”

“…Several basal shapes of wind reduction region (triangle, rectangular, and half‐ellipse) in the lee of plants (Leenders et al, ; Okin, ; Raupach, ) have been proposed. Recent observations (Leenders et al, ; Mayaud et al, ) and simulations (Sadique, Yang, Meneveau, & Mittal, ; Yang et al, ), however, indicated that the half‐ellipse shape proposed by Leenders et al () is likely to be more reasonable for porous shrub vegetation element. The semiminor axis of the half‐ellipse is set to be D /2.…”

“…The change of ground shear wind speed along the central line in the first part for single element vegetation could thus be expressed as Equation . Here, according to Mayaud et al (), u *0 = (1.46 × θ − 0.4076) u * , b = 1.05 × θ + 0.1627, where u *0 is the lowest value of wind shear speed at the leeward edge of vegetation element, and θ is the vegetation porosity. In the case of windbreak, the change of ground wind shear speed within windbreak could also be described as Equation , while the ground wind shear speed in the lee of windbreak is expressed by Equation (Vigiak et al, ).…”

“…When the effect of local wind reduction is not considered, the vegetation porosity is equal to 1. Five values of porosity are 0.3, 0.4, 0.5, 0.6, and 0.7, respectively, on the basis of recent field observations (Mayaud et al, ). Besides, five vegetation heights, H = 0.5, 1.0, 1.5, 2.0, and 2.5 m, are employed.…”

“…However, vegetation element (or clumps) with low frontal area ratio, for instance, windbreak‐like clumps (many elements standing in a line closely), could be observed in nature, considering the diversity of vegetation arrangement. Comparisons of parameterized leeward wind recovery functions (Mayaud et al, ) suggest that remarkable difference in leeside wind speed variation exists between single element and windbreak under identical conditions (vegetation height, porosity, and incoming wind strength). Nevertheless, it is unclear whether this remarkable difference in wind speed could cause considerable change of seed dispersal kernel.…”