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

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 (theta). Spatial variations in mean wind velocity around eight isolated vegetation elements in Namibia (three grass clumps and five shrubs) were recorded at 0.30m height, using a combination of sonic and cup anemometry sampled at a temporal frequency of 10seconds. Wind flow recovery in the lee of the elements was parameterized in an exponential form, (u(ref)-u(0)).1-e(-bx/h) + u(0). The best-fit parameters derived from the field data were u(0)=u(ref)(0.0146 theta - 0.4076) and b = 0.0105 theta + 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. (c) 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.