Wind tunnel simulation on the windbreak effect of Nitraria tangutorum nebkhas in Ulan Buh desert-oasis ecotone of China

Nebkhas can be one of the most widespread biogenic landform types in the transition zone of a desert oasis. Desertification can be further prevented to improve the ecological functions for the soil fertility and sustainable development of oasis areas. The existing research focused mainly on the morphological, physicochemical and surface erosion of nebkhas. It is still lacking on the windbreak effects of nebkhas at different evolution stages and vegetation coverages, particularly from the perspective of wind speed field. This study aims to investigate the distribution of wind speed and windbreak effect of nebkhas under different evolution stages and vegetation coverages, in order to control the soil wind erosion. A single and two Nitraria tangutorum nebkhas in Ulan Buh desert-oasis ecotone were selected as the research objects. The wind speed distributions in vertical and horizontal planes of nebkhas were measured at four evolution stages (embryonic, developing, stable and activating stage), and three vegetation coverages (0, 40%, and 70%) using wind tunnel simulation tests. A systematic investigation was implemented to explore the effects of the evolution stage and vegetation coverage on the windbreak indicators, including the relative wind speed and windbreak efficiency. It was found that there was quite a similar variation in the wind speed around nebkhas at the initial wind speeds of 6, 8, and 10 m/s. The wind speed varied significantly in the range of 0-0.8H (H was the height of Nitraria tangutorum nebkhas) in the vertical direction and –H-5H from the center of nebkhas in the horizontal direction. The wind speed gradually recovered to the corresponding value without the place of nebkhas above the height of 1.2H. A comparison was made on the wind speed variations in the upstream deceleration zone, upper acceleration zone, bilateral jet stream area and leeward deceleration zone. It was found that all nebkhas in the leeward deceleration zone shown the most significant variation in the wind speed at different evolution stages. The maximum reduction of wind speed reached 90% for Nitraria tangutorum nebkhas at the stable stage with a vegetation coverage of 70%. Additionally, the experimental results show that both the evolution stage and vegetation coverage of nebkhas posed great effects on the wind speed distribution around the nebkhas and the windbreak efficiency. Specifically, there was a more significant reduction in the wind speed of Nitraria tangutorum nebkhas at the stable stage, indicating greater windbreak efficiency. However, a relatively weak reduction was observed in the nebkhas at an embryonic stage. The interaction was also considered on two nebkhas that were arranged in parallel to the wind direction. The more significant windbreak efficiency was found in the lowland between two nebkhasis, compared with a single one. And the windbreak efficiency of the second nebkha in the downstream region was slightly smaller than that of the lowland area between the two nebkhas. All nebkhas at different evolution stages show that the windbreak efficiencies around a single and two nebkhas increased with the increase of vegetation coverage. The finding can provide a theoretical basis for decision-making on the measures to control the soil wind erosion in the Ulan Buh desert-oasis ecotone.