乌兰布和荒漠绿洲过渡带白刺灌丛沙堆防风效应风洞模拟

灌丛沙堆作为荒漠绿洲过渡带广泛存在的生物地貌类型，其在不同演化阶段和植被盖度下的风速分布及防风效应对土壤风蚀灾害控制具有重要意义。为揭示灌丛沙堆演化阶段及植被盖度对周围风场结构特征及防风效应指标的影响规律，该研究以乌兰布和沙漠荒漠绿洲过渡带单个和两个白刺（Nitraria tangutorum）灌丛沙堆为对象，利用风洞模拟试验测定其在4个不同演化阶段（雏形阶段、发育阶段、稳定阶段、活化阶段）和3种不同植被盖度（0、40%及70%）工况下竖直和水平面的风速分布，探讨演化阶段及植被盖度对相对速率、防风效能等防风效应指标的影响。研究发现，不同演化阶段的白刺灌丛沙堆在0～0.8H（H为灌丛沙堆高度）高度范围内、距沙堆中心-H～5H水平范围内风速有显著变化；对比坡前减速区、上方加速区、两侧急流区和后方减速区的风速变化发现，白刺灌丛沙堆在后方减速区的风速变化最为显著，处于稳定阶段的灌丛沙堆在植被盖度为70%时的风速最大可减少90%；不同演化阶段中，稳定阶段灌丛沙堆对风速的削减更为显著，防风效能较大，而处于雏形阶段的灌丛沙堆的防风能力相对较弱；在两个平行于风向排列的灌丛沙堆的相互影响下，沙堆之间丘间低地的防风效能比单个灌丛沙堆有所提高，且第二个灌丛沙堆下游区域的防风效能略小于两个灌丛沙堆间的丘间低地；各演化阶段单个和两个灌丛沙堆周围的防风效能均随植被盖度增大而增大。该研究结果可为乌兰布和荒漠绿洲过渡带土壤风蚀防治提供参考。

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.