干旱半干旱区落叶期农田防护林防风效果的风洞试验研究

探究落叶期不同结构乔灌木农田防护林带的空气动力学特征,合理配置林带结构对于减轻风季(10月-翌年5月)干旱、半干旱区农田土壤风蚀灾害具有重要意义。该研究利用风洞模拟技术,以"2行1带"式乔灌木林带为研究对象,按照1:100的缩尺比例,设计了不同疏透度、不同行距的林带模型,分析不同结构林带的风速廓线、防风效能和有效防护比,揭示落叶期不同结构林带空气流场特征和防风效果。结果表明:乔、灌木林带对空气流场垂直结构产生了不同影响,乔木林带对气流具有分层作用,风影区位于林带后5倍树高处(5H),灌木林带对气流具有抬升作用,风影区紧邻林带;疏透度较低的林带风速削弱作用更明显,疏透度45%、58%的乔木林带主要防风效能区间分别为20%～25%和15%～20%,疏透度36%、54%的灌木林带主要防风效能区间分别为10%～20%和5%～15%;在疏透度相近的条件下,行距对乔木林带防护效果的影响较小,在相同防风效能下不同行距乔木林带的累积有效防护比相差不到5个百分点,行距对灌木林带防护效果能产生较明显的影响,4cm行距灌木林带相比2.5cm行距林带在相同防风效能下的累积有效防护比高18～25个百分点;本研究中,乔木林带和灌木林带最优配置均为低疏透度(乔木45%、灌木35%)宽行距(乔木6 cm、灌木4 cm)模式。因此,在干旱、半干旱地区构建农田防护林带时,应当充分考虑林木落叶期的结构特征,针对乔木、灌木各自的特点有针对性地进行搭配,在合理的林带疏透度范围内,适当增大林带行距,以提高林带的防风能力,保障林木的健康生长。

Wind-tunnel tests study of shelter effects of deciduous farmland shelterbelts in arid and semi-arid areas

Shelterbelts can be expected to effectively reduce wind erosion of farmland soil in the wind season in arid and semi-arid lands. In this study, a systematic investigation was made to explore the aerodynamic characteristics, such as wind profile, shelter efficiency and effective protection ratios, of arbor and shrub farmland shelterbelts with deciduous features using wind-tunnel simulation. Based on the 2-row-1-belt pattern, 8 arbor/shrub shelterbelt models were manufactured at a scale of 1:100, according to different porosity (arbors of 45% and 58%, while shrubs of 35% and 54%), and row spacing (arbors of 4 and 6 cm, while shrubs of 2.5 and 4 cm). An attempt was made to analyze the influence of shelterbelt structure on the windproof capacity of farmland shelterbelts with deciduous aspect by measuring the mentioned aerodynamic characteristics, where the wind field of empty wind-tunnel (CK) was measured as well. The results showed that the arbor and shrub shelterbelts affected the vertical structure of the wind field in different ways. Specifically, the airflow was stratified by the arbor belts with the wind shadow located 5H behind the belts, while lifted by the shrub belts with the wind shadow adjacent to the belts. Porosity exerted more impact than row spacing on the windproof capacity of shelterbelts. The shelterbelts with low porosity provided greater shelter efficiency than those with high porosity, where the major shelter efficiency intervals of dense arbors, sparse arbors, dense shrubs, and sparse shrubs were 20%-25%, 15%-25%, 10%-20%, and 5%-15%, respectively. In the deciduous shelterbelts with similar porosity, the row spacing exerted little effect on the windproof capacity of arbor belts, but influenced that of the shrub belts. The difference in the cumulative effective protection ratio of arbor belts with different row spacing was generally within 5 percentage points at the major shelter efficiency. Nevertheless, the cumulative effective protection ratio of shrub belts with a row spacing of 4 cm was 18-25 percentage points higher than that of shrub belts with a row spacing of 2.5 cm. The shelterbelt porosity, which can be adjusted by plant spacing, played an important role in the windbreak. Furthermore, the shelter efficiency of shelterbelts with low porosity (arbors of 45% and shrubs of 35%) was greater than that with high porosity (arbors of 58% and shrubs of 54%). The proposed model of low porosity (arbors of 45% and shrubs of 35%) and wide row spacing (arbors of 6 cm and shrubs of 4 cm) was suitable for a kind of shelterbelt structure considering simultaneously the windproof capacity and the site conditions in arid and semi-arid lands. Therefore, a strong recommendation was made to scientifically match the structural characteristics of deciduous shelterbelts. Within a reasonable density range, the row spacing can be used to improve the windproof capacity of farmland shelterbelts.