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.