Local effects of living windbreaks
2004
Souto, L.F.M.A.R. (Universidade dos Acores, Terra Cha (Azores). Dept. de Ciencias Agrarias) | Meneses, J.F. (Instituto Superior de Agronomia, Lisbon (Portugal))
The primary purpose of living windbreaks is the reduction of the wind speed. Changes in the velocity and direction of airflow lead to changes in the microclimate. Microclimate is the result of various interacting factors, most of which can be affected by shelter at detectable levels. The effectiveness of windbreaks depends upon their internal and external structure. The most important structural feature is porosity. Maximum wind reductions are closely related to low porosity. However, barriers with low porosity generate more turbulence to leeward then medium or high ones. The horizontal extent of windbreak effects is proportional to windbreak height (h). Wind reductions are detectable on the windward side of the barrier from 2 h to 5 h, and extend to leeward about 20 h, if significant reductions of the normal wind speed (10% - 30%) occur. The principal effects of natural windbreaks on microclimate are reviewed. These are: wind speed, turbulence and their dependence on the porosity and height of the barrier; solar and thermal radiation around the windbreak; air temperature during the day and at night; humidity and snow deposition. Complementary effects of windbreaks are discussed, such as wind and hydraulic erosion, crop protection and plant growth, livestock protection of buildings and people. Disadvantages of windbreaks result from tree-crop competition and pests. Windbreaks allow the practice of agriculture in many regions of the world, namely on island and coastal areas, which are perticularly sensitive due to the high wind speeds and salt content of the wind. Windbreaks contribute to sustainable agricultural systems by maintaining biological diversity and improving the aesthetic value of the landscape. A better understanding of their dynamics will certainly contribute to the improvement of sheltered areas and to the establishment of more efficient windbreaks, with less negative impacts to the countryside
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