Effect of wind speed and direction on forces acting on shade nets covering orchard trees

摘要

The use of nets and screens to cover orchards has greatly expanded during the last decade, since the nets protect the trees from excessive radiation, strong winds, hail and insects. Furthermore, recent studies have shown that, in certain orchards, thenets can significantly reduce the water consumption of trees. The nets are usually stretched above the trees and are fixed to the supporting poles by cables that run all around the net circumference. In this study, experiments were done to determine theforces acting on a horizontal shade net that covered an orchard (232x100 m2) in which peach trees were grown. A net section of 40x6 m2, at an upwind edge of the net-house, was used to measure the forces. Sixty load cells that were evenly distributed onthe section circumference were used to attach the net to the net-house cables. Results show that the force acting on the net at the point where the load cell is attached to the net is a second-order polynomial of wind speed. The forces acting on the netwere stronger at the points where the net was attached to a cable at the outer edge of the net-house. At points where the net was attached to inner cables of the net-house, the forces were smaller. Extrapolating the second-order polynomial, it was shownthat, at a wind speed of 150 km h1, the largest force acting at the point where the load cell was attached to the net could be 50 kg.