Tanny, J., Inst. of Soil, Water/Environ. Sci., Agricultural Research Organization, Volcani Center, P. O. B. 6, Bet Dagan 50250, Israel
Cohen, S., Inst. of Soil, Water/Environ. Sci., Agricultural Research Organization, Volcani Center, P. O. B. 6, Bet Dagan 50250, Israel

Characterising aerodynamic properties of agricultural screens covering crops is important for the estimation of resistance to exchanges of mass and momentum in the crop. In this study a small shade net, used to investigate crop response to reduced radiation, was extended at a height of 3 m (just above the tree tops) in a citrus orchard. The net was 2 m wide and 9 m long. A wind/temperature mast was installed between the shaded area and an adjacent control (unshaded) area enabling a comparison between the wind speed and temperature profiles for wind blowing over the canopy and over the net with data from the same mast. The aerodynamic properties of the wind profile, namely the friction velocity, the roughness length, the zero-plane displacement, and the resistance are calculated and presented with respect to the stability of the surface layer above the orchard. The analysis is limited to the transient state since the wind does not reach equilibrium over the relatively small net. It is shown that under neutral and stable conditions the shade net acts to inhibit turbulence, to reduce the roughness length, to displace the wind profile upwards, just above the net and to increase the aerodynamic resistance. However, as the boundary layer above the net becomes thermally unstable the friction velocity and the roughness length increase and the zero-plane displacement decreases relative to their values under stable conditions. The shade net has a strong influence on the wind within the foliage where it reduces the wind speed by about 40% as compared to the foliage when unshaded. © 2003 Silsoe Research Institute. All rights reserved. Published by Elsevier Science Ltd.