Effects of windbreak structure on shelter characteristics

Abstract

Eleven windbreaks (Monterey cypress, Monterey pine, tuart, sugar gum, black wattle and mixed native species) were studied in south-west Victoria. Windrun (U) was measured with cup anemometers set above ground (Z) at 1.5 m and 0.5 m (eight windbreaks), 1.5 m (two windbreaks), or 0.5 and 0.25 m (0.95 m mesh). The U pattern (U/U0, where U0 is open windspeed) was: (1) to windward, a shelter effect to at least –3 H, (where H means windbreak height) with a reduction of 20% or more at –1 H and (2) to lee, Xmin (position of minimum windspeed, Umin) from 1–7 H; Umin (minimum U/Uo) varied with windbreak density; and XS (distance where U/U0 is <0.8) varied from 8–25 H. Optical porosity (βO) was 0.16–0.53 compared with 0.24–0.61 for aerodynamic porosity (βA), obtained from the ratio of Umin and U0. βA from present data and calculated for windbreaks of previous studies, showed a curvilinear effect of windbreak porosity (βA, %) on Xmin, but little effect on XS: Xmin (H) = 3.008LnβA – 6.6 (adjusted r2 = 0.33, P < 0.0001, r.s.d. = 1.8); XS (H) = 12.2+ 0.307βA – 0.0047βA2 (adjusted r2 = 0.07, P < 0.08, r.s.d. = 4.1). Effective shelter is best obtained by establishing tall, dense windbreaks and excluding browsing livestock.