Effects of windbreak structure on shelter characteristics

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/U-0, where U-0 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, X-min (position of minimum windspeed, U-min) from 1-7 H; U-min (minimum U/U-o) varied with windbreak density; and X-S (distance where U/U-0 is < 0.8) varied from 8-25 H. Optical porosity (beta(O)) was 0.16-0.53 compared with 0.24-0.61 for aerodynamic porosity (beta(A)), obtained from the ratio of U-min and U-0. beta(A) from present data and calculated for windbreaks of previous studies, showed a curvilinear effect of windbreak porosity (beta(A), %) on X-min, but little effect on X-S: X-min (H) = 3.008Ln beta(A) - 6.6 (adjusted r(2) = 0.33, P < 0.0001, r.s.d. = 1.8); X-S (H) = 12.2+ 0.307 beta(A) - 0.0047 beta(2)(A) (adjusted r(2) = 0.07, P < 0.08, r.s.d. = 4.1). Effective shelter is best obtained by establishing tall, dense windbreaks and excluding browsing livestock.