Aerodynamic properties of partial canopies

Abstract Roughness lengths ( z o ) and displacement heights ( d ) are often assumed to be simple functions of canopy height. This study was designed to evaluate these canopy aerodynamic properties for cotton ( Gossypium hirsutum L.) having partial ground cover. Both z o and d were calculated from wind profile measurements for neutral conditions as defined with a Richardson number over irrigated and rainfed cotton in 1984 and 1985. Throughout the season z o started at 0.01 m and increased to a maximum of 0.20 m then declined to a value of 0.06 m when the canopy reached 70% ground cover. This change throughout the season was caused by the individual rows first acting as discrete bluff-bodies and then as a more uniform surface as the foliage elements overlapped as the canopy developed. Displacement heights increased linearly throughout the season as the canopy increased in height and leaf area.