Content area
Abstract
Wind speeds were measured within and above grain sorghum canopies planted in north-south oriented rows at Mead, Nebraska in 1982 and 1983. In both years, a 0.76m row spacing was used in the main field. The plant population ha('-1) was about 276,000 in 1982 and 118,500 in 1983. A secondary field was used in 1983 with double row spacing and 95,000 plants ha('-1). Data were analyzed for periods when wind direction (measured at 3m) ranged from southeast to southwest.
The roughness parameter (z(,o)) and zero-plane displacement (d) increased with increasing crop height (h). Information on z(,o)/h and d/h was developed for various stages of crop growth. Mean values of z(,o)/h and d/h for full crop development (under moderate wind speed) were 0.096 and 0.61 in 1982. Corresponding values in 1983 were 0.074 and 0.54. In both years, z(,o)/h and d/h decreased with increasing wind speed. Mean values of the crop drag coefficient (C(,d)) for the fully developed canopies under moderate wind speed were 0.055 in 1982 and 0.035 in 1983.
Wind speeds decreased monotonically with depth in the upper and middle portions of the canopies. Reversal in wind speed gradient and presence of a secondary wind speed maximum was observed in the lower portion of the canopy. This secondary maximum was quite pronounced in the wide-row canopy under windy conditions. The penetration of flow into the canopies was increased with change from stable to unstable conditions.
Several models of within-canopy air flow were evaluated against the observations made in this study. Only in the upper portion of the denser canopy did the empirical models (Inoue, 1963; Cionco, 1965; Cowan, 1968; Landsberg and James, 1971; Thom, 1971) predict wind speeds reasonably well. The second order closure model (Wilson and Shaw, 1977) predicted wind speeds midway between two rows within the denser canopy fairly well. The secondary maximum observed in the lower portion of the canopy was accurately predicted by the model. The prediction capability of both empirical and second order closure models was poor for the sparser canopy (1983).
