MOMENTUM ROUGHNESS AND ZERO-PLANE DISPLACEMENT OF RIDGE-FURROW TILLED SOIL
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Estimation of heat and mass transfer between the atmosphere and the ground surface by flux-profile relationships may be influenced by the values chosen for zero-plane displacement d and momentum roughness z0m. This study was conducted to determine the effect of wind direction on d and z0m of a ridge-furrow tilled soil. Fifteen-minute average wind-speed and air-temperature profiles, together with wind direction, measured over a 28-day period above a ridge-furrow tilled field near College Station, TX, were used to determine d and z0m as a function of wind direction relative to ridge orientation. The difference in height h between the ridge tops and furrow bottoms was 0.22 m and the spacing between ridges was 1 m. A trade-off between d and z0m that did not appreciably alter the fit of theoretical to measured wind-speed profiles made separation of d and z0m difficult. No strong relationship was found between d or z0m and wind direction when both were allowed to vary and the best fits of theoretical to measured windspeed profiles were found by a least sum of error squares technique. When z0m was held constant and d allowed to vary, the best overall fit was with z0m h = 0.032 and d/h varying from about 0.091 to 1.2 when the wind direction was parallel and perpendicular to the ridges, respectively. When d was held constant and z0m allowed to vary, the best overall fit was with d/h = 0.85 and z0m h varying from 0.011 to 0.055 when the wind direction was parallel and perpendicular to the ridges, respectively. The implication to estimation of heat or mass transport between the atmosphere and the surface of a ridge-furrow tilled soil where d and z0m are required in the calculations is that wind direction should be considered. 1991.
