农田休闲期垄作地形对近地表风场的影响

垄作是旱作农田常用的保护性耕作技术,而其防风抗蚀的机制尚需研究。通过野外试验,对垄脊高25cm,垄沟宽1.5m的垄作田上风向、垄间及下风向地带0～4m的风流场进行了同步观测,对不同观测位置的时均风速、空气动力学粗糙度、摩阻速度和风速脉动进行了对比分析。结果表明,土垄间和下风向近地表0～1m内时均风速比上风向平坦地表明显降低,其中高0.3m处降低幅度最大。水平方向上风速降低幅度、空气动力学粗糙度和摩阻速度均随观测点与垄作地表距离的增大而减小。受风速递增的影响,风速的绝对脉动强度随高度呈对数关系递增。风速的相对脉动强度在0～1m内随高度增大而降低,1m以上基本无变化。高0.05m处风速的相对脉动强度在水平方向上随与垄作田距离的减小而增大,其中垄间最大,时均风速相同时对地面剪切力最大。

EFFECTS OF MICRORELIEF OF RIDGE-TILLAGE ON WIND FIELD NEAR THE SURFACE OF FIELDS IN FALLOW

Ridge-tillage is an effective farming practice for soil and water conservation, extensively used in rainfed cropland in semi arid regions, however, its mechanism of controlling wind erosion is unclear and calls for further research.In the experiment field with ridges 25 cm high and 1 5 m in between, synchronous observation was made of wind profiles from 0 m to 4 m at windward, bed between ridges and leeward.Aerodynamic parameters of these sites, including hourly average wind velocity, aerodynamic roughness, friction velocity, and absolute and relative fluctuation intensity of wind velocity, were analyzed.Results indicate that hourly average velocities of the layer of 0~1 m at the site in between ridges and the leeward site were obviously lower than at the windward site, and the biggest drop was observed at the height of 0.3 m.Horizontally, the parameters, such as extent of the decrease in wind velocity, aerodynamic roughness and friction velocity, were all decreased with the distance of the observation point from the ridge.And all the three parameters were the biggest at the site between ridges whereas the smallest at the windward site, 1.5 m from the ridge.Increasing with wind velocity, absolute fluctuation intensity was a logarithm function of the height in the vertical direction whereas relative fluctuation intensity of wind velocity decreased with height in the 0~1 m layer of the profile, and changed little in the layers above 1 m, but increased with the decrease in distance of the observation site leeward from the ridge at the height of 0.05 m.It is the highest at the site between the ridges, bringing the shear stress of airflow on soil surface up to the peak, which is obviously unfavorable to wind erosion control.