AERODYNAMIC CHARACTERISTICS OF STANDING CORN STUBBLE

Maintenance of crop residues on the soil surface is promoted as a mana gement practice for reducing soil erosion by wind and water. Crop resi due on the soil surface, however, also influences turbulent exchange p rocesses that affect surface water and energy balances. At present, th ere are very limited data on the aerodynamic properties of residue-cov ered surfaces. The objective of this research was to derive estimates of the aerodynamic properties (momentum roughness length z(0), zero-pl ane displacement d, and drag coefficient C-d) of fields with standing corn (Zea mays L.) stubble. Wind speed and air temperature measurement s were made at six heights above two no-tillage fields near Ames, IA, in the spring and fall of 1994 and in the spring of 1995. Both sites h ad standing corn stubble approximately 0.3 m tall at a density of grea ter than 60 000 stalks ha(-1) and nearly 95% residue cover. An iterati ve, least-squares linear regression technique was applied to wind prof iles obtained under neutral atmospheric conditions to determine z(0), d, and the friction velocity (u). Mean values of z(0) and d for each of the three measurement periods varied from 11.8 to 24.3 and 119 to 1 69 mm, respectively. The ratios z(0)/h(s) and d/h(s), where hs is the mean stubble height, averaged 0.058 and 0.53, respectively, for all pr ofiles. The magnitude of z(0) and d and their weak dependence on wind speed are consistent with turbulent dow over a surface with sparse, ri gid roughness elements. The z(0) and C-d were higher for the fall meas urement period, suggesting that the loose, fresh residue presented a c omparatively rough surface that was then smoothed by weathering and co mpaction beneath snow.