Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 10, 1996
Publication Date: N/A
Interpretive Summary: Producers have been encouraged to maintain a layer of crop residue on the soil surface to help protect the soil from wind and water erosion. Many studies have addressed how effective crop residue is in reducing erosion. However, crop residue also affects soil temperature, heat transfer, and evaporation. Several methods for predicting evaporation and heat flow rely on knowledge of the change in wind speed with height above the residue-covered surface. After this type of measurement has been made, parameters from the analysis can be used to estimate evaporation and heat transfer from the surface. The purpose of this study was to make such wind profile measurements over a field of corn stubble. Wind speed measurements were made over a no-tillage field in Iowa during spring and fall periods in between cropping seasons. Profiles obtained under suitable conditions were analyzed and showed that the stubble surface was rougher than a bare soil but not as rough as a crop canopy. These differences can be attributed to the relatively sparse density of the standing stalks. The results of this study can now be used to improve estimates of heat flow and evaporation from the residue-covered surface.
Technical Abstract: Maintenance of an appreciable amount of crop residue on the soil surface is promoted as a management practice for reducing soil erosion by wind and water. Crop residue on the soil surface, however, also influences turbulent exchange processes that impact the surface water and energy balances. Presently, there is very limited data on the aerodynamic properties of residue-covered surfaces. The objective of this research was to derive estimates of the aerodynamic properties (momentum roughness length zo, zero-plane displacement d, and drag coefficient Cd) of fields with standing corn (Zea mays L.) stubble. Wind speed and air temperature measurements were made at six heights above two no-tillage fields near Ames, Iowa, in the spring and fall of 1994 and in the spring of 1995. Both sites had standing corn stubble approximately 0.3 m-tall at a density of greater than 60 000 stalks ha**-1 and nearly 95% residue cover. An iterative, least-squares linear regression technique was applied to wind profiles obtained under neutral atmospheric conditions to determine zo, d, and the friction velocity (u*). Mean values of zo and d for each of the three measurement periods varied from 9.8 to 24.2 and 119 to 183 mm, respectively. The ratios zo/hs and d/hs, where hs is the mean stubble height, averaged 0.053 and 0.59, respectively, for all profiles. The magnitude of zo and d and their weak dependence on wind speed are consistent with turbulent flow over a surface with sparse, rigid roughness elements. The momentum roughness length and drag coefficient were higher for the fall measurement period suggesting that the loose, fresh residue presented a comparatively rough surface that was then smoothed by weathering and compaction beneath snow.