MODELING THE ROUGHNESS PROPERTIES OF ARTIFICIAL SOIL CLODS

Abstract

Knowledge of the aerodynamic roughness length (Z₀) resulting from management practices that include soil clods, vegetation, and ridges is essential to describing their protective roles in wind erosion. The usual method of obtaining Z₀ from a logarithmic wind profile has limited accuracy because the probes used to measure wind profiles do not reveal well the subtle characteristics of the very near-surface airflow. To evaluate the roughness properties of soil clods in this study, Z₀ is derived by direct measurement of the drag on cloddy surfaces. Z₀ values, in response to different wind velocities and artificial soil clod coverage, were obtained in a wind tunnel. Z₀ was found to be the function of both clod coverage and wind velocity, increasing with clod coverage by a function of Z₀ = a + bS_clnS_c(R² > 0.81, P < 0.001) and decreasing with wind velocity by a function of Z₀ = c + d exp (−V/k) (R² > 0.68, P < 0.001), where S_c and V are clod coverage and wind velocity and a, b, c, d, and k are regression constants. A multivariate predictive equation was developed by regressing Z₀ on S_c and V. The predicted values matched those from the wind tunnel reasonably well (R² = 0.92). These results demonstrate a need for distinguishing the concept of aerodynamic roughness that is a dynamic dimension of the interaction between near-surface airflow and surface rougher than that of surface roughness that is dependent exclusively on surface conditions.