Modeling the roughness properties of artificial soil clods

Knowledge of the aerodynamic roughness length (Z(0)) 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 Z(0) 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, Z(0) is derived by direct measurement of the drag on cloddy surfaces. Z(0) values, in response to different wind velocities and artificial soil clod coverage, were obtained in a wind tunnel. Z(0) was found to be the function of both clod coverage and wind velocity increasing with clod coverage by a function of Z(0) = a + bS(c)lnS(c)(R-2 > 0.81, P < 0.001) and decreasing with wind velocity by a function of Z(0) = c + d exp (-V/k) (R-2 > 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 Z(0) on S-c and V. The predicted values matched those from the wind tunnel reasonably well (R-2 = 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.