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The usual concept of surface roughness and zero plane displacement breaks down when one considers a vegetative canopy as the lower boundary for the planetary boundary layer. The roughness of many vegetative canopies derived from the log law changes with wind speed as do the zero-plane displacements. Ideally one would like to express the aerodynamic roughness of vegetation in terms of its height, density, and drag characteristics. Restricting considerations to the turbulent transfer of momentum, a model was developed that will predict the canopy wind profile within semi-rigid canopies. For the purpose of this report, a canopy is defined as that layer spanning the region from the ground surface to the top of the plant.
Ronald M. Cionco (Sun,) studied this question.