MOMENTUM-TRANSFER TO PLANT CANOPIES - INFLUENCE OF STRUCTURE AND VARIABLE DRAG

Theory is developed to assess momentum transfer to vegetation canopies displaying a 'dynamic' response to variations in the level of mean wind-speed. With the assumption of an exponential variation of wind-speed with height in the upper layers of a plant canopy and the use of a wind-speed dependent drag coefficient C//d, theory is developed to describe the profiles of shear-stress and momentum diffusivity within canopies of contrasting vertical structure. In a uniform canopy it is shown that the wind-speed and diffusivity profiles are dissimilar except for the case of C//d independent of wind-speed. For canopies with a vertical structure of Gaussian form, the momentum diffusivity in the upper layers may exceed the interface value, but decreases in lower layers. From analysis of wind profiles measured within and above a cotton canopy, it is shown that the effective drag coefficient of foliage elements increases with wind-speed.