DETERMINATION OF AERODYNAMIC PARAMETERS OF CROP CANOPIES WITH A CENTER-OF-PRESSURE METHOD

Using data on leaf area density and wind profiles above and within canopies of wheat,rice,soybean and corn,the center-of-pressure method (CPM),originally proposed by Thom (1971),is first validated in the field.A physically-based model for directly calculating zero-displacement height (d) is derived.The comparison between the friction velocity (u*) estimated with CPM and that with eddy correlation technique shows that CPM not only works well in the field,but also produces more steady and accurate estimates of aerodynamic parameters (which are hardly affected by atmospheric thermal stability),than those with the widely-used log-profile fitting method in diabatic atmosphere.The results presented in this paper also demonstrate that the ratios of d,z0 to crop height h usually vary with canopy architecture,atmospheric stratification and turbulent exchange intensity,and are not just constants as commonly assumed or used.d/h raises with an increase of relative height of the maximum foliage layer and wind extinction coefficient within the canopy.Only for crops with short stem and moderate foliage density,can the relations d=0.64h,z0=0.08h keep stable.In addition,for long stem crops or sparse canopies,the fairly large shear stress at the soil surface and the variation of the exponent in the relationship between drag coefficient and wind speed undoubtedly influence the accuracy of CPM to a certain extent.