AERODYNAMIC PARAMETERS OF CROP CANOPIES ESTIMATED WITH A CENTER-OF-PRESSURE TECHNIQUE

The Center-of-Pressure technique (CPT),originally proposed by Thom in 1971,is firstly verified in the field,based on the canopy architecture and microclimate data of four crops. The comparison of u.estimated by CPT with that by eddy-correlation method shows that CPT not only works well in the field,but also gives more steady and accurate results,which are hardly affected by atmospheric stratification stability,than that of the log-profile fitting method which is frequently used in practice. In addition,a physical model for directly calculating d is approximately developed. The results yield that the ratio of d,zato h,in general,varies with the canopy structure,turbulence intensity and the stability of flow above and within the canopy;d/h increases with the in crease of α(the relative height of the maximum foliage density layer) and Yu(the wind extinction coefficient within canopies).Only for short stem ctops and when their canopies reach the moderate foliage density and stable structure,d and zo approximately and steadily approach to 0.64h and 0.08h,respectively. Besides,concerned with very sparse or tall crops,the fairly large stresses at the soil surface and variability of m undoubtedly influence the accuracy of CPT to a certain extent.