AERODYNAMIC CHARACTERISTICS OF CORN AS DETERMINED BY ENERGY-BALANCE TECHNIQUES

Aerodynamic resistance to heat transfer (r(ah)) needed to calculate se nsible heat flux (H) used in energy balance modeling can be estimated from momentum aerodynamic resistance with corrections for atmospheric stability. This study compared r(ah) and H modeled by four commonly us ed resistance methods with r(ah) and H measured indirectly through ene rgy balance techniques. Three momentum aerodynamic parameters were cal culated: roughness length, Z(om); zero plane displacement, d; and fric tion velocity, U. Corn (Zen mays L.) was grown on east-west rows (0.7 5m wide) in 1989 and 1990 at Bushland, TX, in two contiguous 5-ha fiel ds where two weighing lysimeters were located and micrometeorological measurements were made. Sensible heat flux was indirectly measured as a residual of the energy balance and then used to calulate aerodynamic resistance. Momentum aerodynamic parameters were calculated from near -neutral condition wind-speed profiles using a least squares procedure . The momentum parameter relationships to crop height (CH) were d = 0. 73 x CH(r(2) = 0.59) and Z(om) = 0.12 x CH (r(2) = 0.96). While no r(a h) model performed well, the best linear fit (r(2) = 0.75, y = 1.O8x 4.2) between measured (x) and modeled (y) r(ah) occurred under stable atmospheric conditions; for measured and modeled H, the best linear f it (r(2) = 0.84, y = 0.93x + 62.1) occurred under all atmospheric cond itions. Measured r(ah) in neutral and unstable conditions was not clos ely associated with wind speed. Performance of a model with a limited stability factor nas improved by increasing the magnitude of the facto r, These results suggest that r(ah) models may be sensitive to atmosph eric stability and local conditions such as fetch and leaf area.