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United States Department of Agriculture

Agricultural Research Service

Title: Aerodynamic Characteristics of Corn As Determined by Energy Balance Techniques

Authors
item Tolk J A,
item Howell T A,
item Steiner J L,
item Krieg D R, - TEXAS TECH UNIVERSITY

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 14, 1994
Publication Date: N/A

Interpretive Summary: Calculation of evapotranspiration (ET) rates for energy balance modeling requires estimation of the aerodynamic resistances for heat and for water vapor exchanges. These exchanges are routinely estimated from momentum aerodynamic resistance based on wind profile characteristics with corrections for atmospheric stability. Measurements of ET using a large weighing lysimeter, net radiation, soil heat flux, profiles of wind, vapor pressure, and air temperature were made on corn nearing anthesis in 1990 at Bushland, TX. Four commonly used aerodynamic resistance equations were then evaluated using aerodynamic resistance measured through energy balance techniques. The equations represented different applications of the Richardson number, which is a stability correction based on wind speed and the difference between canopy and air temperatures. Momentum aerodynamic parameters of roughness length (Zom), zero plane displacement (d), and friction velocity (U*) were also determined from near-neutral wind speed profiles using a least squares technique. All four equations overestimated the aerodynamic resistance calculated from sensible heat flux. The equations with very limited to no stability corrections produced the best linear fit when comparing calculated with measured sensible heat flux. The momentum parameter relationships to crop height (CH) were d=0.71*CH and Zom=0.12*CH. With increasing friction velocity, d/CH declined and Zom increased. These results, along with low values of calculated aerodynamic resistance, suggest a turbulent regime best characterized by momentum aerodynamic parameters.

Technical Abstract: Aerodynamic resistances for heat and for water vapor are required to estimate latent and sensible heat fluxes of a cropped surface and are routinely estimated from the momentum aerodynamic resistance based on wind profile characteristics with corrections for atmospheric stability. Four commonly used aerodynamic resistance equations were evaluated using aerodynamic resistance measured through energy balance techniques. Momentum aerodynamic parameters of roughness length (Zom), zero plane displacement (d), and friction velocity (U*) were also determined. Corn (Zea Mays L.) was grown on 0.75 m wide E-W beds in 1990 at Bushland, TX, in two contiguous 5-ha fields. A weighing lysimeter, with a 9-m**2 surface area, is centered in each field. Net radiation, soil heat flux, and profiles of air temperature, vapor pressure, and wind were measured at each lysimeter. Aerodynamic resistance used to evaluate the four equations was calculated from sensible heat flux which was determined as a residual of the energy balance equation. The momentum aerodynamic parameters were calculated from near-neutral condition wind speed profiles by a least squares procedure. All four equations overestimated the aerodynamic resistance calculated from sensible heat flux. The equations with limited stability corrections produced the best linear fit when comparing calculated with measured sensible heat flux. The momentum parameter relationships to crop height (CH) were d=0.71*CH and Zom=0.12*CH. With increasing friction velocity, d/CH declined and Zom increased. These results, along with low values of calculated aerodynamic resistance, suggest a turbulent regime best characterized by momentum aerodynamic parameters.

Last Modified: 7/23/2014
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