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Accuracy of Lysimetric, Energy Balance, and Stability-corrected Aerodynamic Methods of Estimating Above-canopy Flux of CO₂¹

Measurement of carbon dioxide exchange is necessary to indicate rates of photosynthesis in the field. Methods which are accurate over short periods of 15 min to 1 hour can indicate the influence of changing weather conditions on crop photosynthetic activity. Three micrometeorological methods—the lysimetric, energy balance, and stability-corrected aerodynamic—can be used for field determination of the flux of CO₂ to a crop surface. The objective of this paper is to present a detailed comparison of the performances of the three methods. Measurements were made in an oat field at Mead, Neb. Results obtained with the three methods were in good agreement, both on a short-period and on a daily basis. The calculated CO₂ flux agreed reasonably well with estimates from other oat-photosynthesis studies. Midday CO₂ flux rates ranged from about 2 to 3 x 10^–7 g cm^–2 sec^–1. The three micrometerological methods were subjected to detailed error analysis to evaluate the relative influence of errors in measurement of the constituent input parameters used to calculate the flux of carbon dioxide, namely water vapor flux and gradients of vapor pressure and CO₂ concentration in the lysimetric method; net radiation, soil heat flux and gradients of vapor pressure, air temperature and CO₂ concentration in the energy balance method; and gradients of windspeed, air temperature and CO₂ concentration in the stability-corrected aerodynamic method.

Key Words: Micrometerology • Microclimate • Energy balance • Lysimeters • Aerodynamic • CO₂ Flux • Error analysis • Field-photosynthesis • Stability correction • Exchange coefficients • Turbulent transfer

² Assistant Professor and Professor of Agricultural Meteorology, Dep. of Agric. Eng., Institute of Agric. and Natural Resources, Univ. of Nebraska, Lincoln, NE 68503.

Received for publication May 23, 1974.