Photosynthesis Under Field Conditions. VIII. Analysis of Windspeed Fluctuation Data to Evaluate Turbulent Exchange Within a Corn Crop

Turbulent transfer within a crop of corn was characterized by an aerodynamic approach. The distributions of windspeed within an immature and a mature crop were measured using both cup and heated thermocouple anemometers. A combination of the statistical and mixing-length theories was employed to analyze the wind data for transfer coefficients. Eulerian time scales of turbulence were calculated from the windspeed fluctuations of selected 30-second periods of semi-steady wind. Momentum transfer coefficients were determined from the scale of turbulence by equating it to the mixing length and assuming isotropy. The resulting values were as large as 8,000 cm2 sec-1. The shearing stress profiles calculated with these values had unrealistically sharp maximums just below the top of the crop. This anomaly was considered an artifact of the method resulting from extreme anisotropy in the turbulent shear flow. The K values obtained by the statistical mixing length method were approximately ten times larger than those determined by logarithmic profile analysis. The latter values were considered more nearly correct and were used as a basis for correcting the other values. The results showed that the magnitude of turbulent transfer is several orders of magnitude greater than molecular diffusion even at levels deep within the crop. The transfer coefficient showed a marked attenuation with depth below the top of the corn but remained a function of windspeed at all heights

Photosynthesis Under Field Conditions. XA. Assessing Sources and Sinks of Carbon Dioxide in a Corn (Zea mays L.) Crop Using Momentum Balance Approach

In a previous communication we described a meteorological method of assessing the vertical distribution of photosynthesis and respiration activity in a corn crop. The method, however, is tedious. Here we describe a much simpler method. Results indicate that all the leaves of a corn crop appear to follow the same near-linear light response curve, at least above the compensation point. Below the compensation point, all leaves evidently respire very little

The Energy Budget at the Earth's Surface: Assessing Sources and Sinks of Carbon Dioxide in a Corn Crop Using a Momentum Balance Approach

In two previous communications (Wright and Lemon 1966a, 1966b) an aerodynamic method was described for evaluating the source and sink distribution of carbon dioxide in plant communities. Some data were presented for a corn crop, giving quantitative information about photosynthetic fixation and respiration release of carbon dioxide, layer by layer, within the crop. In the method, CO2 concentration profiles and windspeed measurements of the bulk air were made within and above the crop. The analysis of the windspeed measurements to calculate diffusivity coefficients required tedious analysis of windspeed fluctuations and the application of complicated statistical and mixing length theories. It is our purpose to present here a simpler method, requiring vertical profiles of mean windspeed, vertical profiles of mean carbon dioxide concentration and representative vertical profiles of the foliage surface area density of the plant community

Photosynthesis Under Field Conditions. IX. Vertical Distribution of Photosynthesis Within a Corn Crop

The vertical distribution of the photosynthetic fixation of carbon dioxide within a crop of corn was calculated from carbon dioxide profile data and transfer coefficients obtained by analysis of windspeed measurements. Infrared analyzers were used to measure the carbon dioxide concentration at several heights within and above the crop. The calculated total fixation for the day was approximately 60 g CO? m-? (equivalent to 470 pounds of sugar per acre per day). The results demonstrated the importance of the upper leaves in the fixation of carbon dioxide and showed the increased fixation by the lower leaves during periods of high light penetration. There was some indication that a coupling existed between the level of windspeed and fixation under conditions of high light and relatively low windspeed. With refinement in technique the method could be used to obtain more quantitative estimates of the distribution of photosynthesis in other crops

Photosynthesis Under Field Conditions. XB. Origins of Short-Time CO2 Fluctuations in a Cornfield

Studies of the vertical distributions of CO? fluctuation in a cornfield were made in the 4-to 0.25-cycle/min frequency range. Amplitude of fluctuations decreased with height above the ground. Frequency in this range appeared rather constant, however. Sources and sinks for CO? within the cornfield contribute to the fluctuations; however, eddy structure originating inside and/or outside the cornfield plays an important role too

Photosynthesis Under Field Conditions. IX. Vertical Distribution of Photosynthesis Within a Corn Crop

The vertical distribution of the photosynthetic fixation of carbon dioxide within a crop of corn was calculated from carbon dioxide profile data and transfer coefficients obtained by analysis of windspeed measurements. Infrared analyzers were used to measure the carbon dioxide concentration at several heights within and above the crop. The calculated total fixation for the day was approximately 60 g CO? m-? (equivalent to 470 pounds of sugar per acre per day). The results demonstrated the importance of the upper leaves in the fixation of carbon dioxide and showed the increased fixation by the lower leaves during periods of high light penetration. There was some indication that a coupling existed between the level of windspeed and fixation under conditions of high light and relatively low windspeed. With refinement in technique the method could be used to obtain more quantitative estimates of the distribution of photosynthesis in other crops

Photosynthesis Under Field Conditions. XA. Assessing Sources and Sinks of Carbon Dioxide in a Corn (Zea mays L.) Crop Using Momentum Balance Approach

In a previous communication we described a meteorological method of assessing the vertical distribution of photosynthesis and respiration activity in a corn crop. The method, however, is tedious. Here we describe a much simpler method. Results indicate that all the leaves of a corn crop appear to follow the same near-linear light response curve, at least above the compensation point. Below the compensation point, all leaves evidently respire very little

Photosynthesis Under Field Conditions. VIII. Analysis of Windspeed Fluctuation Data to Evaluate Turbulent Exchange Within a Corn Crop

Turbulent transfer within a crop of corn was characterized by an aerodynamic approach. The distributions of windspeed within an immature and a mature crop were measured using both cup and heated thermocouple anemometers. A combination of the statistical and mixing-length theories was employed to analyze the wind data for transfer coefficients. Eulerian time scales of turbulence were calculated from the windspeed fluctuations of selected 30-second periods of semi-steady wind. Momentum transfer coefficients were determined from the scale of turbulence by equating it to the mixing length and assuming isotropy. The resulting values were as large as 8,000 cm2 sec-1. The shearing stress profiles calculated with these values had unrealistically sharp maximums just below the top of the crop. This anomaly was considered an artifact of the method resulting from extreme anisotropy in the turbulent shear flow. The K values obtained by the statistical mixing length method were approximately ten times larger than those determined by logarithmic profile analysis. The latter values were considered more nearly correct and were used as a basis for correcting the other values. The results showed that the magnitude of turbulent transfer is several orders of magnitude greater than molecular diffusion even at levels deep within the crop. The transfer coefficient showed a marked attenuation with depth below the top of the corn but remained a function of windspeed at all heights