Transpiration of water from growing canopies is necessary of optimal growth. Likewise, carbon dioxide is a necessary component for growth. Interactions between water vapor and carbon dioxide are critical components in plant growth responses to microclimatic variations. However, there has not been an attempt to link these with flux measurements and the recent advances in measurement methods open potential new ways at examining water-carbon interactions in cropping systems. We have been conducting season-long studies within corn canopies to couple water vapor ad carbon dioxide fluxes to improve our understanding how crop respond to different stresses. These studies have been conducted within a production-sized field (35 ha) using a combination of energy balance measurements and water vapor and carbon dioxide fluxes. These measurements included a net radiometer positioned at 3 m above the canopy, soil heat flux plates at 0.1 m below the soil surface, windspeed and wind direction sensors at 1 m above the canopy, surface temperature with an infrared thermometer oriented at a 45° angle from nadir, and a three-dimensional sonic anemometer, and an infrared carbon dioxide and water vapor fast response sensor. These sensors were sampled at 10 Hz with a 15-minute average computed. Biological measurements included dry and wet biomass, leaf area, height, phonological stage, and carbon and nitrogen content in the above ground biomass. These measurements were made throughout the growing season.

Combining water vapor and carbon dioxide provides an analysis of water use efficiency with a physiological basis. Water use efficiency can also be calculated using total seasonal transpiration amounts relative to crop biomass or crop yield. Use of the short-term water vapor/carbon dioxide relationships provides insight into the changes in water loss to carbon dioxide uptake throughout a day. Throughout a day in which soil water is limiting water use efficiency begins to decrease. When these stresses are cumulative we can observe reductions in relative growth rates. In corn, when the stress occurs during the grain-filling period the reduction in grain field reduces the water use efficiency. This approach allows us to identify the interactions between microclimatic changes and management practices, e.g., nitrogen management. This use of flux measurements in conjunction with plant physiological measurements opens new insights in crop response.