Linkages Between CO2 and H2O Fluxes over Corn and Soybean Canopies

Measurements of CO2 and H2O vapor exchanges are becoming routine observations over many surfaces. These data are generally used to estimate the fluxes of two critical gases as part of the energy exchanges between a vegetated surface and lowest boundary-layer of the atmosphere. The value in these types of observations extends beyond that contained in the flux measurements to assessing the value of these parameters in defining canopy resistance for CO2 and H2O exchanges and the dynamics of canopy level responses throughout a growing season. We have been evaluating the relationship between CO2 and H2O fluxes over the same field of a corn and soybean rotation in central Iowa that is representative of the Upper Midwest Corn Belt region. Continuous long-term measurements of the CO2 and energy balance components have been conducted since 2001. The field is divided in half so that we obtain data over each crop every year. A complete energy balance station is located in each field. Turbulent fluxes of heat, water vapor and CO2 are measured with eddy covariance at 10 Hz. The remaining available energy and various micrometeorological ancillary measurements are sampled at 0.1 Hz. Preliminary data are retrieved from the data acquisition systems via cell phone at night. The data are then processed into various time intervals after a rigorous screening and quality control procedures. These data analyzed to evaluate diurnal relationships between CO2 and H2O vapor fluxes over both canopy surfaces. The diurnal relationship between CO2 and H2O vapor fluxes responds to the physiological state of the plant and local micro-meteorological conditions. There is an asymmetric relationship between CO2 and H2O vapor fluxes within a given day in which significant patterns are observed during a dry down cycle of a crop. These patterns are unique for corn versus soybeans crop and are related to physiology of the crop. Use of these relationships can promote increased understanding in energy balance studies that can explain vegetative response to environmental stresses.