Spatial and temporal variation of water, energy and carbon fluxes from tower and aircraft measurements

Tower-based eddy covariance measurements of carbon dioxide (CO2) latent heat (LE) and sensible heat (H) were made during the Soil Moisture Experiment 2002 (SMEX02) over production scale corn and soybean fields in central Iowa in the summer of 2002. Ancillary energy balance components (net radiation and soil heat flux) measurements were also made at the same locations as the eddy covariance.

The 12 locations (6 corn and 6 soybean fields) represent a typical Midwest corn and soybean production watershed over. Pre and post study inter-comparison measurements of all 12-eddy covariance systems over uniform alfalfa and grass surfaces resulted in a coefficient of variation (CV) for H, latent heat LE and CO2 of approximately 0.1.

Large field scale variation of turbulent flux components was evident for both corn and soybean sites during the early phase of vegetation development. Sources of variation were attributed to type and stage of vegetation development in the early part of the season (time of planting and biomass accumulation) and variable soil water content resulting from soil differences and variable precipitation events across the watershed. Spatial variation decreased as the growing season progressed but still remained evident throughout the measurement period. This was attributed mainly to soil water content differences.

Cumulative totals of CO2 uptake and water use were found to vary significantly when comparing corn to corn and soybean to soybean fields. An important result is that the fact that variation of LE and CO2 fluxes exist across corn and soybean fields despite the relative homogeneous appearance of the surfaces.