Experimental test of density and energy-balance corrections on carbon dioxide flux as measured using open-path eddy covariance
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Eddy covariance is the most direct technique for measuring water, C, and energy fluxes above crops and managed ecosystems. When using open-path gas analyzers, corrections for air density fluctuations due to heat and water vapor flux must be applied, and these corrections are often larger in magnitude than the CO2 fluxes. Lack of energy balance closure, a common problem when using eddy covariance, implies that CO2 fluxes often are underestimated. Research was conducted to evaluate performance of the density corrections by making eddy covariance measurements above a large parking lot where CO2 and water vapor fluxes were almost zero. Uncorrected and corrected flux measurements were compared to the "known" values to deter-mine accuracy. Data also were collected from a tallgrass prairie and a cedar forest to examine how density corrections and adjustments for energy balance closure affected daily C balances. Raw measurements from the parking lot showed apparent, density-induced, downward CO2 fluxes (i.e., apparent photosynthesis) of approximately-0.4 mg m-2 s-1 that were correlated with sensible heat. On average, the daily uncorrected CO2 flux was-12.7 g m-2 d-1, but the density correction reduced and changed the direction of the flux to 1.8 g m -2 d-1, which was very close to independent chamber measurements of 2.8 g m-2 d-1. Density corrections in the forest and prairie changed average daily CO2 fluxes by 20 to 80%. Energy balance closure averaged 80 and 95% in the prairie and forest, respectively. Corrections based on energy balance closure changed daily C balances by 16 to 35%. A plethora of post-measurement corrections, coupled with lack of energy balance closure, signals the need for additional research before eddy covariance can be routinely applied in agronomic research.