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Experimental Test of Density and Energy‐Balance Corrections on Carbon Dioxide Flux as Measured Using Open‐Path Eddy Covariance
Author(s) -
Ham J. M.,
Heilman J. L.
Publication year - 2003
Publication title -
agronomy journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.752
H-Index - 131
eISSN - 1435-0645
pISSN - 0002-1962
DOI - 10.2134/agronj2003.1393
Subject(s) - eddy covariance , flux (metallurgy) , sensible heat , energy balance , atmospheric sciences , environmental science , latent heat , water vapor , carbon dioxide , covariance , physics , chemistry , meteorology , mathematics , ecosystem , statistics , thermodynamics , ecology , organic chemistry , biology
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 CO 2 fluxes. Lack of energy balance closure, a common problem when using eddy covariance, implies that CO 2 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 CO 2 and water vapor fluxes were almost zero. Uncorrected and corrected flux measurements were compared to the “known” values to determine 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 CO 2 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 CO 2 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 CO 2 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.