COMPARISON OF FLUX MEASUREMENTS WITH OPEN-PATH AND CLOSED-PATH GAS ANALYZERS ABOVE AN AGRICULTURAL FIELD AND A FOREST FLOOR

Comparison was made of the flux measurements of a closed-path CO2/H2O analyzer and an open-path H2O analyzer above a clover field and the fo rest floor of a Douglas-fir stand. The attenuation of the gas concentr ation fluctuations caused by the sampling tube of the closed-path anal yzer resulted in underestimation of the H2O flux above both surfaces. The degree of underestimation above the clover field depended on wind speed, but was smaller than that calculated from the transfer function for laminar flow in a circular tube and the scalar cospectrum in the neutral and unstable surface layer. Above the forest floor CO2 fluctua tions led those of H2O by approximately 0.75 s. The implications of th is are discussed regarding the determination of the time delay caused by the sampling tube of the closed-path analyzer. The day-time CO2 eff lux from the forest floor, averaged over three days, was 0.043 mg/(m2s ).