Inter-comparison of Open and Closed-Path Eddy Covariance Fluxes Over an Irrigated Alfalfa Field in Davis, CA

The eddy covariance technique is widely used to measure gas exchange, sensible heat fluxes, latent heat fluxes and momentum fluxes integrated over a unit area. This technique requires fast measurements of both the scaler and the wind components. Typically, the measurement system consists of an ultrasonic anemometer and either an open or closed-path gas analyzer. Open-path flux measurements have proven to be challenging because the density corrections associated with temperature and water vapor are relatively large and difficult to measure. Conversely, closed-path systems tend to attenuate fast changes in temperature, water vapor, and to some extent CO₂, which leads to underestimating fluxes. In an attempt to account for these shortcomings, two new types of gas analyzers have been developed in recent years: an open-path with a co-located sonic anemometer and a closed path with a short intake tube and small sample cell. We evaluated the performance of these systems in a field experiment over a wide range of flux regimes and crop growing stage. Measurements were analyzed to investigate the spectral attenuation of the closed path analyzer as a function of relative humidity. Furthermore, the measurement of the density terms in the open-path system were examined and a comparison of CO₂ and H₂O fluxes between the two systems is presented.