Validation of a simple footprint tool for trace gas flux measurements above agricultural fields

We present a user-friendly tool for footprint calculations of flux measurements in the surface layer. The calculations are based on the analytical footprint model by Kormann and Meixner (Boundary-Layer Meteorology, 2001). The footprint density function of a flux sensor is determined using readily available data from standard eddy covariance measurements. This footprint density function is integrated over defined surface areas given as quadrangular polygons representing e.g. agricultural fields. The footprint tool was implemented in an EXCEL spreadsheet in order to facilitate the data input and handling. We illustrate the use and performance of the tool by applying it to miscellaneous flux measurement situations for different trace gases in an agricultural environment at the Swiss FLUXNET site Oensingen.

(i) CO₂ flux measurements with three eddy covariance system. Two flux towers were positioned in the centre of two neighbouring fields that showed a contrasting CO₂ flux during the study period. The third tower was located near the border of the two fields and was frequently influenced by both fields to a varying degree. The calculated footprint fractions were used to reproduce the third tower flux from the first two systems.

(ii) EC measurements of a complex NH₃ emission pattern were measured after sequential slurry application on an arable and a grassland field. The emission of the entire field was calculated using the fooprint model for different sections of the field with non-simultaneous slurry application. It was then compared to an independent measurement method based on the mass balance approach.

(iii) An artificial flux generation system was built on the field to mimic a limited area source of CH₄. The grid of about 300 m² was placed within the main footprint area of an EC system including a Quantum Cascade Laser Spectrometer for CH₄. The uniform release of gas across the source area was achieved by using identical flow orifices distributed over the grid. This allowed for an independent verification of the EC flux system and the corresponding footprint correction.

All three applications indicated a good performance of the simple footprint tool. Thus it can be used as a routine quality check for flux monitoring stations influenced by distinct surface areas with differing vegetation covers, growth states, and/or land-use.