Impact of early termination of vegetation around paired eddy covariance towers in a divided winter wheat field

Eddy covariance-based gas fluxes are dependent on the vegetation conditions under the tower. An assumption when used in agricultural fields is that the field is effectively homogeneous so as the footprint changes with crop and measurement heights and the fluxes measured by the tower are representative of the entire field. Flux towers in two neighboring fields managed as one field had an inadvertent experiment occur in 2019 when the grower-cooperator chemically terminated approximately 8.1 hectares due to issues with Jointed Goatgrass in both fields. In the north field, the terminated area (approx. 4.05 hectares) encompassed the tower and aligned with the prevailing winds and scale of the flux footprint. The sprayed-out area was less than 10% of the total field area so a minority of the field had a disproportionate impact on the assumed field-scale measurements. In the southern field, the terminated area was a skinny but elongated patch of the field (approx. 3 hectares or 11% of the field) off-axis of the flux footprint leading to a relatively minimal impact on the flux measurements. Being in the middle of the growing season, this caused the latent heat and carbon fluxes to drop in the northern field, as the crop and weeds were no longer active around the tower though per visual observation, the rest of the field was green. Using the southern tower as a comparison, the underestimation of carbon and water fluxes will be estimated by comparing the pre- and post-termination flux values. Results from this inadvertent experiment will underline the importance of knowledge of the flux footprint and evolution of site vegetation characteristics over the course of a growing season as being vital to understanding the carbon and water fluxes in agricultural fields.