Experimental validation of footprint models for eddy covariance CO2 flux measurements above grassland by means of natural and artificial tracers

- Validation of footprint models via tracer experiments has been done.
- Best agreement was found between Lagrangian models and experimental data.
- Roughness inclusion improved the agreement between models and experimental data.
- Roughness improved agreement more than atmospheric stability.
- Limited footprint extension found in neutral and unstable stratifications.

Footprint models, which simulate source area for scalar fluxes, are fundamental tools for a correct interpretation of micromoeteorological flux measurements and ecosystem exchange inferred from such data. Over the last two decades models of varying complexity have been developed, but all of them suffer from a significant lack of experimental validation. In this study two different experimental tests have been conducted with the aim of offering validation: a manipulation of the vegetation cover and an artificial tracer emission. In the first case the extension of the flux source has been changed progressively by successive cuts of vegetation, while in the second case by varying the distance of a tracer emission line respect to the measurement point. Results have been used to validate two analytical and a numerical footprint models. The experimental data show a good agreement with footprint models and indicate a limited extension of the flux source area, with approximately 75% of the sources confined within a range of 10-20 times the effective measurement height, i.e. the measurement height above the zero plane displacement. Another interesting result was the strong dependence on the surface roughness of both experimental estimates and numerical simulations of footprint. The effect of surface roughness on experimental results and models outputs was comparable to the effect of atmospheric stability. This indicates that surface roughness and turbulence conditions may play a significant role in source area location, in particular above inhomogeneous surfaces with change in roughness, as in the case of the manipulation experiment. Consequently a careful site specific quantification of these parameters seems to be fundamental to obtain realistic footprint estimates and significantly improve eddy covariance flux interpretation at complex sites.