Visualization of The Concept
As a practical example, let's consider an instrument measuring a flux of water (evapotranspiration) few meters above the surface in the situation with no wind. In such a case, the instrument would measure evapotranspiration generated just underneath the instrument location and brought upwards by mostly non-turbulent exchange. Now lets consider situation with a strong wind: it would blow air located under the instrument away, would bring in air generated somewhere upwind and brought upwards to a large degree due to turbulent exchange. So, the water flux footprint was just under the instrument in the first case, and was somewhere upwind in the second case. In Figure 1, the general concept of the flux footprint is visualized: the darker the red color – the more contribution that is coming from the surface area certain distance away for the instrument. So, most of the contribution usually comes, not from underneath the instrument or from kilometers away, but rather from somewhere in between. Size and shape of the footprint is also a dynamic area – it changes with time.
Read more about this topic: Flux Footprint
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—José Ortega Y Gasset (1883–1955)