Footprint modeling for vegetation atmosphere exchange studies: A review and perspective

The footprint of a turbulent flux measurement defines its spatial context. With the onset of long-term flux measurement sites over forests and other inherently inhomogeneous areas, and the development of the FLUXNET program, the need for flux footprint estimations has grown dramatically. This paper provides an overview of existing footprint modeling approaches in the critical light of hindsight and discusses their respective strengths and weaknesses. The second main objective of this paper is to establish a formal connection between micrometeorological measurements of scalar fluxes and their mass conservation equation, in a surface-vegetation-atmosphere volume. An important focus is to identify the limitations of the footprint concept and to point out situations where the application of footprint models may lead to erroneous conclusions, as much as to demonstrate its utility and power where warranted. Finally, a perspective on the current state-of-the-art of footprint modeling is offered, with a list of challenges and suggestions for future directions.