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Research Project: Understanding Water-Driven Ecohydrologic and Erosion Processes in the Semiarid Southwest to Improve Watershed Management

Location: Southwest Watershed Research Center

Title: Representativeness of Eddy-Covariance flux footprints for areas surrounding AmeriFlux sites

item CHU, H. - Lawrence Berkeley National Laboratory
item LUO, X. - Lawrence Berkeley National Laboratory
item OUYANG, Z. - Stanford University
item CHAN, W.S. - Lawrence Berkeley National Laboratory
item DENGEL, S. - Lawrence Berkeley National Laboratory
item BIRAUD, S.C. - Lawrence Berkeley National Laboratory
item TORN, M.S. - Lawrence Berkeley National Laboratory
item METZGER, S. - Neon, Inc
item KUMAR, J. - Oak Ridge National Laboratory
item ARAIN, M.A. - McMaster University
item ARKEBAUER, T.J. - University Of Nebraska
item BALDOCCHI, D. - University Of California
item BERNACCHI, D. - University Of Nebraska
item BLACK, T.A. - University Of British Columbia
item BLANKEN, P.D. - University Of Colorado
item BOHRER, G. - The Ohio State University
item BRACHO, R. - University Of Florida
item BROWN, S. - University Of Guelph
item BRUNSELL, N.A. - University Of Kansas
item CHEN, J. - Michigan State University
item CHEN, X. - Pacific Northwest National Laboratory
item CLARK, K. - Us Forest Service (FS)
item DESAI, A.R. - University Of Wisconsin
item DUMAN, T. - University Of New Mexico
item DURDEN, T. - Neon, Inc
item FARES, S. - National Research Council - Italy
item FORBRICH, I. - Woods Hole Marine Biological Laboratory
item GAMON, J.A. - University Of Alberta
item GRIFFIS, T. - University Of Minnesota
item HELBIG, M. - Dalhousie University
item HOLLINGER, D. - Us Forest Service (FS)
item HUMPHREYS, E. - Carleton University - Canada
item IKAWA, H. - National Agriculture And Food Research Organization (NARO), Agricultrual Research Center
item IWATA, H. - Shinshu University
item JU, Y. - The Ohio State University
item Knowles, John
item KNOX, S. - University Of British Columbia
item KOBAYASHI, H. - Japan Agency For Marine-Earth Science And Technology (JAMSTEC)
item KOLB, T. - Northern Arizona University
item LAW, B. - Oregon State University
item LEE, X. - Yale University
item LITVAK, M. - University Of New Mexico
item LIU, H. - Washington State University
item MUNGER, J.W. - Harvard University
item NOORMETS, A. - Texas A&M University
item NOVICK, K. - Indiana University
item OBERBAUER, S.F. - Florida International University
item OECHEL, W. - San Diego State University
item OIKAWA, P. - California State University
item PAPUGA, S.A. - Wayne State University
item PENDALL, E. - University Of Western Australia
item PRAJAPATI, P. - Texas A&M University
item Prueger, John
item QUINTON, W.L. - Wilfrid Laurier University
item RICHARDSON, A.D. - Northern Arizona University
item RUSSELL, E.S. - Washington State University
item Scott, Russell - Russ
item STARR, G. - University Of Alabama
item STAEBLER, R. - Environment And Climate Change Canada
item STOY, P. - University Of Wisconsin
item STUART-HAËNTJENS, E. - Us Geological Survey (USGS)
item SONNENTAG, O. - Universite De Montreal
item SULLIVAN, R.C. - Argonne National Laboratory
item SUYKER, A. - University Of Nebraska
item UEYAMA, M. - Osaka Prefecture University
item VARGAS - University Of Delaware
item WOOD, J.D. - University Of Missouri
item ZONA, D. - San Diego State University

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/31/2021
Publication Date: 2/14/2021
Citation: Chu, H., Luo, X., Ouyang, Z., Chan, W., Dengel, S., Biraud, S., Torn, M., Metzger, S., Kumar, J., Arain, M., Arkebauer, T., Baldocchi, D., Bernacchi, D., Black, T., Blanken, P., Bohrer, G., Bracho, R., Brown, S., Brunsell, N., Chen, J., Chen, X., Clark, K., Desai, A., Duman, T., Durden, T., Fares, S., Forbrich, I., Gamon, J., Griffis, T., Helbig, M., Hollinger, D., Humphreys, E., Ikawa, H., Iwata, H., Ju, Y., Knowles, J.F., Knox, S., Kobayashi, H., Kolb, T., Law, B., Lee, X., Litvak, M., Liu, H., Munger, J., Noormets, A., Novick, K., Oberbauer, S., Oechel, W., Oikawa, P., Papuga, S., Pendall, E., Prajapati, P., Prueger, J.H., Quinton, W., Richardson, A., Russell, E., Scott, R.L., Starr, G., Staebler, R., Stoy, P., Stuart-Haëntjens, E., Sonnentag, O., Sullivan, R., Suyker, A., Ueyama, M., Vargas, Wood, J., Zona, D. 2021. Representativeness of Eddy-Covariance flux footprints for areas surrounding AmeriFlux sites. Agricultural and Forest Meteorology. 301-302, Article 108350.

Interpretive Summary: Tower-based measurements are the primary tool with which to benchmark satellite- or model-based fluxes of energy, water, and/or nutrient exchange between Earth’s surface and the atmosphere. These fluxes encompass the processes of photosynthesis, respiration, and evaporation that are critically important to water availability, ecosystem health, and greenhouse gas absorption and emission. However, the value of these benchmarking comparisons depends on the degree to which flux measurements from a single tower location represent the aggregated flux conditions over an area that is represented by a 106 – 108 m2 satellite- or model-based grid cell. As a result, the current study quantified how land cover and vegetation characteristics contribute to heterogeneity within the source area that contributes to measured tower-based fluxes (known as the flux “footprint”; typically ranges from 103 – 107 m2) at 214 sites across 13 vegetation classes. This analysis indicates that footprint heterogeneity over space and time can introduce a bias of between 4% and 20% into comparisons between tower and satellite- or model-derived data. Accordingly, we propose a set of representativeness indices to identify sites and/or time periods that may be suitable for specific applications. These indices are designed to promote “footprint awareness”, in order to guide effective model and data benchmarking.

Technical Abstract: Large datasets of greenhouse gas and energy surface-atmosphere fluxes measured with the eddy- covariance technique (e.g., FLUXNET2015, AmeriFlux BASE) are widely used to benchmark models and remote-sensing products. This study addresses one of the major challenges facing model-data integration: To what spatial extent do flux measurements taken at individual eddy-covariance sites reflect model- or satellite-based grid cells? We evaluated flux footprints—the temporally dynamic source areas that contribute to measured fluxes—and the representativeness of these footprints for target areas (e.g., within 250–3000 m radii around flux towers) that are often used in flux-data synthesis and modeling studies. We examine the land-cover composition and vegetation characteristics, represented here by the Enhanced Vegetation Index (EVI), in the flux footprints and target areas across 214 AmeriFlux sites, and evaluate potential biases as a consequence of the footprint-to-target-area mismatch. The flux footprints varied across sites and through time depending on the measurement heights, underlying vegetation- and ground-surface characteristics, and turbulent state of the atmosphere. Monthly 80% footprint climatologies ranged four orders of magnitude from 103–107 m2. Few eddy-covariance sites are located in a truly homogeneous landscape. Thus, the common model-data integration approaches that use a fixed-extent target area across sites introduce biases on the order of 4%–20% for EVI and 6%–20% for the dominant land cover percentage. And these biases were site-specific functions of measurement heights, target area extents, and land-surface characteristics. We advocate that flux datasets need to be used with footprint awareness, especially in research and applications that benchmark models and data products with explicit spatial information. We propose a simple representativeness index based on our evaluations that can be used as a guide to identify site-years suitable for specific applications and to provide general guidance for data use.