Vast networks of meteorological sensors ring the globe measuring atmospheric state variables, like temperature, humidity, wind speed, rainfall, and atmospheric carbon dioxide, on a continuous basis. These measurements serve earth system science by providing inputs into models that predict weather, climate and the cycling of carbon and water. And, they provide information that allows researchers to detect the trends in climate, greenhouse gases, and air pollution. Yet, to understand how and why atmospheric state variables may vary in time and space, biogeoscientists need to quantify the fluxes of trace gases between land and the atmosphere, e.g., the number of moles per unit area per unit time.
The eddy covariance method is currently the standard method used by biometeorologists to measure fluxes of trace gases between ecosystems and atmosphere. Fluxes are measured by computing the covariance between the vertical velocity and target scalar mixing ratios at each individual node (site). Key attributes of the eddy covariance method are its ability to measure fluxes directly, in situ, without invasive artifacts, at a spatial scale of hundreds of meters, and on time scales spanning from hours, days, years, and now, decades.
Today, eddy covariance measurements of carbon dioxide and water vapor exchange are being made routinely on all continents. The flux measurement sites are linked across a confederation of regional networks in North, Central and South America, Europe, Asia, Africa, and Australia, in a global network, called FLUXNET. This global network includes more than eight hundred active and historic flux measurement sites, dispersed across most of the world’s climate space and representative biomes (Figure 1, 2). Within this larger network, smaller meso-networks target specific land use types, such as urban areas, inland water systems, within a region. Many of these locales serve as focal points for sets of ecosystem-scale ‘manipulative’ studies. There, comparative flux measurements are being made at satellite-sites that may differ by plant functional type, biophysical attributes, biodiversity, time since disturbance (e.g., fire, logging, wind throw, flooding, or insect infestation), or management practices (e.g., fertilization, irrigation, or cultivation). Distinct scientific attributes of the flux network include its ability to detect emergent scale properties of ecosystem metabolism at local to regional and global scales and quantify temporal and spatial variability in carbon, water and energy fluxes.
FLUXNET is organized through the Regional Networks that contribute data to FLUXNET datasets available at the FLUXNET-Fluxdata webiste (https://fluxnet.fluxdata.org/), hosted at the Lawrence Berkeley National Laboratory (USA). Here the data that have been shared by the Regional Networks and processed and harmonized to share with the FLUXNET communities. Fluxdata website offers a number of tools in addition to the data access such communication and ideas sharing platforms, documentation, and support to the FLUXNET data users.
The FLUXNET portal is interconnected with the Regional Networks, making FLUXNET one of the largest ecosystem network and environmental experiment in the world. The high level of collaboration and integration of all the participants ensures the robustness and the best possible use of the resources available.
Figure 1. The spatial representativeness of the FLUXNET network (existing towers labeled as blue dots), which is mapped relative to a set of quantitative ecoregions (white-black colors). Distance in data space to the closet ecoregion containing a site quantifies how well the FLUXNET network represents each ecoregion in the map. Environments in the darker ecoregions are poorly represented by this network. (Jitendra Kumar, Forrest M. Hoffman, William W. Hargrove, in prep)
Figure 2. Distribution of FLUXNET sites across temperature and precipitation ranges (a.k.a., Whittaker’s biome classification), compared to land surface from the terrestrial globe. The length of the record of sites is represented in the circle sizes and colors (a-c). Panel (a) shows the sites included in the La Thuile 2007 Dataset; panel (b) shows the sites included in the FLUXNET2015 Dataset; panel (c) shows all sites present in FLUXNET; and, panel (d) compares the distribution of land surface, FLUXNET sites, and sites in the FLUXNET2015 Dataset across these temperature-precipitation ranges. (Pastorello et al., 2017, EOS)
Note on FLUXNET portals. Up until 2017, the FLUXNET-ORNL website, hosted at the Oak Ridge National Laboratory (USA), maintained a catalog of all the existing and past eddy covariance sites globally, giving access to historical collection such the Marconi dataset and providing useful tools such updated images, MODIS cutout, and references. The Marconi dataset, other historic FLUXNET-related data products, and the MODIS cutout tools are still available online, and a new version of the site catalog is being discussed. For more information on current and past FLUXNET datasets please see https://fluxnet.fluxdata.org/data/.