FLUXNET DATA SYSTEM – Required and optional variables – An Overview
FLUXNET Data System submissions include both continuous data and ancillary metadata. The continuous data include computed fluxes, gas concentrations, and meteorological variables, while the ancillary data includes information on the instruments, measurement locations, and vegetation and management characteristics.
This spreadsheet lists all the Mandatory, Strongly Suggested, and Possible variables for continuous and ancillary data. They are also summarized in more detail below. (PDF of continuous data requirements available here, and PDF of ancillary data requirements available here)
A core set of “Mandatory” variables must be submitted in order for data to be processed by ONEFlux – the code that was used to produce, for example, the FLUXNET2015 data product, the AmeriFlux-FLUXNET data product and the ICOS Level2 products. There is also a set of “Strongly Suggested” variables which are available at most sites, and which we highly encourage PIs to consider including in their submissions because doing so will lead to better products and more involvement in synthesis papers. Finally, for the continuous data, there is also a set of “Possible” variables – for example, methane fluxes.
Despite the obvious differences among flux towers networks worldwide, PIs are requested to follow a set of standard requirements for data uploads. First, data must be in .csv files using a standard set of variable names. It is very important that the column headers adopt the standard naming convention, that the variables are presented using the standardized units, and that the timestamp adopts the standard format. The formats vary across regional networks. Towers located in North and South America should consult the guidance from AmeriFlux (see here). Towers located in Australia and New Zealand should contact Peter Isaac ([email protected]) or Cacilia Ewenz ([email protected]). Towers located in Europe and in other countries outside of the domains of AmeriFlux and OzFlux should follow the template available here.
For the ancillary data, standard templates exist and can be accessed from the regional networks. To access the AmeriFlux template, click here. To access the template for sites in Europe and places outside the domains of AmeriFlux and OzFLux, click here. If your tower is in Australia or New Zealand, please contact Peter Isaac ([email protected]) or Cacilia Ewenz ([email protected]) for details on ancillary data requirements.
Note, if a tower team has already submitted data to a regional network or FLUXNET for ONEFlux processing, including the FLUXNET CH4 Version 2.0 activity, and you have been notified that the ONEFlux processing was successful, then no further action is required for inclusion in the FLUXNET Data System. However, PIs may want to review the list of suggested and possible variables to see if any additional data could be contributed from their site. In this case, we encourage resubmission of the entire dataset to the appropriate networks.
Continuous Data
Mandatory Data: The minimum, mandatory set of variables required for inclusion in the FLUXNET data system is similar to what has been required in the past for the ONEFLUX processing. This includes computed carbon (FC), water (LE), and sensible heat (H) fluxes, information on CO2 concentration, radiation, humidity, and air temperature, and a minimum set of information necessary to characterize turbulence (e.g. friction velocity, or USTAR).
In some cases, PIs may select from within a set of “alternative” variables that describe the same (mandatory) data feature. For example, the CO2 storage is a variable needed in the processing. If the site is equipped with a vertical CO2 profile then the calculated storage flux is the best choice. In case the profile is not available (e.g. in case of short towers) then the CO2 concentration measured at the top of the tower is needed, either as the mole fraction in wet air or the mole fraction in dry air. In this case, the storage should not be calculated and submitted (possible only if measured with a profile system). Likewise, PIs may choose to submit either relative humidity or vapor pressure deficit, and they may choose to submit either photosynthetically active radiation (PAR) or incident shortwave solar radiation. When alternative choices exist, they are indicated in the spreadsheet using a number system that begins with the letter A. For example, relative humidity and vapor pressure deficit are both labeled “A1” in the spreadsheet, which means submission of one or the other is required. Even if only one of a set of “alternative” variables is required, PIs may certainly choose to submit more than one from that set. Or in other words, it is fine and encouraged to submit both relative humidity and vapor pressure deficit.
Strongly Suggested Variables: The list of strongly suggested variables are not mandatory for ONEFlux processing, but they add substantial value to tower datasets and would likely enhance the end-use of the FLUXNET data system, help to reduce uncertainty and increase the possibilities to be involved in synthesis papers. They include variables like water vapor concentration, soil heat flux, outgoing shortwave and incident and outgoing longwave radiation, storage fluxes of sensible and latent heat flux, precipitation and soil moisture data, information describing the flux footprint, wind characteristics necessary for footprint calculation, and information on the raw data quality control. Even if not strictly required for the ONEFlux processing, PIs that have these data available are encouraged to submit them.
Possible Variables: These variables represent information that, if available, adds substantial information value and increases the usefulness of the FLUXNET Data System. These include information on methane and nitrous oxide fluxes and related gas concentrations, additional information about raw data quality control, additional water cycle and below canopy measurements, and additional spectral and radiation measurements. Note that towers that support methane measurements have a concurrent opportunity to participate in the FLUXNET-CH4 v2 data release. If you have already shared data with that initiative, your data will be automatically incorporated into the FLUXNET Data System. Likewise, if you share methane flux data through the procedures described here, your data will automatically be incorporated into that release.
Ancillary Data
Mandatory data: The required ancillary data includes critical information about each tower’s PI team, instrument models, ecosystem classification, canopy height, the measurement height of the eddy covariance instruments, and the height or depth of several other variables (for example, all the soil measurements).
Strongly Suggested data: These variables include leaf area index measurements, information on the species present at each site, management and disturbance activity, and stand age (for forests only).