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FLUXNET Requirements

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byLukas Hörtnagl
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Overview of required data and data format for files uploaded to EFDC / FLUXNET for post-processing and subsequent data sharing.

 

Which data do we upload to FLUXNET / EFDC?

  • We upload the adjusted *full_output* file from EddyPro (Level-1 flux calculations, for an overview of the different flux levels see here: Flux Processing Chain). The EddyPro *full_output* file still has to be adjusted following the EFDC guidelines (note the required timestamp and variable names with _1_1_1, also only upload most relevant flux variables from this file, not all of them).
  • Despite its name, the EddyPro *_fluxnet_* file cannot directly be uploaded for sharing in FLUXNET.
  • In some cases, it might be necessary to remove some clearly wrong data in the uploaded file before upload, e.g. when we are sure that a sensor was clearly defective but still produced data and erroneous fluxes were calculated. In this case, file is adjusted before upload (erroneous fluxes set to -9999).
  • Important: FLUXNET needs fluxes that are not oulier-removed and not u*-filtered and not gap-filled and not partitioned because they apply their scripts to perform all of these steps! Only when data are clearly wrong it is recommended to set the data to -9999 in the data files before upload.
  • In FLUXNET / EFDC, the understand the uploaded data as Level-2 data. In our case, we can directly upload our Level-1 data directly in most cases.
    • If the Level-1 data contains erroneous fluxes that need to be removed, those fluxes are set to -9999 and the adjusted file is stored as Level-2 data on our server and also uploaded to FLUXNET.
    • Keep in mind that we upload our Level-1 data, although FLUXNET / EFDC always understands uploaded data as (their) Level-2:

Level-2 data are original data submitted by the PI and checked/filtered only for out-of-range values or clearly wrong data. The variables available are listed in the table here below and are provided as ASCII file with missing values reported as -9999 and timestamp indicating the end of the averaging period. The file produced has in the first line the variables codes.
– Source: EFDC

 

Metadata

  • the site FluxID in the form CC-SSS (two character country code, three character site identifier within country) – e.g., CH-Dav
  • the latitude and longitude for the site in the WGS 84 decimal format with at least four decimal points resolution – e.g., 42.5378 / -72.1715
  • timezone of the site (time series, if timezone changed; timestamps are all local standard time, no daylight savings) – e.g., UTC-5
  • height of the gas analyser – e.g., 30.0 m

 

File Format

  • CSV file
  • Missing values or error code: -9999
  • Timestamp in the format: YYYYMMDDhhmm (timestamp must show END of averaging interval)
  • Yearly range needed: one file for each year. First timestamp in the respective file for e.g. 2020 is 202001010030, last timestamp is 202101010000
  • Time resolution: 30min (60min also accepted)
  • Variable names need suffix, e.g. _1_1_1, _1_2_1, …

 

Flux Variables

from the EddyPro *full_output* file, renamed according to EFDC / FLUXNET variable codes

CO2

  • FC (µmolCO2 m-2 s-1): Carbon Dioxide (CO2) turbulent flux (without storage component)
  • FC_SSITC_TEST (adimensional): Quality check – CO2 flux – * see the note
  • SC (µmolCO2 m-2 s-1): Carbon Dioxide (CO2) storage flux measured with a vertical profile system, optional if tower shorter than 3 m
  • CO2 (µmolCO2 mol-1): Carbon Dioxide (CO2) mole fraction in moist air

H2O

  • LE (W m-2): latent heat turbulent flux, without storage correction
  • LE_SSITC_TEST (adimensional): Quality check – latent heat flux – * see the note
  • SLE (W m-2): Latent heat storage below flux measurement level
  • H2O (ppt: mmolH2O mol-1): Water (H2O) vapor mole fraction

H

  • H (W m-2): sensible heat turbulent flux, without storage correction
  • H_SSITC_TEST (adimensional): Quality check – sensible heat flux – * see the note
  • SH (W m-2): Heat storage in air below flux measurement level

Wind & Footprint

  • USTAR (m s-1): friction velocity
  • WD (°): wind direction
  • WS (m s-1): horizontal wind speed
  • FETCH_70 (m): Fetch at which footprint cumulated probability is 70%
  • FETCH_90 (m): Fetch at which footprint cumulated probability is 90%
  • FETCH_MAX (m): Fetch at which footprint cumulated probability is maximum

* note: for the quality flag definition please refer to this document: Quality Flags Definition

 

Meteo Variables

from meteo data files, renamed according to EFDC / FLUXNET variable codes

Mandatory

  • TA (°C): air temperature
  • RH (%): relative humidity (range 0–100%)
  • PA (kPa): atmospheric pressure
  • SW_IN (W m-2): incoming shortwave radiation

Not Mandatory

  • G (W m-2): ground heat flux, not mandatory, but needed for the energy balance closure calculations
  • NETRAD (W m-2): net radiation, not mandatory, but needed for the energy balance closure calculations
  • TS (°C): soil temperature
  • PPFD_IN (µmolPhotons m-2 s-1): incoming photosynthetic photon flux density
  • P (mm): precipitation total of each 30 or 60 minute period
  • LW_IN (W m-2): incoming (down-welling) longwave radiation
  • SWC (%): soil water content (volumetric), range 0–100%

 

Example: FLUXNET Processing Steps

The processing steps in FLUXNET are approximately like this:

  1. Level-1 fluxes are uploaded by PI
  2. Level-3 produced by FLUXNET
    1. calculation of NEE (storage correction, gives NEE, Papale et al., 2006)
    2. extension of QC flags, quality checks (spikes, u* filtering flags to filter for low-turbulence conditions, creates additional data gaps)
  3. Level-4 produced by FLUXNET
    1. application of QC flags, e.g. spike removal (Papale et al., 2006)
    2. gap-filling
    3. partitioning (using NEE: Gross Primary Production, GPP; Terrestrial Ecosystem Respiration, TER)
    4. aggregation to different time resolutions