Time-varying (dynamic) metadata

Data processing and flux calculation with EddyPro is based not only on raw data, but also on the metadata describing the site characteristics, the eddy covariance station setup and the raw data files themselves.

Metadata may vary along the dataset. For example, think about the growing canopy of an agricultural field, which may also imply an adjustment of the measurement height of the instruments. Maintenance and movement of instruments may imply a different relative positioning and orientation of the instruments. Also, instruments can be added or changed during the data collection, or parameters such as the acquisition frequency and the duration of the raw file, can change. All these possible changes must be taken into account for proper data processing and flux computation and correction. The ideal way to do that is to collect raw data in LI-COR’s .ghg format. .ghg files embed the metadata inside each raw file so, if the data logging software is updated with the current meta-information during data acquisition (and that’s by far the most suitable moment for doing that!), all raw files will embed the correct metadata, and EddyPro will be able to correctly process them without any further intervention from you.

However, if you collected raw files in a format other than .ghg, or if you didn’t update the data logging software with the proper meta-information during acquisition of your .ghg dataset, EddyPro provides you with the possibility of taking time-varying metadata into account. This will require a little effort on your part, to create a dynamic metadata file prior to using EddyPro. The dynamic metadata file is a comma separated text file featuring a header with the names of the contained variables, and a sequence of records starting with a timestamp, followed by the actual values.

Using the dynamic metadata does not imply that the “.metadata” file must not be created. Indeed, the “.metadata” file is used to initialize all the necessary metadata. On the contrary, the dynamic metadata file needs to contain only the parameters that undergo at least one modification along the dataset. Table 1 lists all metadata that can be provided in a dynamic metadata file, including the standardized variables names—indispensable for correctly interpreting the file content – and a short description of the variable meaning.

The associations between the data in the metadata file and the corresponding flux averaging periods is based on matching time stamps. The timestamp must be provided as date and time. EddyPro will attempt to match all available timestamp information. If, for the current flux averaging period, there is no matching dynamic metadata contained in the file, EddyPro will use the closest preceding one. This means that you don’t need to create a file that contains metadata for all flux averaging periods. Basically, you need to create a new metadata line each time a modification occurs for any metadata parameter.

As an example, imagine a dataset that starts on the 1^st of January, with the CO₂ analyzer placed 20 cm at the North of the anemometer. On the 15^th of February, at 10:30AM, the analyzer is moved and placed 30 cm eastward of the anemometer. The acquisition ends at the end of February and this was the only notable modification that occurred during the two months. Valid metadata in this situation would read:

where the first line is the header containing standardized labels (see the following table). The second line indicates that, starting January 1^st, 2011 at 1:00AM, the CO₂ analyzer is placed 20 cm northward of the master anemometer, and 0 cm eastward. These settings will bypass any different information provided in the alternative, “master” (and static) metadata file.

The third line says that, starting February 15^th, 2011 at 10:30AM, the CO₂ analyzer was moved, and is now aligned with the master anemometer in the South-North direction, but displaced 30 cm eastward.

Variable	Label	Units/Format/ Values	Description and notes
Date	date	yyyy-mm-dd	Year, month, and day of the current data.
Time	time	HH:MM	Hour and minute of the current data. The minute must correspond to the end of the period the metadata refer to.
File length	file_length	minutes	The duration in minutes of the raw files.
Acquisition frequency	acquisition_frequency	Hz	Frequency of acquisition of raw data from gas analyzer(s) and anemometer(s)
Canopy height	canopy_height	m	Height of the canopy
Displacement height	displacement_height	m	Displacement height
Roughness length	roughness_length	m	Roughness length
Manufacturer of the master anemometer	master_sonic_manufacturer	variable†	See list of supported manufacturers.
Model of the master anemometer	master_sonic_model	variable‡	See list of supported models and how to compose the name.
Height of the master anemometer	master_sonic_height	m	This height also defines the "measurement height" used in calculations.
Format of wind data	master_sonic_wformat	'uvw' 'polar_w' 'axis'	The 'axis' option (wind components aligned with paths of the transducers pairs) applies only to Gill anemometers.
North alignment for wind data	master_sonic_wref	'spar''axis'	Only applies to Gill anemometers (learn more...).
North misalignment for wind data	master_sonic_north_offset	deg from North	Any misalignment between "anemometer North" and geographic North, in degrees from North (positive if measured eastward).
Manufacturer of gas analyzer for XXX	xxx_irga_manufacturer	variable*	See list of supported manufacturers.
Model of gas analyzer for XXX	xxx_irga_model	variable**	See list of supported manufacturers.
Northward separation of XXX analyzer from master anemometer	xxx_irga_northward_separation	cm	Positive if gas analyzer or sample inlet is placed North of anemometer.
Eastward separation of XXX analyzer from master anemometer	xxx_irga_eastward_separation	cm	Positive if gas analyzer or sample inlet is placed East of the master anemometer.
Vertical separation of XXX analyzer from master anemometer	xxx_irga_vertical_separation	cm	Positive if gas analyzer or sample inlet is placed higher than anemometer
Length of the tube of the sampling line for XXX analyzer	xxx_irga_tube_length	cm	Only applicable to closed/enclosed path gas analyzers.
Diameter of the tube of the sampling line for XXX analyzer	xxx_irga_tube_diameter	mm	Only applicable to closed/enclosed path gas analyzers.
Flow rate in the sampling line of XXX analyzer	xxx_irga_flowrate	lit min-1	Only applicable to closed/enclosed path gas analyzers.
Extinction coefficient in water	xxx_irga_kw	m3 g-1 cm-1	Only applicable to Krypton/Lyman-α hygrometers./
Extinction coefficient in oxygen	xxx_irga_ko	m3 g-1 cm-1	Only applicable to Krypton/Lyman-α hygrometers.
XXX analyzer longitudinal path length	xxx_irga_vpath_length	m	Path length of the gas analyzer in the main direction (direction between source and sensor).
XXX analyzer transversal path length	xxx_irga_hpath_length	m	Path length of the gas analyzer in the cross direction.
XXX analyzer time response	xxx_irga_tau	m	Analyzer's time response as provided by the manufacturer.

†Supported Anemometer Manufacturers:

Manufacturer	Label
Gill Instruments, Ltd.	gill
Campbell Scientific®, Inc.	csi
Metek GmbH	metek
R. M. Young	young
Others	other_sonic

‡Supported Anemometer Models:

Model	Label
Gill R2	r2
Gill R3-50	r3_50
Gill R3-100	r3_100
Gill R3A-100	r3a_100
Gill HS-50	hs_50
Gill HS-100	hs_100
Gill WindMaster®	wm
Gill WindMaster Pro	wmpro
Metek USA-1	usa1_standard
Metek USA-1 fast	usa1_fast
Campbell® Scientific CSAT3	csat3
Campbell® Scientific CSAT3B	csat3b
R. M. Young	81000
R. M. Young	81000V
R. M. Young	81000RE
R. M. Young	81000VRE
Others	generic_sonic

*Supported Gas Analyzer Manufacturers:

Manufacturer	Label
LI-COR, Inc.	licor
others	other

*Supported Gas Analyzer Models:

Model	Label
LI-COR LI-6262	li6262
LI-COR LI-7000	li7000
LI-COR LI-7500	li7500
LI-COR LI-7500A	li7500a
LI-COR LI-7500RS	li7500rs
LI-COR LI-7500DS	li7500ds
LI-COR LI-7200RS	li7200rs
LI-COR LI-7200	li7200
LI-COR LI-7700	li7700
Generic Open Path	generic_open_path
Generic Closed Path	generic_closed_path
Open Path Krypton Hygrometer	open_path_krypton
Close Path Krypton Hygrometer	closed_path_krypton

The model label must be created by adding an “instrument order number” to the labels listed above. Imagine creating two lists of instruments, one for anemometers and one for gas analyzers. The first instrument of the list that you describe takes the number 1, and the label becomes “instrumentlabel_1”. The second takes number 2, and so on. Note that in the dynamic metadata file (different from the alternative metadata file) you can only describe 1 anemometer. This feature is needed to provide maximum flexibility. As an example, think about the following suite of measurements and corresponding instruments:

• Wind and temperature from a Gill R3A-100
• CO₂ concentration from an LI-7500RS
• H₂O concentration from a different LI-7500RS
• CH₄ concentration from an LI-7700

In this case, you can provide dynamic metadata for 4 different instruments, and the instrument model label will take the following values:

• master_sonic_model = r3a_100_1
• co2_irga _model = li7500_1
• h2o_irga_model = li7500_2
• ch4_irga_model = li7700_3

with the advantage the instruments for CO₂ and H₂O, although of the same model, are differentiated by EddyPro thanks to the attached number.