Metadata and Data units

Victor Granda (Sapfluxnet Team)



Metadata numeric variables come in different units depending on the variable. One easy way to check for a variable units is to use the describe_md_variable function:

#> Description:
#> Elevation above sea level
#> Values:
#> Contributor defined | 
#> Units:
#> meters
#> Type:
#> Numeric
#> Description:
#> Mean stand age at the moment of sap flow measurements
#> Values:
#> Contributor defined | 
#> Units:
#> years
#> Type:
#> Numeric

As it can be seen, site elevation is in meters and stand age is in years.

Nevertheless, and for the sake of completion, here you can find a complete list of the metadata variables and their units:

variable description type units
1 si_name Site name given by contributors Character None
2 si_country Country code (ISO) Character Fixed values
3 si_contact_firstname Contributor first name Character None
4 si_contact_lastname Contributor last name Character None
5 si_contact_email Contributor email Character None
6 si_contact_institution Contributor affiliation Character None
7 si_addcontr_firstname Additional contributor first name Character None
8 si_addcontr_lastname Additional contributor last name Character None
9 si_addcontr_email Additional contributor email Character None
10 si_addcontr_institution Additional contributor affiliation Character None
11 si_lat Site latitude (i.e. 42.36) Numeric Latitude, decimal format (WGS84)
12 si_long Site longitude (i.e. -8.23) Numeric Longitude, decimal format (WGS84)
13 si_elev Elevation above sea level Numeric meters
14 si_paper Paper with relevant information to understand the site as DOI links or DOI codes Character DOI link
15 si_dist_mgmt Recent and historic disturbance and management events that affected the measurement years Character Fixed values
16 si_igbp Vegetation type based on IGBP classification Character Fixed values
17 si_flux_network Logical indicating if site is participating in the FLUXNET network Logical Fixed values
18 si_dendro_network Logical indicating if site is participating in the DENDROGLOBAL network Logical Fixed values
19 si_remarks Remarks and commentaries useful to grasp some site-specific peculiarities Character None
20 si_code sapfluxnet site code, unique for each site Character Fixed value
21 si_mat Site annual mean temperature, as obtained from WorldClim Numeric Celsius degrees
22 si_map Site annual mean precipitation, as obtained from WorldClim Numeric mm
23 si_biome Biome classification as per Whittaker diagram, based on mat and map obtained from WorldClim Character sapfluxnet calculated
24 st_name Stand name given by contributors Character None
25 st_growth_condition Growth condition with respect to stand origin and management Character Fixed values
26 st_treatment Treatment applied at stand level Character None
27 st_age Mean stand age at the moment of sap flow measurements Numeric years
28 st_height Canopy height Numeric meters
29 st_density Total stem density for stand Numeric stems/ha
30 st_basal_area Total stand basal area Numeric m2/ha
31 st_lai Total maximum stand leaf area (one-sided, projected) Numeric m2/m2
32 st_aspect Aspect the stand is facing (exposure) Character Fixed values
33 st_terrain Slope and/or relief of the stand Character Fixed values
34 st_soil_depth Soil total depth Numeric cm
35 st_soil_texture Soil texture class, based on simplified USDA classification Character Fixed values
36 st_sand_perc Soil sand content, % mass Numeric % percentage
37 st_silt_perc Soil silt content, % mass Numeric % percentage
38 st_clay_perc Soil clay content, % mass Numeric % percentage
39 st_remarks Remarks and commentaries useful to grasp some stand-specific peculiarities Character None
40 st_USDA_soil_texture USDA soil classification based on the percentages provided by the contributor Character sapfluxnet calculated
41 sp_name Identity of each measured species Character Scientific name without author abbreviation, as accepted by The Plant List
42 sp_ntrees Number of trees measured of each species Numeric number of trees
43 sp_leaf_habit Leaf habit of the measured species Character Fixed values
44 sp_basal_area_perc Basal area occupied by each measured species, in percentage over total stand basal area Numeric % percentage
45 pl_name Plant code assigned by contributors Character None
46 pl_species Species identity of the measured plant Character Scientific name without author abbreviation, as accepted by The Plant List
47 pl_treatment Experimental treatment (if any) Character None
48 pl_dbh Diameter at breast height of measured plants Numeric cm
49 pl_height Height of measured plants Numeric m
50 pl_age Plant age at the moment of measure Numeric years
51 pl_social Plant social status Character Fixed values
52 pl_sapw_area Cross-sectional sapwood area Numeric cm2
53 pl_sapw_depth Sapwood depth, measured at breast height Numeric cm
54 pl_bark_thick Plant bark thickness Numeric mm
55 pl_leaf_area Leaf area of eachvvmeasured plant Numeric m2
56 pl_sens_meth Sap flow measures method Character Fixed values
57 pl_sens_man Sap flow measures sensor manufacturer Character Fixed values
58 pl_sens_cor_grad Correction for natural temperature gradients method Character Fixed values
59 pl_sens_cor_zero Zero flow determination method Character Fixed values
60 pl_sens_calib Was species-specific calibration used? Logical Fixed values
61 pl_sap_units Uniformized sapfluxnet units for sapwood, leaf and plant level Character Fixed values
62 pl_sap_units_orig Original contribution units (at sapwood or plant level) Character Fixed values
63 pl_sens_length Length of the needles or electrodes forming the sensor Numeric mm
64 pl_sens_hgt Sensor installation height, measured from the ground Numeric m
65 pl_sens_timestep Subdaily time step of sensor measures Numeric minutes
66 pl_radial_int Character Fixed values
67 pl_azimut_int Character Fixed values
68 pl_remarks Remarks and commentaries useful to grasp some plant-specific peculiarities Character None
69 pl_code sapfluxnet plant code, unique for each plant Character Fixed value
70 env_time_zone Time zone of site used in the TIMESTAMPS Character Fixed values
71 env_time_daylight Is daylight saving time applied to the original timestamp? Logical Fixed values
72 env_timestep Subdaily timestep of environmental measures Numeric minutes
73 env_ta Location of air temperature sensor Character Fixed values
74 env_rh Location of relative humidity sensor Character Fixed values
75 env_vpd Location of relative vapour pressure decifit sensor Character Fixed values
76 env_sw_in Location of shortwave incoming radiation sensor Character Fixed values
77 env_ppfd_in Location of incoming photosynthetic photon flux density sensor Character Fixed values
78 env_netrad Location of net radiation sensor Character Fixed values
79 env_ws Location of wind speed sensor Character Fixed values
80 env_precip Location of precipitation sensor Character Fixed values
81 env_swc_shallow_depth Average depth for shallow soil water content measures Numeric cm
82 env_swc_deep_depth Average depth for deep soil water content measures Numeric cm
83 env_plant_watpot Availability of water potential values for the same measured plants during the sap flow measurements period Character Fixed values
84 env_leafarea_seasonal Availability of seasonal course leaf area data and level Character Fixed values
85 env_remarks Remarks and commentaries useful to grasp some environmental-specific peculiarities Character None


Sap flow units

Sap flow units depends on the database level chosen:

\[Plant:~\frac{cm³}{h}\] \[Sapwood:~\frac{cm³}{cm²_{sa}·h}\] \[Leaf:~\frac{cm³}{cm²_{la}·h}\]

Environmental variables

Environmental variables come in the following units:

Variable Description Units
1 env_ta Air temperature ºC
2 env_rh Air relative humidity %
3 env_vpd Vapour pressure deficit kPa
4 env_sw_in Shortwave incoming radiation W m-2
5 env_ppfd Incoming photosynthetic photon flux density micromols m-2 s-1
6 env_netrad Net radiation W m-2
7 env_ws Wind speed m s-1
8 env_precip Precipitation mm timestep-1
9 env_swc_shallow Shallow soil water content cm3 cm-3
10 env_swc_deep Deep soil water content cm3 cm-3



TIMESTAMP variable comes as provided by the contributors and timezone is applied per the site metadata description:

#> Loading required package: timechange
#> Attaching package: 'lubridate'
#> The following objects are masked from 'package:base':
#>     date, intersect, setdiff, union

# timezone provided by contributor
get_env_md(ARG_TRE) %>% pull(env_time_zone)
#> [1] "13UTC-03:00, P"

# timezone in the TIMESTAMP
get_timestamp(ARG_TRE) %>% tz()
#> [1] "Etc/GMT+3"

As you can see, the final timezone is in GMT format.


When inspecting the solar TIMESTAMP variable, timezone declared is UTC:

get_solar_timestamp(ARG_TRE) %>% tz()
#> [1] "UTC"

This is so because solar timestamp is only referred to the sun position in the sky, rendering the timezone useless in this case. So UTC was chosen as timezone to be able to join and compare global data.