Practical example: from raw gas
concentration data to clean ecosystem gas fluxes.
In this example we will process a dataset from the Plant Functional
Traits Course 6 (PFTC6; Norway, 2022). Net ecosystem exchange (NEE),
ecosystem respiration (ER), air and soil temperature and
photosynthetically active radiation (PAR) were recorded over the course
of 24 hours at an experimental site called Liahovden, situated in an
alpine grassland in southwestern Norway. Those data are available in the
Fluxible R package.
To work with your own data, you
need to import them as a dataframe object in your R session. For import
examples, please see
vignette("data-prep", package = "fluxible").
Model fitting
We fit a model and obtain the slope at \(t_0\), which is needed for the flux
calculation, with the flux_fitting function.
In this example we use the exp_zhao18 model (Zhao et al., 2018). The
exp_zhao18 is a mix of an exponential and linear model -
thus fitting all fluxes independantly from curvature - and includes
\(t_0\) as a fitting parameter (its
implementation is described in
vignette("zhao18", package = "fluxible")). A similar model
but with the option to manually set \(t_0\) is exp_tz. Other
available models are: linear for a linear fit,
quadratic for a quadratic fit, and exp_hm for
the original HM model (Hutchinson and Mosier,
1981).
The conc_df argument is the dataframe with gas
concentration, date and time, and start and end of each measurement,
ideally produced with flux_match (see
vignette("data-prep", package = "fluxible") for
requirements to bypass flux_match). Then
f_conc and f_datetime are, similarly as in
flux_match, the gas concentration and corresponding
datetime column. The arguments f_start, f_end,
and f_fluxid are produced by flux_match. They
indicate, respectively, the start, end and unique ID of each
measurement. The model chosen to fit the gas concentration is provided
with fit_type. The user can decide to restrict the focus
window before fitting the model with the start_cut and
end_cut arguments. For the models quadratic,
exp_tz, and exp_hm, t_zero needs
to be provided to indicate how many seconds after the start of the focus
window should the slope be calculated. Arguments cz_window,
b_window, a_window and roll_width
are specific to the automatic fitting of the exp_zhao18 and
exp_tz models and are described in
vignette("zhao18", package = "fluxible"). We recommend
keeping the default values.
slopes_liahovden <- flux_fitting(
conc_df = conc_liahovden, # the output of flux_match
f_conc = conc, # gas concentration column
f_datetime = datetime, # date and time column
f_start = f_start, # start of each measurement, provided by flux_match
f_end = f_end, # end of each measurement, provided by flux_match
f_fluxid = f_fluxid, # unique ID for each measurement, provided by flux_match
fit_type = "exp_zhao18", # the model to fit to the gas concentration
start_cut = 0, # seconds to prune at the start before fitting
end_cut = 0 # seconds to prune at the end of all measurements before fitting
)
Quality checks and visualizations
The function flux_quality is used to provide diagnostics
about the quality of the fit, potentially advising to discard some
measurements or replace them by zero.
The main principle is that the user sets thresholds on diagnostics
(depending on the model used) to flag the measurements according to the
quality of the data and the model fit. Those quality flags are then used
to provide f_slope_corr, a column containing the advised
slope to use for calculation. The force_ arguments allow
the user to override this automatic flagging by providing a vector of
fluxIDs. The ambient_conc and error arguments
are used to detect measurements starting outside of a reasonable range
(the mean of the three first gas concentration data points is used,
independantly from the focus window). The minimal detectable slope is
calculated as \(\frac{2 \times \text{instr
error}}{\text{length of measurement}}\) and is used to detect
slopes that should be replaced by zero. Other arguments are described in
the function documentation (displayed with ?flux_quality).
The function flux_flag_count provides a table with the
counts of quality flags, which is convenient for reporting on the
dataset quality, and can also be done on the final flux dataset. This
table is also printed as a side effect of flux_quality.
flags_liahovden <- flux_quality(
slopes_df = slopes_liahovden,
f_conc = conc,
# force_discard = c(),
# force_ok = c(),
# force_zero = c(),
# force_lm = c(),
# force_exp = c(),
ambient_conc = 421,
error = 100,
instr_error = 5
)
#>
#> Total number of measurements: 138
#>
#> ok 109 79 %
#> zero 27 20 %
#> discard 2 1 %
#> force_discard 0 0 %
#> start_error 0 0 %
#> no_data 0 0 %
#> force_ok 0 0 %
#> force_zero 0 0 %
#> force_lm 0 0 %
#> no_slope 0 0 %
flux_flag_count(flags_liahovden)
#> # A tibble: 10 × 3
#> f_quality_flag n ratio
#> <fct> <int> <dbl>
#> 1 ok 109 0.790
#> 2 zero 27 0.196
#> 3 discard 2 0.0145
#> 4 force_discard 0 0
#> 5 start_error 0 0
#> 6 no_data 0 0
#> 7 force_ok 0 0
#> 8 force_zero 0 0
#> 9 force_lm 0 0
#> 10 no_slope 0 0
The function flux_plot provides plots for a visual
assessment of the measurements, explicitly displaying the quality flags
from flux_quality and the cuts from
flux_fitting. Note that different values than the default
can be provided to scale_x_datetime and
facet_wrap by providing lists of arguments to
scale_x_datetime_args and facet_wrap_args
respectively.
flags_liahovden |>
# we show only a sample of the plots in this example
dplyr::filter(f_fluxid %in% c(54, 95, 100, 101)) |>
flux_plot(
f_conc = conc,
f_datetime = datetime,
f_ylim_upper = 600, # upper limit of y-axis
f_ylim_lower = 350, # lower limit of x-axis
y_text_position = 450, # position of text with flags and diagnostics
facet_wrap_args = list( # facet_wrap arguments, if different than default
nrow = 2,
ncol = 2,
scales = "free"
)
)
Output of flux_plot for fluxid 54,
95, 100 and 101.
To export the plots as pdf without printing them in one’s R session,
which we recommend for large datasets, the code looks like this:
flux_plot(
slopes_df = flags_liahovden,
f_conc = conc,
f_datetime = datetime,
print_plot = FALSE, # not printing the plots in the R session
output = "pdfpages", # the type of output
f_plotname = "plots_liahovden" # filename for the pdf file
)
If the argument f_plotname is left empty (the default),
the name of the slopes_df object will be used
(flags_liahovden in our case). The pdf file will be saved
in a folder named f_quality_plots.
Based on the quality flags and the plots, the user can decide to run
flux_fitting and/or flux_quality again with
different parameters. Here we will cut the last 60 seconds of the fluxes
(cutting the last third). We also detected a flux that do not look
correct. Sometimes measurements will pass the automated quality control
but the user might have reasons to still discard them (or the opposite).
That is what the force_discard, force_ok,
force_lm and force_zero arguments are for.
In our example, for the measurement with fluxID 101, the exponential
model is not providing a good fit (resulting in the flux being
discarded) due to some noise at the start of the measurement. We take
the decision to force the use of the linear model instead, because it
seems to fit much better (given that the flux looks quite flat, we could
also force it to be zero). This is achieved with
force_lm = 101. Several fluxIDs can be provided to the
force_ arguments by providing a vector:
force_zero = c(100, 101).
The function flux_fitting is run again, with an end cut
of 60 seconds:
fits_liahovden_60 <- conc_liahovden |>
flux_fitting(
conc,
datetime,
fit_type = "exp_zhao18",
end_cut = 60 # we decided to cut the last 60 seconds of the measurements
)
Then flux_quality again, forcing the use of the linear
model for fluxID 101:
flags_liahovden_60 <- fits_liahovden_60 |>
flux_quality(
conc,
force_lm = 101 # we force the use of the linear model for fluxid 101
)
#>
#> Total number of measurements: 138
#>
#> ok 127 92 %
#> zero 8 6 %
#> discard 2 1 %
#> force_lm 1 1 %
#> force_discard 0 0 %
#> start_error 0 0 %
#> no_data 0 0 %
#> force_ok 0 0 %
#> force_zero 0 0 %
#> no_slope 0 0 %
And finally flux_plot again to check the output:
flags_liahovden_60 |>
dplyr::filter(f_fluxid %in% c(54, 95, 100, 101)) |>
flux_plot(
conc,
datetime,
f_ylim_upper = 600,
f_ylim_lower = 350,
y_text_position = 450,
facet_wrap_args = list(
nrow = 2,
ncol = 2,
scales = "free"
)
)
Output of flux_plot for fluxid 54,
95, 100 and 101 after refitting with a 60 seconds end cut.
Flux calculation
Now that we are satisfied with the fit, we can calculate fluxes with
flux_calc, which applies the following equation:
\[
\text{flux}=C'(t_0)\times \frac{P\times V}{R\times T\times A}
\]
where
flux: the flux of gas at the surface of the plot (mmol m-2
h-1)
slope: slope estimate (ppm s-1)
P: pressure (atm)
V: volume of the chamber and tubing (L)
R: gas constant (0.082057 L atm K-1 mol-1)
T: chamber air temperature (K)
A: area of chamber frame base (m2)
The calculation is using the slope, which can either be
f_slope (provided by flux_fitting and not
quality checked) or f_slope_corr which is the recommended
slope after quality check with flux_quality. Here the
volume is defined as a constant for all the measurements but it is also
possible to provide the volume as a separate column
(setup_volume). The cols_ave arguments
indicates which column(s), i.e. the environmental data, should be
averaged for each flux. When setting the argument
cut = TRUE (default), the same cut that was applied in
flux_fitting will be used.
The cols_sum and cols_med do the same for sum
and median respectively. In the output, those columns get appended with
the suffixes _ave, _sum and _med
respectively. Here we recorded PAR and soil temperature in the same
dataset and would like their average for each measurement. The
cols_keep arguments indicates which columns should be kept.
As flux_calc transforms the dataframe from one row per
datapoint of gas concentration to one row per flux, the values in the
columns specified in cols_keep have to be constant within
each measurement (if not, rows will be repeated to accomodate for non
constant values). Other columns can be nested in a column called
nested_variables with cols_nest
(cols_nest = "all" will nest all the columns present in the
dataset, except those provided to cols_keep).
The units of gas concentration, conc_unit, can be ppm or
ppb. The units of the calculated flux can be \(mmol/m^2/h\)
(flux_unit = "mmol") or \(\mu
mol/m^2/h\) (flux_unit = "micromol"). Temperature in
the input can be in Celsius, Kelvin or Fahrenheit, and will be returned
in the same unit in the output.
fluxes_liahovden_60 <- flux_calc(
slopes_df = flags_liahovden_60,
slope_col = f_slope_corr, # we use the slopes provided by flux_quality
f_datetime = datetime,
temp_air_col = temp_air,
conc_unit = "ppm", # unit of gas concentration
flux_unit = "mmol", # unit of flux
temp_air_unit = "celsius",
setup_volume = 24.575, # in liters, can also be a variable
atm_pressure = 1, # in atm, can also be a variable
plot_area = 0.0625, # in m2, can also be a variable
cols_keep = c("turfID", "type"),
cols_ave = c("temp_soil", "PAR")
)
#> tibble [138 × 10] (S3: tbl_df/tbl/data.frame)
#> $ f_fluxid : Factor w/ 138 levels "1","2","3","4",..: 1 2 3 4 ..
#> $ temp_soil_ave : num [1:138] 6.96 7.01 6.83 6.83 2.5 ...
#> $ PAR_ave : num [1:138] 24.242 1.05 28.809 0.403 48.062 ...
#> $ turfID : chr [1:138] "4 AN1C 4" "4 AN1C 4" "27 AN3C 27" ""..
#> $ type : chr [1:138] "NEE" "ER" "NEE" "ER" ...
#> $ f_slope_corr : num [1:138] -0.2258 0.0718 -0.3718 0.2433 -0.2865..
#> $ f_temp_air_ave: num [1:138] 3.21 3.3 3.15 2.96 2.81 ...
#> $ datetime : POSIXct[1:138], format: "2022-07-27 05:37:30" ...
#> $ f_flux : num [1:138] -14.09 4.48 -23.22 15.2 -17.91 ...
#> $ f_model : chr [1:138] "exp_zhao18" "exp_zhao18" "exp_zhao1"..
Gross Primary Production calculation
CO2 flux chambers and tents can be used to measure net
ecosystem exchange (NEE) and ecosystem respiration (ER) if the user
manipulates the light levels in the chamber. The difference between the
two is the gross primary production (GPP), which cannot be measured
isolated from ER but is often a variable of interest.
The function flux_gep calculates GPP as \(GPP = NEE - ER\) and returns a dataset in
long format, with NEE, ER and GPP as flux type and filling any variables
specified by the user (cols_keep argument) with their
values corresponding to the NEE measurement. Other type of flux than ER
and NEE, if present in the dataset (e.g. light response curves, soil
respiration) are kept.
Each NEE and ER measurements need to be paired together for this
calculation. The id_cols argument specifies which columns
should be used for pairing (e.g., date, campaign). Since those
measurements were done continuously for 24 hours, we added a pairID
column, which is pairing each NEE measurement with its following ER
measurement. This pairs the NEE and ER measurements of the same plot and
same round of measurement because we systematically measured ER after
NEE for each plot.
library(tidyverse)
fluxes_liahovden_60 <- fluxes_liahovden_60 |>
mutate(
f_fluxid = as.integer(f_fluxid),
pairID = case_when(
type == "NEE" ~ f_fluxid,
type == "ER" ~ f_fluxid - 1
),
f_fluxid = as_factor(f_fluxid),
pairID = as_factor(pairID)
)
gpp_liahovden_60 <- flux_gpp(
fluxes_df = fluxes_liahovden_60,
type_col = type, # the column specifying the type of measurement
f_datetime = datetime,
id_cols = c("pairID", "turfID"),
cols_keep = c("temp_soil_ave", "PAR_ave"), # or "none" (default) or "all"
nee_arg = "NEE", # default value
er_arg = "ER" # default value
)
Structure of the flux dataset including GPP:
#> tibble [207 × 7] (S3: tbl_df/tbl/data.frame)
#> $ datetime : POSIXct[1:207], format: "2022-07-27 05:37:30" ...
#> $ type : chr [1:207] "GPP" "NEE" "ER" "GPP" ...
#> $ f_flux : num [1:207] -18.57 -14.09 4.48 -38.42 -23.22 ...
#> $ temp_soil_ave: num [1:207] 6.96 6.96 7.01 6.83 6.83 ...
#> $ PAR_ave : num [1:207] 24.24 24.24 1.05 28.81 28.81 ...
#> $ pairID : Factor w/ 69 levels "1","3","5","7",..: 1 1 1 2 2 ..
#> $ turfID : chr [1:207] "4 AN1C 4" "4 AN1C 4" "4 AN1C 4" "27 "..
References
Hutchinson, G.L. and Mosier, A.R. (1981),
“Improved
Soil Cover Method for Field Measurement of
Nitrous Oxide Fluxes”,
Soil Science Society
of America Journal, Vol. 45 No. 2, pp. 311–316.
Zhao, P., Hammerle, A., Zeeman, M. and Wohlfahrt, G. (2018),
“On the
calculation of daytime CO2 fluxes measured by automated
closed transparent chambers”,
Agricultural and Forest
Meteorology, Vol. 263, pp. 267–275.