Junbin Zhao (2019). FluxCalR: a R package for calculating CO₂ and CH₄ fluxes from static chambers. Journal of Open Source Software, 4(43), 1751, https://doi.org/10.21105/joss.01751

1. Added an argument pa for air pressure input (unit: atm).
2. Added an argument metric with an option of using RMSE, other than R2 (default), as evaluation metric for selecting regressions.

(Acknowledge to Thomas Gremmen who made suggestions for the updates!)

Used plotly package for the final checkup graph, so that it can be zoomed in for more accurate screenings.

1. Separated the interactive manual time cue selection from the main function for a more flexible workflow.
2. Fixed the timestamp mismatch issue.
3. Added the option in the argument cue_type of taking the start and end of a measurement at the same time as the time cues.
4. Added the ext argument for users to define the window range for choosing the best regression.

The static close chamber technique is a widely used approach for measuring greenhouse gas fluxes from different sources in an ecosystem (e.g. soil respiration). While the method itself is easy and cheap to implement, the calculation of fluxes from the measured gas concentrations includes many tedious processes. Previous programs that can calculate the fluxes measured using the static chamber method were either compatible to only one designated system (e.g., Licor Trace Gas Analyzer Platform, Flux Puppy), unable to calculate fluxes for multiple gases (e.g., Flux Puppy), or hard to handle continuously measured concentrations (e.g., R package ‘flux’).

This R package FluxCalR is designed to calculate CO₂ and CH₄ gas fluxes measured with static chamber method. It provides an easy way to calculate multiple flux measurements from one continuously measured data file exported by most gas analyzers (e.g. LGR Ultraportable Gas Analyzers). Specifically, users can use the function LoadLGR() or LoadOther() to load the raw data file that includes the gas concentration records. Then, create a separate data frame by SelCue() that includes the time cues that each measurement starts and/or ends corresponding to the time in the raw data file. Based on the time cues, the fluxes of all the measurements can be calculated and exported using the function FluxCal(). In addition, the package has several features that facilitate the process of flux calculation:

• An interactive way of selecting the measurement time cues - The package includes the function SelCue() that one can use to select the time cues (start or end of each measurement) manually by clicking on an interactive graph of the entire gas concentration time series from the raw data file. The data frame returned from the function can be directly used as the argument df_cue in the function FluxCal() for the flux calculation. The time cue data frame can also be saved (default) and used again to get reproducible results.

• Flexible input - Depending on the measurement protocol and requirement of the users, the FluxCal() function is flexible in the sense that 1) it takes either “Start”, “End” or both “Start_End” of each measurement (argument cue_type) as time cues to identify each measurement; 2) it calculates “CO2”, “CH4” or both “CO2_CH4” fluxes at the same time (argument cal); 3) the temperature used for calculation can be either measured by the analyzer (default), measured by a separate sensor, or a constant number defined by the user.

• A dynamic measurement window - The flux calculation automatically scans through a range of 1.5x (default) of the measurement window and the best linear regression (largest R²) is used to calculate the final fluxes. This protocol optimize the representativeness and reproducibility of the calculated flux. This dynamic range can be changed in the argument ext in the function FluxCal().

• A checkup graph after the calculation - By default (argument check_plot), a graph is plotted after the calculation with regression lines plotted on the CO₂ and/or CH₄ concentration time series for checkup purposes. This post-calculation feature will help users identify any flux calculation that is based on the unwanted section of the data.

• Easy to be integrated into workflows - The FluxCal() returns a data frame with the calculated flux and identifier of each measurement (e.g., plot ID, see argument Other) that can be integrated into users’ data processing/analyzing workflow. At the same time, it also saves the data frame to a “.csv” file that can be loaded by R again later or futher processed in other programs.

First, make sure the package remotes is installed in R. If not, install the package by:

install.packages("remotes")

Then, install the FluxCalR package (with vignettes) in R by:

remotes::install_github("junbinzhao/FluxCalR",build_vignettes = TRUE)

The functioning of the package is based on other R packages: dplyr, lubridate, assertthat, magrittr, birk, plotly and htmlwidgets, and they will/must be installed before using the functions in the FluxCalR package.

Note: in case the installation fails in Rstudio, try to install the package in the original R program and then load the package in Rstudio.

To report problems, seek support or contribute, please contact the author Junbin Zhao (junbinzhao1985@gmail.com). Requests/suggestions for new features are also welcome.

This is a basic example which shows how to calculate fluxes from raw data exported from the LGR Ultraportable Gas Analyzer with manually selected time cues.

First, we’ll need to load the raw data file exported from LGR into the R program by the function LoadLGR(). The users just need to assign the directory (including the file name) and timestamp format. Then the function will take care the rest process and convert the data file into a data frame that can be used in the next steps. Here, we take the example data file “Flux_example_1_LGR.txt” comes with the package.

library(FluxCalR)
#### data from LGR
# get the directory of the example LGR raw data
example_data1 <- system.file("extdata", "Flux_example_1_LGR.txt", package = "FluxCalR")
example_data1 # check the directory
#> [1] "C:/Users/juzh/Documents/R/win-library/4.0/FluxCalR/extdata/Flux_example_1_LGR.txt"

# load the data
Flux_lgr <- LoadLGR(file = example_data1,
                    time_format = "mdy_HMS")

After loading the data, one can use the function SelCue() to manually identify the data for each of the measured fluxes within the entire dataset. After executing the function, a interactive graph of CO2 (or CH4, see the argument flux) concentration time series will pop up for the user to click on the corresponding points (either end or start of the measurements). Selecting the End (see the argument cue) points are recommended here because they are usually associated with sudden drops or increases in the gas concentrations when removing the chamber, making them easier to be identified than the starting points.

# manually select the end of each measurement as time cues
time_cue <- SelCue(Flux_lgr,flux = "CO2",cue = "End",save = F)

This is how the “time_cue” data frame looks like. Instead of having it created by the function SelCue(), one can also manually prepare such a file and load it into R as a data frame. But the column name (“End” or “Start”) and time format must be in line with the “time_cue” here, or see example files "Time_&Ta_1.csv" and "Time&_Ta_2.csv" at https://github.com/junbinzhao/FluxCalR/tree/master/inst/extdata).

time_cue
#>    Num      End
#> 1    1 12:08:47
#> 2    2 12:14:06
#> 3    3 12:23:58
#> 4    4 12:29:55
#> 5    5 12:37:55
#> 6    6 12:42:25
#> 7    7 12:50:26
#> 8    8 12:55:53
#> 9    9 13:04:51
#> 10  10 13:09:16
#> 11  11 13:20:53
#> 12  12 13:27:26

After having the time cues saved as a data frame ‘time_cue’, we can do the calculations now using the function FluxCal()! In the function, there are 5 arguments that have to be assigned by the users without default: data (the data frame we got from LoadLGR()), win (the window size for flux calculation, unit: minute),vol (chamber volumn in l),area (chamber base area in m^2) and df_cue (the data frame we got from SelCue()). After the function is executed with the arguments assigned, the calculation will be done and a check-up graph with all the regression lines drawn on top of the gas concentration time series will pop up by default for the user to visually check if there is any mistake occured to the calcualtions.

# calculate the fluxes over a 3-minute window using the manually selected cues
Flux_output1 <- FluxCal(data = Flux_lgr,
                        win = 3,
                        vol = 208,area = 0.26,
                        df_cue = time_cue,
                        cue_type = "End",
                        output = FALSE) # no output file

This is how the output data frame looks like and the last column (“Flux”) includes the calculated flux rates:

head(Flux_output1)
#>   Num       Date    Start      End Gas  Slope   R2    Ta   Flux
#> 1   1 2016-11-21 12:05:43 12:08:47 CO2 -0.034 0.99 29.07 -1.097
#> 2   2 2016-11-21 12:11:01 12:14:06 CO2  0.032 0.99 30.20  1.028
#> 3   3 2016-11-21 12:20:08 12:23:12 CO2  0.019 0.97 31.57  0.608
#> 4   4 2016-11-21 12:25:19 12:28:23 CO2  0.017 0.95 32.11  0.543
#> 5   5 2016-11-21 12:34:04 12:37:09 CO2  0.009 0.79 32.84  0.287
#> 6   6 2016-11-21 12:39:20 12:42:24 CO2  0.050 0.99 33.20  1.591

For more details of the functions and examples, please check the vignette (browseVignettes("FluxCalR")) and the help documents (?LoadLGR(),?LoadOther(),?SelCue(),?FluxCal()).