Methane simulation

[lucia]

Hello everyone, I have a few questions to ask you.
The site I simulated was located at 37°18′N,100°15′E. The vegetation type is alpine steppe.
1. I'm simulating methane flux and I'm not sure which variable to use to represent net methane flux from grid cells. I'm currently using FCH4 and I'd like to know if this can represent the entire net methane emission .
2. I'm having some issues with the simulated methane flux. The actual measurements in this area show that more methane is absorbed, especially during the growing season, so the methane flux should be negative. However, the FCH4 values simulated by CLM are positive during the growing season throughout the year, which suggests that they represent methane emission rather than absorption？The figure below shows the actual values on the left and the simulated values on the right. I have converted the units to µg·m-2·h-1.



My initial hypothesis is that this result may be related to local rainfall, and that the high rainfall in that year may have led to anaerobic conditions in the area, resulting in methane emissions. However, when I output the precipitation data, I found no significant difference between the precipitation in that year and the precipitation in other years when simulating methane absorption using CLM.

I noticed the parameter 'ZWT0' in the parameter file finundated_inversiondata_0.9x1.25_c170706.nc. I found that the value of 'ZWT0' for the grid corresponding to the simulated site was 2.97, so I changed it to 0.01. However, I did not observe any change in the simulated result of FCH4. At the same time, I noticed that the simulated result of 'ZWT' was 8.6m, which seems to be consistent with the local conditions. The groundwater level did not put the soil into an anaerobic environment.But I don't know why the simulated methane emissions are in a positive value.
I would like to ask if you have any good suggestions, I will be very grateful.

 

[slevis]

I think you are pursuing a good process of elimination. If you are running out of ideas, you may find it useful to go into the code and write out the values of various (or all) parts of the calculation, until you identify the source of the problem.

 

[lucia]

Thank you, slevis. However, I have encountered some tricky problems again, and I am urgently seeking help. I hope everyone can give me any suggestions.

I have another site's methane results that I simulated, covering the period from 1989 to 2018 with a time scale of one hour. I noticed that in 2007, there was suddenly a shift from absorbing methane to emitting methane, and the emission rate was very large. I can't understand why this is happening.



I have outputted every result of the methane module, and here are my findings. Based on the simulation results, the reason for the sharp increase in FCH4 in 2009 may be due to an increase in CH4PROD in the soil, which led to an increase in CH4_SURF_DIFF_UNSAT, CH4_SURF_AERE_UNSAT, and CON_CH4_UNSAT.




My initial guess is that the coverage of snowfall may have created some anaerobic conditions, but after outputting the parameters related to snowfall, I found that they did not differ significantly from the data before that time. I still don't know the reason for the sharp increase in methane production in the soil in 2009. What other data should I output to investigate the cause?
The picture shows my output result, and I would appreciate any advice you could give me. Thank you very much

 

[oleson]

Unfortunately, I don't have any expertise with the methane model and so I can't offer any hypothesis, and I'm not sure that anyone else in our group has expertise either since it was contributed by an outside group, but I will ask around.
I assume you are aware of the technical documentation on the methane model:

2.25. Methane Model — ctsm CTSM master documentation

As suggested by @slevis , if you think the issue is related to CH4PROD, then you could start by grepping for CH4PROD in the code:

this%ch4prodg_grc(begg:endg) = spval
call hist_addfld1d (fname='CH4PROD', units='gC/m2/s', &
avgflag='A', long_name='Gridcell total production of CH4', &
ptr_lnd=this%ch4prodg_grc)

Furthermore, ch4prodg_grc is created from ch4_prod_tot:

call c2g( bounds, &
ch4_prod_tot(begc:endc), ch4prodg(begg:endg), &
c2l_scale_type= 'unity', l2g_scale_type='unity' )

So then you can look for the calculation of ch4_prod_tot:

ch4_prod_tot(c) = ch4_prod_tot(c) + (finundated(c)*ch4_prod_depth_sat(c,j) + &
(1._r8 - finundated(c))*ch4_prod_depth_unsat(c,j))*dz(c,j) * catomw

Since this appears to be a single point simulation, it would be straightforward to print out values of ch4_prod_tot and the variables involved in its calculation.

 

[lucia]

Thank you for your reply, Oleson. I'll take a look at the code in the methane-producing module, and look for possible causes.

 

[lucia]

Hello everyone, I encountered some problems while searching for the reasons.
I printed out all the output results from 1989 to 2018 (a total of over 400, at monthly time scales), and plotted over 400 output variables to see if there were any variables that showed a significant change in the same time period as FCH4 in 2007. By comparing them, I found five variables that may be related to the sudden methane emissions: FCOV, FSAT, QFLX_SUB_SNOW, ZWT, and ZWT_CH4_UNSAT. These results indicate an increase in impermeable area around 2007, which is an anaerobic environment, including ZWT_CH4_UNSAT decreasing from 8.6 to 0.06 and QFLX_SUB_SNOW suddenly increasing.




My guess is that this period of time may have resulted in anaerobic conditions due to increased rainfall or snow cover, but when I output the variables RAIN and SNOEDP ('gridcell mean snow height') related to precipitation, I found that they did not show much variation compared to previous data. I'm really not sure why anaerobic conditions occurred in this situation.



I would like to ask if it is reasonable that my site experienced anaerobic conditions around 2007. Also, I am not sure what ZWT exactly represents. My understanding is that it represents the depth of the water table. Is it reasonable that my ZWT has been consistently at 0.92m? If this location were in a non-anaerobic state, should the ZWT be 8.6? Please find attached the output results of my over 400 variables. Any suggestions would be greatly appreciated. Thank you very much.

 

[slevis]

I'm afraid that I am also unable to help with your questions. I hope that colleagues who know more about the methane model will offer their insights. Good luck in your investigations.