Fan 2019 ctsm

[juliusvira]

Description of changes

Includes the FANv2 (Flow of Agricultural Nitrogen) process model for simulating ammonia volatilization from fertilizers and manure.

Specific notes

Are answers expected to change (and if so in what way)?

Connecting FAN to the CLM biogeochemistry (nitrogen pools) can be toggled from namelist. If not connected, answers should not change. If connected, C and N cycling will change.

Any User Interface Changes (namelist or namelist defaults changes)?

Added FAN namelist with some options and path to the FAN stream file.

Testing performed, if any:

The model has been tested with a 3-year (2009-2011) transient run for the following:

• C and N related variables do not change if FAN is off or not coupled to the biogeochemistry
• FAN produces NH3 emissions as expected
• Running with biogeochemistry coupling enabled works technically and conserves N.
  Scientific evaluation of the biogeochemistry coupling is to be done.

Other remarks:

• FAN has the capability to move N from crop to soil column within a grid cell, depending on the fract_spread_grass namelist variable.
• The N balance check currently essentially checks separately for N conservation in FAN and in the rest of CLM. The advantage is that this works the same way regardless of whether FAN is coupled to the biogeochemistry or not. The drawback is that mass conservation between FAN and CLM is not checked. There could be a few ways to fix this.

Edit: link to slides comparing various FAN coupling options:
https://drive.google.com/file/d/1CROPPXphlwhWCMbLKl_RKUaaSehUomHg/view?usp=sharing

[wwieder]

@ekluzek now that you've created a branch tag for this PR, does it remain open?

[ekluzek]

I think we leave it open as a reminder that we eventually want to bring it into main CTSM. But as we have plenty of open PRs we could close it for now. Let's talk about this Thursday.

[wwieder]

as with #2290, it seems like this won't make the cut for CLM6. Removing this PR from the highlight board. Sorry.