CH4 tuning for CLM6 & WIEMIP

[wwieder]

@swensosc has a wetland CH4 branch that has both methane changes and hillslope hydrology modifications that we want to use for WIE-MIP.

I've done a test showing that running this branch produces nearly identical results to a 5.4.024 baseline control (but halves global CH4 production).

I'd love to consider merging Sean's branch to master to avoid having to maintain a long-term branch.

[slevis-lmwg]

@swensosc could you link the corresponding PR here?

[slevis-lmwg]

Next: Discuss prioritization at ctsm software and science meetings 3/26/26

[swensosc]

here is the pr: #3893

[swensosc]

associated with the methane code changes, we would like to change some of the default methane parameters.

[slevis-lmwg]

I will put a time in the calendar for @wwieder @slevis-lmwg @swensosc to review this work.