Update values of fates_hydro_solver and fates_hydro_vg_alpha_node

[jennykowalcz]

This PR changes the default values of the parameters fates_hydro_solver and fates_hydro_vg_alpha_node.

Description:

The default for hydro_solver is changed from 1 (Taylor 1D) to 2 (Picard 2D) to increase the hydro model stability and avoid having it crash under very dry conditions. See discussion 1187.

The default for fates_hydro_vg_alpha_node is changed from 0.005 to 0.12 MPa^-1. The previous small value led to unrealistic values of stem conductivity fraction over the Amazon Basin when using the van Genuchten hydraulic transfer function for plants (i.e. fates_hydro_htftype_node = 2 for all organs). See issue 1228. The new larger value was chosen to approximate the results from the default TFS hydraulic transfer function (fates_hydro_htftype_node = 1) with default hydro parameter values.

Disclaimer: I have not tested this is a global run, just regionally over tropical South America.

Collaborators:

@rgknox

Expectation of Answer Changes:

The change to fates_hydro_vg_alpha_node will change model output significantly if you are using the van Genuchten hydraulic transfer function.

Checklist

If this is your first time contributing, please read the CONTRIBUTING document.

All checklist items must be checked to enable merging this pull request:

Contributor

• The in-code documentation has been updated with descriptive comments
• The documentation has been assessed to determine if updates are necessary

Integrator

• FATES PASS/FAIL regression tests were run
• Evaluation of test results for answer changes was performed and results provided

Documentation

• Technical Note update:
• User's Guide update:

Test Results:

CTSM (or) E3SM (specify which) test hash-tag:

CTSM (or) E3SM (specify which) baseline hash-tag:

FATES baseline hash-tag:

Test Output: