ctsm5.4.042

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Tag name: ctsm5.4.042
Originator(s): slevis (Samuel Levis,UCAR/TSS,303-665-1310)
Date: Fri May 22 04:07:30 PM MDT 2026
One-line Summary: Get hillslope_fsat_equals_zero .true. for use_hillslope

Purpose and description of changes
----------------------------------

hillslope_fsat_equals_zero was intended to be .true. for use_hillslope = .true. but was coming back false:
- First because of the lack of this line in CLMBuildNamelist.pm:
$nl_flags->{'use_hillslope'} = $nl->get_value('use_hillslope');
- And since ctsm5.4.040 due to the placement of said line in sub setup_logic_params_file, which is called after sub setup_logic_hillslope

Here I'm moving that line into sub setup_logic_hillslope, which fixes the problem without breaking anything else.

Significant changes to scientifically-supported configurations
--------------------------------------------------------------

Does this tag change answers significantly for any of the following physics configurations?
(Details of any changes will be given in the "Answer changes" section below.)

[Put an [X] in the box for any configuration with significant answer changes.]

[x] clm6_0 hillslope hydrology

[ ] clm5_0

[ ] ctsm5_0-nwp

[ ] clm4_5


Bugs fixed
----------
List of CTSM issues fixed (include CTSM Issue # and description) [one per line]:
Resolves #4030 Bug: hillslope_fsat_equals_zero is .false. when use_hillslope = .true.

Notes of particular relevance for users
---------------------------------------
Changes made to namelist defaults (e.g., changed parameter values):
Now get the correct hillslope_fsat_equals_zero for use_hillslope = .true..

Testing summary:
----------------

[PASS means all tests PASS; OK means tests PASS other than expected fails.]

build-namelist tests (if CLMBuildNamelist.pm has changed):

derecho - OK (2 expected failures)

regular tests (aux_clm: https://github.com/ESCOMP/CTSM/wiki/System-Testing-Guide#pre-merge-system-testing):

derecho ----- OK
izumi ------- OK

Answer changes
--------------
Changes answers relative to baseline: Yes

Summarize any changes to answers, i.e.,
- what code configurations: use_hillslope = .true.
- what platforms/compilers: all
- nature of change: larger than roundoff

No simulations have been run with the bug-fix, yet.

Other details
-------------
Pull Requests that document the changes (include PR ids):
https://github.com/ESCOMP/ctsm/pull/4046

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