Bug: hillslope_fsat_equals_zero is .false. when use_hillslope = .true.

[slevis-lmwg]

Brief summary of bug

This line in CLMBuildNamelist.pm does not behave as intended:
add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'hillslope_fsat_equals_zero', 'use_hillslope'=>$nl_flags->{'use_hillslope'} );

General bug information

CTSM version you are using: ctsm5.4.039

Does this bug cause significantly incorrect results in the model's science? Maybe

Configurations affected: use_hillslope = .true.

Details of bug

I was encountering the same problem in #4029 when trying to pick up a different paramfile for hillslope hydrology. I appear to have solved it by changing the nl_flags to get_value as follows:

add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'paramfile',
              'phys'=>$nl_flags->{'phys'},
              'use_hillslope'=>$nl->get_value('use_hillslope'),
              'lnd_tuning_mode'=>$nl_flags->{'lnd_tuning_mode'},
              'use_flexibleCN'=>$nl_flags->{'use_flexibleCN'} );

[slevis-lmwg]

@swensosc I don't know if this matters much, but from what I can tell you expect hillslope_fsat_equals_zero = .true. when use_hillslope = .true., and it does not seem to be the case. I posted a possible solution above that I tried successfully for a different example, not this one.

[swensosc]

yes, I think that should be the default behavior now.

…

On Mon, May 18, 2026 at 8:11 PM Samuel Levis ***@***.***> wrote: *slevis-lmwg* left a comment (ESCOMP/CTSM#4030) <#4030 (comment)> @swensosc <https://github.com/swensosc> I don't know if this matters much, but from what I can tell you expect hillslope_fsat_equals_zero = .true. when use_hillslope = .true., and it does not seem to be the case. I posted a possible solution above that I tried successfully for a different example, not this one. — Reply to this email directly, view it on GitHub <#4030?email_source=notifications&email_token=AGRN57CPQZTLZNA6KSFZSS343O7EDA5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTINBYGM4DEOBXGYYKM4TFMFZW63VHNVSW45DJN5XKKZLWMVXHJLDGN5XXIZLSL5RWY2LDNM#issuecomment-4483828760>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AGRN57BSFV5UOQEPQ4RANI343O7EDAVCNFSM6AAAAACZDTVWO6VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHM2DIOBTHAZDQNZWGA> . Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>. You are receiving this because you were mentioned.Message ID: ***@***.***>

[slevis-lmwg]

As recommended in the code review in #4029, I changed the setup_logic_params_file code snippet shown above (in CLMBuildNamelist.pm) as follows:

  $nl_flags->{'use_hillslope'} = $nl->get_value('use_hillslope');
  add_default($opts, $nl_flags->{'inputdata_rootdir'}, $definition, $defaults, $nl, 'paramfile',
              'phys'=>$nl_flags->{'phys'},
              'use_hillslope'=>$nl_flags->{'use_hillslope'},
              'lnd_tuning_mode'=>$nl_flags->{'lnd_tuning_mode'},
              'use_flexibleCN'=>$nl_flags->{'use_flexibleCN'} );

This didn't solve the issue with hillslope_fsat_equals_zero ending up false; however, setup_logic_hillslope gets called before setup_logic_params_file. Would moving $nl_flags->{'use_hillslope'} = $nl->get_value('use_hillslope'); from setup_logic_params_fileto setup_logic_hillslope solve it?

[slevis-lmwg]

I will try the fix in aux_clm. If it doesn't change answers, I will open a "b4b-dev"-based PR.

[slevis-lmwg]

I made the recommended change and ran this test successfully, showing that answers changed relative to the baseline:
./create_test SMS_D_Mmpi-serial_Ld5.5x5_amazon.I2000Clm60Bgc.derecho_gnu.clm-HillslopeC -c /glade/campaign/cgd/tss/ctsm_baselines/ctsm5.4.041
Now submitted aux_clm on derecho to confirm that nothing broke:
./run_sys_tests -s aux_clm -c ctsm5.4.041 --skip-generate