Question on commented-out mixed and head form options in SoilWaterMovementMod.F90

[gidonhan]

Hello,

I am currently examining the file src/biogeophys/SoilWaterMovementMod.F90 in CTSM, specifically the implementation of the Richards equation in the soil hydrology module.

In the code, I see that four solution options are defined for soil water movement. Among them, the Zeng & Decker (2009) approach and the soil-moisture-form option appear to be selectable and actively used. However, the mixed form and head form options are entirely commented out. I also tried to locate the implementation of soilwater_mixed_form elsewhere in the codebase, but I was unable to find it.

This led me to wonder about the background of these two commented-out options, and I would appreciate clarification on the following points:

Were the mixed-form and head-form approaches fully developed at some point but later commented out because they were deemed difficult to apply robustly or consistently within CTSM?

Alternatively, were these options left commented out because they were still under development, with only a structural placeholder defined for potential future implementation?

I am asking in order to better understand the design decisions behind the soil hydrology solver in CTSM, and to assess whether these formulations are considered deprecated, experimental, or simply unimplemented.

Thank you very much for your time and for maintaining the CTSM code.

Best regards,
Gidon Han
Ph.D. Candidate
Department of Civil and Environmental Engineering
KAIST

[swensosc]

These options are commented out because they have not been developed yet, so a placeholder was implemented in case someone wishes to develop these options.

[gidonhan]

Thank you very much for kind answer!