Add explanation of Soil_Inertia1.m

[EntingTang]

Description

Add reference and explanation of Soil_Inertia1.m

Checklist

• [x] Add a reference to related issues.
• [x] @mentions of the person or team responsible for reviewing proposed changes.
• [x] This pull request has a descriptive title.
• [ ] Code is written according to the guidelines.
• [x] The checks by MISS_HIT style checker and linter, below the pull request, are successful (green).
• [ ] Documentation is available.
• [ ] Add changes to the changelog file under section Unreleased.
• [x] Model runs successfully.
• [ ] Ask a meinatainer to re-generate exe file if matlab codes are changed. About how to create an exe file, see exe readme.

[Crystal-szj]

@EntingTang Good job 👍. The explanation makes the code more readable.

[EntingTang]

In addtion to the added explanations, I found three points for potential improvements (reference: Inertial1_Murray and Verhoef (2007).pdf):

1. "QC" (quartz content) is a user-defined parameter that varies based on soil type (see Table 1 in the reference). If data for QC are not available, QC can be assumed equal to "fss" (sand fraction).
2. "gamma_s" is dependent on "fss" (sand fraction): if fss>0.4, gamma_s=0.96 while fss≤0.4, gamma_s=0.27.

However, the parameter fss is currently not utilized in the code. If we have this data from the global dataset, it would be better to use the fss here and might improve the accuracy of calculation on GAM.

3. A bug at Line 21: lambda_s = (lambda_qc^(QC)) * lambda_d^(1 - QC);, where the "lambda_d" should actually be "lambda_o" (see Eq.(18) in the reference). lambda_o represents the thermal conductivity of other minerals. The value of lambda_o is determined by QC: If QC>0.2, lambda_o=2.0 W m^-1 K^-1; otherwise lambda_o=3.0 W m^-1 K^-1.

Please correct me if there are any misunderstandings or omissions in my interpretation. If the above-mentioned points are valid, shall I open an issue to address them? @SarahAlidoost @yijianzeng @Yunfei-Wang1993

[SarahAlidoost]

@EntingTang and @Crystal-szj Hi, are you still working on this pull request? can the changes be finalized and merged?