van Genuchten - Mualem model
I am reviewing the RPCAP keyword with the G-M model (7). There are recurring terms Sl, Slr, and Sgr that are not defined in the User Manual. I understand "irreducible water saturation" is Slr. In the example INFILE I have this number is equivalent to 0.00096. What is the significance of this term and the others that are undefined in the TOUGH manual? I am looking at Mualem 1976 and the lambda number is clearly defined for a number of soil types. Please help me figure out what these other water saturation values are.
Sl is liquid saturation (the fraction of the pore space containing liquid phase). Usually "liquid" means aqueous, but not always, it depends on which equation of state (EOS) you are using. This is an output of the code that varies from grid block to grid block.
Slr is residual liquid saturation, aka irreducible water saturation, the liquid saturation below which liquid is immobile. This is an input that varies from material (that is, rock or soil type) to material.
Sgr is residual gas saturation, the gas saturation below which gas is immobile. This is an input that varies from material to material.
Mualem's lambda is sometimes referred to as m in TOUGH documentation, and is usually the same as van Genuchten's m. This parameter controls the shape of the relative permeability and capillary pressure curves. This is an input that varies from material to material.
Have you used the toughio package (https://toughio-dash.lbl.gov)? You can plot these curves and input different values for the Slr, Sgr, andlambda parameters and see the effect on the curves. In toughio, pretend you want to prepare a TOUGH2/TOUGH3 input file, then select ROCKS, and enter parameters in the relative permeability or capillary pressure section, then plot. You will need the TOUGH user's guide to relate RP(1), RP(2), etc. to the actual parameters (in the TOUGH3 User manual, see Appendix A and Appendix B). For example, for van Genuchten (IRP=7), RP(1)=lambda, RP(2)=Slr, RP(3)=Sls (=full water saturation, usually 1), RP(4)=Sgr.
Good luck, Christine