How to account for evaporation Sw initial < Swir
I am trying to model infiltration in unsaturated media using EOS9. I believe that in reality my media started with an initial water saturation that is lower than the irreducible water saturation (calculated in lab), due to evaporation.
Apart from lowering the irreducible water saturation for my media, is there another way to describe this phenomenon without having any unphysical implications?
If you represent infiltration as a mass source of water, then Sw will increase at your infiltration locations and eventually Sw > Swir and liquid will be mobile. I don't think it is really unphysical to start with Sw < Swir. It just means water is present but immobile. You do have to be careful about the capillary pressure function you are using, some of them go to infinity when Sw < Swir.
If you are interested in evaporation, you might want to try using EOS3 instead of EOS9. EOS3 includes a gas phase that is a combination of air and water vapor, thus evaporation can be modeled. In EOS3, Sw can change if there is evaporation or condensation, even if Sw < Swir. In EOS9, the gas phase is not modeled, it is just implied as the lack of a liquid phase. In this case I don't think you can represent evaporation, so if you have Sw < Swir everywhere, that saturation will not change unless you have a water source somewhere.