I show first the physical model. We have to take into account the different transport mechanisms.
First we have capillary water, which is free water. Capillary water moves through advective flow and the driving force is the water pressure gradient.
We may have adsorbed water. Here we have diffusional flow and the driving force is water concentration gradient.
Then we have the chemically adsorbed water, there is no transport.
Then the water vapour, where we may have either advective flow, which is driven by gas pressure gradients, or the diffusional flow, where the driving force is the water vapour concentration gradient.
Finally, dry air. Again we have a advective flow, where we have the gas pressure gradient, and a diffusional flow, where we have the dry air concentration gradient.