Unsaturated flow with dry initial condition

Hi,

I am working with a Richards' equation model that tries to simulate an imbibition of a very dry soil.
As anyone can see, in literature it's reported that Richards' equation has not good convergence with initial dry condition, near to residual saturation value.
Actually, I have experienced that there is a thresh for the initial pressure head below which the solver doesn't converge.

In literature the problem is solved with the primary variable switching techique (using saturation as primary variable in unsaturated porous media) or controlling the steps of the solver modifying the preconditioner algorithm.
Can anybody give me some tips to emulate these procedures?
How can I customize the solver algorithm?

Thanks a lot!

Tommaso Santagata