Coupled heat and groundwater flow

Hi,

I am modelling a permafrost problem, where a porous soil is saturated with groundwater and heated by surface temperature.

I have used 'Heat transfer in porous media' coupled with the 'Darcy equations' to simulate water and heat flow, including the phase change of water (freezing in winter; thawing in summer).

In my model, there are warming "Failed to find consistent initial values. Singular matrix." could you share some experience about how to solve this problem.Thanks.

Does anyone have experience with modelling heat and water flow in a porous medium, or have any ideas about how to do this?

Thanks,
Lin