How to simulate the porous media under the condition of a certain pressure at inlet and outlet

I am going to simulate the porous media with LBM when the inlet and outlet are subjected to pressure. At the same time, I gain the pressure value and field.
The known condition is that water density, dynamic viscosity and the pressure at inlet and out let of models, such as 300kPa, in physics.
According LBM theory, water is not compressible, namely its density is constant, but the pressure is one third of density in lattice, which I did not understand that very much.
Now, I am very puzzled. I want to know how to simulate the pressure condition boundary and how to compute interior pressure with MATLAB language.
Thank in advance.