The porosity of a soft substrate (eg : sand or gravel) or porous rock (eg : limestone) is determined by the size of the interstices between the grains that constitute it, and the communication between the interstices. All things being equal, an incompressible fluid will track even more easily and quickly in situation 1 than in situation 2. If the porosity is known, and the fluid pressure (determined by the height of the water above the groundwater level equilibrium in the case of water trying to infiltrate from the surface), Darcy's Law to calculate the percolation rate, and hence flow. This is theoretical: in reality, the pores can sometimes become clogged (if the fluid is loaded), or expand (if the fluid is for solubilizing the material; acid water in a limestone substrate, for example)