We develop a hydrological model of the Martian crust, including both ancient heavily cratered terrains and younger basaltic and sedimentary terrains. The porosity, permeability, and compressibility are represented as interdependent functions of the effective stress state of the aquifer, as determined by the combination of the lithostatic pressure and the fluid pore pressure. In the megaregolith aquifer model, the crust is modeled as a 2 km thick megaregolith, composed of lithified and fractured impact ejecta, overlying the impact-fractured and partially brecciated basement rock. The hydraulic properties depend primarily upon the abundance of breccia and the compressional state of the fractures. The porosity ranges from approximately 0.16 at the surface to 0.04 at a depth of 10 km, with a sharp discontinuity at the base of the regolith. The permeability varies from approximately 10-11 m2 at the surface to 10-15 m2 at depths of 5 km or more and is strongly dependent upon the fluid pore pressure. The hydrologic properties of basaltic and sedimentary aquifers are also considered. These parameters are used to model the fluid pressures generated beneath a thickening cryosphere during a postulated dramatic cooling of the climate at the end of the Noachian. As a result of a negative feedback between the fluid pore pressure and the permeability, it is more difficult than previously thought to generate pore pressures in excess of the lithostatic pressure by this mechanism. The production of the outflow channels as the result of such a climatic change is deemed unlikely.