Groundwater occurring in the saturated zone below the water table is classified by formation type based on porosity (total void fraction) and specific yield (the fraction of stored water drainable by gravity), yielding four categories — aquifer, aquifuge, aquiclude, and aquitard — that describe a formation's combined storage and transmission capacity. Flow through such porous media is governed by Darcy's Law, which states that discharge is proportional to hydraulic gradient and cross-sectional area via the hydraulic conductivity constant, and is valid only within the laminar-flow regime defined by a pore Reynolds number below approximately one. This belongs to hydrogeology, a subsurface-flow branch of hydrology, and distinguishes unconfined aquifers (water table at atmospheric pressure) from confined aquifers (pressurized flow described by piezometric head).
Groundwater occurring in the saturated zone below the water table is classified by formation type based on porosity (total void fraction) and specific yield (the fraction of stored water drainable by…