water table

   1. The top surface of a body of slowly moving ground water that fills the pore spaces within a rock mass. Above it lies the freely draining vadose zone, and below it lies the permanently saturated phreas. In uniform aquifers, such as sandstone, the water table is a smoothly contoured surface intersecting the ground at rivers and lakes, but in limestone it is more complex. Individual cave conduits may be above or below the water table, and therefore either vadose or phreatic, and the water table cannot normally be related to them. The water table concept does, however, apply to the diffuse drainage of percolation water in the micro-fissure network of limestone, but its detailed structure may be complicated by the presence of conduits. The watertable slope (hydraulic gradient) is low in limestone due to the high permeability, and the level is controlled by outlet springs or local geological features. High flows create steeper hydraulic gradients and hence rises in the water level away from the spring. In France’s Grotte de la Luire, the water level in the cave (and therefore the local water table) fluctuates by 450m [9].

   2. The upper surface of a zone of saturation except where that surface is formed by a confining unit [22].

   3. The upper surface of the zone of saturation on which the water pressure in the porous medium equals atmospheric pressure [22].

   4. The upper boundary of an unconfined zone of saturation, along which the hydrostatic pressure is equal to the atmospheric pressure [10].

   See also potentiometric surface.