Water table

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The water table is the surface in an unconfined aquifer at which the pressure of the water in the void spaces is exactly equal to atmospheric pressure. This surface is usually taken as the boundary between the saturated zone beneath and the unsaturated zone above. It should be noted that small voids in the aquifer will usually be completely filled with water for some distance above the water table, the water being held in place by capillary forces; this water will be at a pressure less than that of the atmosphere, and occupies a zone usually referred to as the capillary zone or capillary fringe.

The word table suggests something that is flat and static. The water table is neither. It usually follows the topography in a subdued way, so that it is at or near the surface in valleys, and deep below the surface beneath hills. The gradient of the water table is an indicator of the hydraulic gradient that causes groundwater movement, with flow occurring away from regions where the water table ...