Water table
The ‘water table’ is the below-ground level that marks the transition between ground that is saturated with water and ground that is not saturated. The upper, unsaturated level, is known as the 'capillary fringe' or 'zone of aeration'. The lower, saturated level, is known as the 'zone of saturation'. An aquifer is a pocket of water that is found below the water table.
As water moves down from the surface it filters through sediments and rocks and causes the water table to fluctuate. The water table typically follows the topography of the above-ground land, but sometimes intersects with the land surface, which may be evident from the presence of spring or oasis.
The level of the water table is influenced by:
- Geology: Heavy, dense rocks are capable of holding less water than light, porous rocks.
- Weather: which is why it tends to fluctuate seasonally.
- Vegetation.
- Snowmelt.
- Extraction: for industrial purposes, drinking water and so on.
- The way land is used: it is common for urban areas to have predominantly impervious surfaces such as roads, car parks and public spaces, and these prevent the seepage of water into the ground. Instead it becomes run off, and the water table can dip as a result.
The SuDS Manual (C753), published by CIRIA in 2015 defines the water table as: 'The point where the surface of groundwater can be detected. The water table may change with the seasons and with annual rainfall.'
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