what is water table?
What is an aquiclude?
Where the water table meets the land surface, a spring might bubble up or seep from the ground and flow into a lake, stream woodland, or the ocean.
The water table is a fundamental reference surface in the study of groundwater. It tends to follow the ground surface, rising under hills and falling at valleys, but the gradient of the water table is usually much less than that of the ground surface.
Under hills the water table is usually at greater depths below the surface than it is below valleys. Where the rocks are very permeable, water can flow through them easily, so the water table will be flatter. Where the water table intersects the ground surface (Diagram Section c), groundwater will flow out as springs, or directly to streams or rivers.
As water sinks into the ground and accumulates (Section a), the water table rises as a horizontal plane in (Section b) until it reaches the ground surface in the valleys in (Section c), where groundwater seeps out as springs and to streams on the surface. With continued infiltration, the water table is no longer horizontal or planar.
Geological and water table map for the Triassic sandstones in part of Nottinghamshire. The topographic ground surface contours are shown in brown and the water table contours in blue. Higher areas of the ground surface and areas where the water table rises have higher contour values. Water table contours are given in metres above ordnance datum (OD). (Section b) North-South cross-section of the outcrop through the Triassic sandstones along the line shown in (Section a). The vertical scale is highly exaggerated.
The water table can be mapped from the elevation (i.e. the height above ordnance datum, which is roughly sea level) of the water in wells. Section b in the diagram is a North-South cross-section across the area in Section a. The general slope of the water table in the diagram is in the same direction as the slope of the ground surface, and undulations of the water table follow undulations of the ground. The water table does not, however, slope as steeply as the ground surface.
A water table has a seasonal rise and fall. There is a lag between the time of maximum infiltration and the highest water table level.
In Britain, for example, the highest rates of infiltration occur in the winter but the water table does not reach its highest level until spring when infiltration rates are lower, because infiltration is a relatively slow process, and it takes time for water to reach the saturated zone.