A water table is the free water surface in an unconfined aquifer. The static level of a well penetrating an unconfirmed aquifer indicates the level of the water table at that point.
The water table is constantly in motion adjusting its surface to achieve a balance between the recharge and outflow from the subsurface storage.
Fluctuations in the water level in a dug well during various seasons of the year, lowering of the groundwater table in a region due to heavy pumping of the wells and the rise in the water table of an irrigated area with poor drainage, are some common examples of the fluctuation of the water table.
In a general sense, the water table follows the topographic features of the surface.
In the water table intersects the land surface the groundwater comes out to the surface in the form of springs or seepage.
Sometimes a lens or localized patch of impervious stratum can occur inside an unconfined aquifer in such a way that it retains a water table above the general water table (Fig. 9.3).
Such a water table retained around the impervious material is known as perched water table. Usually the perched water table is of limited extent and the yield from such a situation is very small. In groundwater exploration a perched water table is quite often confused with a general water table.
The position of the water table relative to the water level in a stream determines whether the stream contributes water to the groundwater storage or the other way about.
If the bed of the stream is below the groundwater table, during periods of low flows in the stream, the water surface may go down below the general water table elevation and the groundwater contributes to the flow in the stream.
Such streams which receive groundwater flow are called effluent streams (Fig. 9.4 (a)).
Perennial rivers and streams are of this kind. If, however, the water table is below the bed of the stream, the stream-water percolates to the groundwater storage and a hump is formed in the groundwater table (Fig. 9.4 (h)).
Such streams which contribute to the groundwater are knows as influent streams. Intermittent rivers and streams which go dry during long periods of dry spell (i.e. no rain periods) are of this kind.
The important properties of an aquifer are its capacity to release the water held in its pores and its ability to transmit the flow easily.
These properties essentially depend upon the composition of the aquifer.
The amount of pore space per unit volume of the aquifer material is called porosity. It is expressed
While porosity gives a measure of the water storage capability of a formation C the water held in the pores is available for extraction by Pumping or draining by gravity.
The poles hold back some water by molecular attraction and surface tension.
The actual volume of water that can be extracted by the force of gravity from a unit of aquifer material is known as the Specific yield Sy.
The fraction of a unit held back in the aquifer is known as specific retention.
Thus Porosity of water