Water Table

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.

AQUIFER PROPERTIES

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.

Porosity

The amount of pore space per unit volume of the aquifer material is called porosity. It is expressed

Specific Yield

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

Other links:

HYDROLOGIC CYCLE
PRECIPITATION
RAIN GAUGE
EVAPORATION
INFILTRATION
GROUNDWATER
DARCY’S LAW
FLOOD FREQUENCY STUDIES
RECURRENCE INTERVAL
GUMBEL’S METHOD
FLOOD ROUTING