Water discharged from an aquifer or recharged into an aquifer represents the change in its storage volume.
It is very easy to determine this change for unconfined aquifers.
Rise or fall of water table in given time is determined experimentally.
Multiplying this by average specific capacity during the time, the change of the storage volume can be found out.
In the case of confined aquifers, change in pressure produces only small change in storage volume.
The hydrostatic pressure within an aquifer partially supports the over burden and partially the solid structure of the aquifer.
When hydrostatic pressure is reduced by pumping water from an quifer, the load on it is increased and aquifer gets consequently compressed and thus forces some water out of it.
In addition, lowering of the pressure causes expansion and subsequent release of water.
The water yielding capacity of an artesian acquifer can be expressed by its storage coefficient, generally denoted by A.
The storage coefficient (A) for a confined aquifer is equal to the volume of the water released from the aquifer of unit cross-sectional area and full height, when piezometric surface depresses by unity.
In general, storage co-efficient is defined as the volume of water than an aquifer releases or stores per unit surface area of the aquifer per unit change in the head normal to that surface.