Following figure illustrates variation in saturation of soil mass after lowering water table from depth Z2 to Z1.
Specific retention is the remaining water in the soil mass after discharge of specific yield; in other word water retained in soil mass after specific yield is specific retention.We know soil is porous medium; thus sum of specific retention and specific yield must comprise porosity of soil mass.
i.e. Porosity = Specific retention + Specific yield
or, Specific retention = Porosity – Specific yield.
We know, when pore water pressure is decreased under change in water table, the effective stress is increased which produce compression on soil skeleton.
Thus some water from soil mass is released which was fully saturated initially; this process is called consolidation in soil mechanics. Water released as volume from each unit volume of the aquifer soil for each unit change in water pressure (head) is known as specific storage.
In ground water hydrology, it is also known as specific mass storativity. When specific storage is multiplied by thickness of water bearing layer, it yields storage co-efficient of the aquifer.