The important properties of an aquifer are:
- Its capacity to release the water held in its pores.
- Its ability to transmit the flow easily.
These properties especially depend upon its composition.
The amount of pore space per unit volume of aquifer is known as porosity.
It is expressed as
Where Vv= volume of voids
Vo= volume of porous medium
The actual volume of water that can be extracted by the force of gravity from a unit volume of aquifer material is known as specific yield, Sy.
The fraction of water held back in the aquifer is known as specific retention, Sr.
Thus Porosity = specific yield + specific retention
Reference : Engineering hydrology by K. Subramanya