**Specific yield:-**

The capacity of a formation to contain water is measured by porosity. However, a high porosity does not indicate that an aquifer will yield large volume of water to a well. The only water which can be obtained from the aquifer is that which will flow by gravity. The specific yield of an aquifer is defined as the ratio expressed as a percentage, of the volume of water which after being saturated, can be drained by gravity to its own volume.

$\text{Specific yield (Sy)} = \frac{\text{Volume of water drained by gravity}}{\text{Total volume}}$

$Sy = \frac{W_y}{V} \times 100$

Where Wy = volume of water drained by gravity. Specific Retention: -

Specific yield is always less than porosity since some water will be retained in the aquifer by molecular and surface tension . The specific retention of an aquifer is the ratio, expressed as a percentage, of the volume of water it will retain after saturation against the force of gravity to its own volume.

$\text{Specific retention (Sr)} = \frac{\text{Volume of water retained}}{\text{Total volume}}$

$Sr = \frac{W_r}{V} \times 100$

Where Wr = volume of water retained.