Storage coefficient - The volume of water given up per unit horizontal area of an aquifer and per unit drop of the water-table or potentiometric surface. It is a dimensionless ratio and always less than unity. In unconfined aquifers it is equal to the specific yield, but in confined aquifers it depends on elastic compression of the aquifer, and is usually less than 10^−3.

Coefficient of permeability is defined as the velocity of flow which will occur through the total cross-sectional area of the soil under a unit hydraulic gradient.