Non-steady state conditions occur where there is a change in the quantity of the flux in storage. In groundwater, this is where water is removed from storage, frequently leading to a reduction in hydraulic head and/or a change in porosity of the formation. The non-steady state equation for groundwater flow requires the introduction of specific storage. Easy enough to characterise for an unconfined aquifer, where the concept is also known as specific yield, but very difficult to characterise for a confined aquifer. A detailed analysis is provided in this Chapter based upon the compressibility of the fluid and formation.

Abstraction from a confined aquifer frequently leads to land subsidence that is impossible to reverse. Concepts of barometric efficiency and loading efficiency are reviewed with examples given for their calculation from field data using Fourier analysis techniques. If porosity is established, then the specific storage can be calculated. Linear poro-elastic theory is reviewed to develop a better understanding of the physically realistic numerical bounds to specific storage.