The property which expresses the capacity of an aquifer to take water into and release water from storage is called “storativity” for a confined aquifer and “specific yield” for an unconfined aquifer.
In a confined aquifer, the pores always remain fully saturated. Water is released from storage in response to a drop in head by two mechanisms:
- Expansion of the water in the pores as water pressure is reduced
- Compaction of the aquifer as water pressure is reduced and stresses between solid grains of the aquifer matrix increase
In an unconfined aquifer, a drop in head results in a release of water from storage by an actual dewatering of pores as the water table declines. As a result of these mechanism differences, the specific yields of unconfined aquifers are much larger than the storativities of confined aquifers.
Watch the simulation videos below and answer the following questions:
Explain, from a hydraulic point of view,
- why unconfined aquifers are generally preferable to confined aquifers for water supply
- why artificial recharge - a method of controlling declining water levels, is more effective for a leaky confined aquifer or an unconfined aquifer than for a confined aquifer.
- why the responses in a confined aquifer and a unconfined aquifer to stream level fluctuations are dramatically different?
