Confined Aquifer Water Yielding Capacity

The question asks how the water yielding capacity of a confined aquifer is expressed. This refers to how much water a section of the aquifer can release or store under changes in hydraulic head.

Storage Coefficient Explained

For a confined aquifer, the primary property that describes the volume of water released from or taken into storage per unit surface area of the aquifer per unit change in hydraulic head is called the Storage coefficient (often denoted as S). It is a dimensionless quantity or sometimes expressed with units of length (though dimensionless is more common). It essentially represents the storage characteristic of a confined aquifer.

When the hydraulic head in a confined aquifer drops, the water is released from storage due to two main factors:

• Compression of the aquifer material (matrix).
• Expansion of the water itself.

The Storage coefficient accounts for both of these effects. A higher storage coefficient means the aquifer can release a larger volume of water for a given drop in head, and vice versa.

Other Aquifer Properties

Let's look at the other options to understand why they are not the primary term for the *water yielding capacity* (storage property) of a confined aquifer:

• Coefficient of transmissibility (T): This property describes the rate at which water flows horizontally through a unit width of the aquifer under a unit hydraulic gradient. It is calculated as the product of the hydraulic conductivity (or coefficient of permeability) and the saturated thickness of the aquifer (\(T = K \times b\)). Transmissibility is related to the *flow* capacity of the aquifer, not its storage capacity.
• Specific retention (\(S_r\)): This term is primarily used for unconfined aquifers or unsaturated zones. It represents the volume of water that a unit volume of saturated porous medium will retain against the force of gravity after being drained. Confined aquifers are typically assumed to be fully saturated under pressure, and water is released due to head changes through compression and expansion, not gravity drainage.
• Coefficient of permeability (K) or Hydraulic Conductivity: This is a property of the aquifer material itself, representing its ability to transmit water. It is the rate of flow of water through a unit area of the medium under a unit hydraulic gradient. While permeability is a component used to calculate transmissibility (\(T = K \times b\)), it does not directly express the *overall* water yielding capacity (storage characteristic under changing head) of the entire confined aquifer layer.

Conclusion

Based on the definitions, the Storage coefficient is the specific parameter used to quantify the water yielding capacity, or more precisely, the storage characteristic, of a confined aquifer when the hydraulic head changes. It tells us how much water is released from or added to storage within the aquifer.