Storage Coefficient | Civil Engineering Optional Notes for UPSC PDF Download

Storage Coefficient (S)

• The storage coefficient or storativity (S) is a dimensionless value representing the volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in hydraulic head.
• The storage coefficient is dimensionless because it represents a ratio of volumes (change in volume of water stored) to areas (cross-sectional area of the aquifer) divided by a length (change in hydraulic head).
• A high storage coefficient indicates that the aquifer has the ability to store and release significant amounts of water relative to its surface area and the change in hydraulic head. Conversely, a low storage coefficient suggests that the aquifer has limited storage capacity.
• The storage coefficient is a crucial parameter in groundwater modeling and management, as it influences various aspects such as groundwater recharge, discharge, and the response of aquifers to pumping or recharge activities. It is often determined through field measurements, pumping tests, or numerical modeling techniques.

Unconfined Aquifers

Specific Yield (Sy)

• Specific yield is the ratio of the volume of water drained by gravity from a saturated rock or soil to the total volume of the rock or soil.
• It represents the drainable porosity of the aquifer material.
• Typical values range from 0.01 for clay to 0.35 for gravel.
• Storage Coefficient Calculation

For unconfined aquifers, the storage coefficient (S) is approximately equal to the specific yield (Sy).

S ≈ Sy

Value Range

• The storage coefficient for unconfined aquifers typically ranges from 0.01 to 0.30.
• Higher values indicate greater porosity and drainable void spaces within the aquifer material.

Water Release Mechanism

• In unconfined aquifers, water is released from or taken into storage primarily by gravity drainage of pore spaces.
• When the water table declines, water drains from the previously saturated pore spaces above the new water table level.

Water Production

• Due to this gravity drainage effect, unconfined aquifers can produce relatively large volumes of water for small declines in the water table compared to confined aquifers.
• This makes unconfined aquifers generally more productive in terms of water supply.

Confined Aquifers

Water Release Mechanism

• Compression of the aquifer material
• Expansion of the water itself

Aquifer Compression

• When pumping lowers the pressure in a confined aquifer, the reduced pressure causes a slight compression or compaction of the aquifer material.
• This compression releases water from the pore spaces within the aquifer material.

Water Expansion

• The water stored in a confined aquifer is under pressure.
• As the pressure decreases due to pumping, the water expands slightly, releasing water from the pore spaces.

Storage Coefficient Value

• The storage coefficient (S) for confined aquifers is much smaller than for unconfined aquifers.
• Typical range: 1 x 10^-5 to 1 x 10^-3
• The smaller value reflects the relatively small amount of water released from storage due to the compression of the aquifer material and the expansion of water.

Aquifer Response

• During pumping, confined aquifers are not dewatered; the water level (piezometric surface) declines, but the aquifer remains saturated.
• The water level decline in a confined aquifer extends over a larger area compared to an unconfined aquifer for the same pumping rate.

Water Production

• Confined aquifers generally produce less water per unit decline in the water level compared to unconfined aquifers due to the lower storage coefficient.
• However, confined aquifers can still be highly productive if they have high transmissivity and are sufficiently thick.

In summary, the storage coefficient in confined aquifers is several orders of magnitude smaller than in unconfined aquifers because water is released primarily through the compression of the aquifer material and the expansion of water, rather than gravity drainage. This smaller storage coefficient affects the aquifer's response to pumping and its water production capabilities.