Formula & Calculator
Aquifer Storativity (Confined Aquifer)
Calculates the storage coefficient of a confined aquifer, representing the volume of water released per unit area per unit head decline.
Interpretation
S = Ss × b. Storativity is the volume of water released from a confined aquifer per unit decline in head per unit area. Ss is specific storage, b is thickness. Used in transient groundwater flow.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| S | Storativity (storage coefficient) | |
| Ss | Specific storage | 1/m |
| b | Aquifer thickness | m |
What it means
Storativity (S) is a dimensionless parameter that measures the amount of water released from or taken into storage in a confined aquifer per unit change in hydraulic head, per unit area. It is the product of specific storage (Ss) and the aquifer thickness (b). Storativity is a key parameter in transient groundwater flow modeling and is used in pumping test analysis to determine aquifer properties. Understanding storativity is essential for predicting water level changes, for groundwater resource evaluation, and for managing aquifer storage and recovery systems.
Worked example
Aquifer Storativity – Two Detailed Examples
Real‑World| Parameter | Value |
|---|---|
| Ss (1/m) | 1.0e-5 |
| b (m) | 20 |
| Parameter | Value |
|---|---|
| Ss | 3.0e-5 |
| b | 40 |
Common mistakes
- Storativity S: S = Ss·b – for confined aquifers.
- Specific storage Ss: Volume of water released per unit volume of aquifer per unit head change – in 1/m.
- b: Aquifer thickness – in metres.
- Units: S is dimensionless – Ss (1/m) × b (m) = dimensionless.
- For unconfined aquifers: Storativity approximates specific yield (Sy) – not this formula.
Applications
Aquifer storativity (confined), S = Ss·b, is the volume of water released from storage per unit decline in head per unit area, where Ss is specific storage and b is aquifer thickness. This parameter is essential for transient groundwater flow modelling and for predicting water‑level changes in response to pumping. Hydrogeologists use storativity to evaluate the response of aquifers to stresses, to design water‑supply systems, and to assess subsidence potential. By measuring S, professionals can manage groundwater resources effectively, especially in areas with high pumping demands. Understanding storativity is crucial for sustainable groundwater management and for avoiding environmental degradation.
- Transient groundwater modelling and flow prediction
- Analysis of aquifer response to pumping and recharge
- Assessment of land subsidence and compaction
- Design of aquifer storage and recovery (ASR) projects
- Environmental impact assessment of groundwater development
Frequently Asked Questions
S = S_s · b, where S_s is the specific storage (1/m) and b is the aquifer thickness (m). Storativity is dimensionless and represents the volume of water released from storage per unit surface area per unit decline in hydraulic head.
Storativity typically ranges from 10⁻⁵ to 10⁻³, with values around 10⁻⁴ common for many sands and gravels. It is much smaller than specific yield in unconfined aquifers.
S_s is the volume of water released from a unit volume of aquifer per unit decline in head. It arises from the compressibility of the aquifer matrix and the water itself.
A lower S means the aquifer releases water more readily (pressure changes propagate faster), leading to quicker drawdown response. Higher S means more water storage, and drawdown spreads more slowly.
Storativity (S) is for confined aquifers – water is released by elastic expansion/compression, typically small. Specific yield (Sy) is for unconfined aquifers – water is released by gravity drainage, typically 0.1–0.3, much larger.
From pumping tests using the Theis (type curve) or Cooper‑Jacob (semi‑log) methods, which analyse drawdown vs. time to estimate T and S.
If there is leakage from a confining layer, the effective S may be larger (due to delayed yield). The simple formula assumes perfect confinement.
Typically, S decreases with depth as the aquifer becomes more confined and less compressible. However, it depends on the lithology and pressure.
Storativity is dimensionless. Specific storage S_s has units of 1/m (or m⁻¹).
S_s = ρg (α + nβ), where α is the aquifer matrix compressibility and β is the water compressibility. Thus, S depends on the elastic properties of the medium.