Formula & Calculator
Well Specific Capacity
Measures a well's efficiency by relating its pumping rate to the resulting drawdown, useful for comparing well performance.
Interpretation
Sc = Q / s. Specific capacity is the pumping rate per unit drawdown. Used to evaluate well performance and efficiency. A key parameter in well design.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| Sc | Specific capacity | m3/day/m |
| Q | Pumping rate | m3/day |
| s | Drawdown | m |
What it means
Specific capacity (Sc) is the ratio of the pumping rate (Q) to the drawdown (s) in a well. It is a measure of well efficiency: a higher Sc means the well yields more water per unit of drawdown. It is used in well design and evaluation, to compare wells, and to estimate transmissivity. It is also used in aquifer tests. Understanding specific capacity helps groundwater engineers select pumping equipment, design well fields, and manage groundwater resources effectively.
Worked example
Well Specific Capacity – Two Detailed Examples
Real‑World| Parameter | Value |
|---|---|
| Pumping Rate Q (m³/day) | 500 |
| Drawdown s (m) | 5 |
| Parameter | Value |
|---|---|
| Q | 800 |
| s | 8 |
Common mistakes
- Specific capacity: Sc = Q / s – the pumping rate per unit drawdown.
- Q: Pumping rate – in m³/day or L/min.
- s: Drawdown in the well – in metres.
- Units: m³/day per metre (m²/day) or equivalent.
- Not constant: Sc decreases with pumping time and is affected by well efficiency.
Applications
Well specific capacity, Sc = Q/s, is the pumping rate per unit drawdown, providing a measure of well efficiency and aquifer productivity. This is used to evaluate well performance, to compare wells, and to estimate aquifer properties. Hydrogeologists use specific capacity data from step‑drawdown tests to determine the well loss and the formation loss components. By monitoring Sc over time, professionals can detect changes in well condition (e.g., clogging) or aquifer behaviour. This parameter is essential for well design, maintenance, and groundwater management.
- Well performance evaluation and efficiency assessment
- Comparison of well yields for water‑supply planning
- Step‑drawdown test analysis for well losses
- Monitoring of well clogging and rehabilitation needs
- Estimation of aquifer transmissivity from specific capacity
Frequently Asked Questions
Sc = Q / s, where Q is the pumping rate (m³/day) and s is the drawdown (m). It is a measure of well performance, indicating the pumping rate per unit drawdown.
Specific capacity has units of m³/day per metre of drawdown, often expressed as m²/day (since Q/s = (m³/day)/m = m²/day).
For a municipal well, specific capacity might range from 1 to 50 m²/day (or higher). A well with Sc < 1 m²/day is considered poor.
Specific capacity generally decreases with higher pumping rate due to non‑linear well losses (turbulence, friction). Thus, it is not a constant.
Approximately, T ≈ Sc (if the well is 100% efficient). In practice, T = Sc × (recovery factor) due to partial penetration, skin effects, etc.
It is used to evaluate the effectiveness of a well, compare different well designs, and estimate the sustainable yield.
- Aquifer properties – T and S.
- Well construction – screen length, slot size, and gravel pack.
- Well losses – due to turbulence and friction.
- Partial penetration – incomplete penetration of the aquifer.
Well losses reduce the effective drawdown, lowering Sc. A lower Sc indicates higher well losses and less efficient well.
By increasing the well diameter, screen length, using a gravel pack, and reducing well losses (e.g., using a submersible pump).
It is a good first‑order indicator, but it must be interpreted with care because it depends on the test conditions (pumping rate, duration) and well efficiency.