Specific capacity describes one observed operating point. Sustainable yield requires a suitable test, time history and local hydrogeologic judgment.
1. Establish one water-level reference
Mark a repeatable point at the casing. Static, pumping and recovery depths must use the same point and positive-down convention.
Record date, recent pumping, weather and nearby abstraction. A level called static immediately after use may still be recovering.
2. Measure flow and level at the same time
A pumping level has meaning only with its discharge rate and elapsed time. Log time, water level and measured flow together.
Route discharge so it cannot rapidly return to the well. Protect the casing from contamination and use suitable electrical protection.
3. Use specific capacity as an operating-point indicator
Specific capacity divides discharge by drawdown. Compare repeated tests only at similar duration and flow.
It is not a fixed aquifer constant. Well loss, screen condition, duration, construction and boundaries can change it.
4. Keep linear projections close to measured conditions
The calculator scales drawdown directly with flow. This nearby screen can be optimistic because well loss often grows faster than flow.
A negative water-cover margin fails the geometric screen. A positive margin still does not check cooling, sand, well bottom, screen position or transients.
5. Size the system from the full evidence
Use a test method and duration appropriate to the well and local requirements. Review recovery, water quality, seasonal history and neighboring wells with a qualified professional.
If peak demand exceeds a defensible source flow, separate source, bulk storage, booster delivery and pressure-tank cycling instead of forcing a larger pump onto the well.
Minimum pumping-test record
| Record | Why it matters | Common ambiguity |
|---|---|---|
| Reference point | Makes depths comparable | Ground or casing datum |
| Elapsed time | Shows continuing drawdown | One “final” level |
| Measured flow | Defines the operating point | Nameplate maximum |
| Water level | Quantifies drawdown | Static level still recovering |
| Recovery levels | Shows response after shutdown | No post-test observations |
Worked example
Screen one nearby flow
Static depth is 12 m. At 30 L/min, pumping depth is 18 m. Screen 40 L/min with intake at 35 m and 5 m protected cover.
Observed drawdown = 18 − 12 = 6 m Specific capacity = 30 / 6 = 5 L/min/m Screened drawdown = 40 / 5 = 8 m Screened level = 12 + 8 = 20 m Water-cover margin = 35 − 5 − 20 = 10 m
The geometry passes this simplified screen, but 40 L/min still needs testing. The 10 m margin does not prove sustainable yield or pump suitability.
What to have ready
- One marked water-level reference
- Level and flow measurements with elapsed times
- Static, pumping and recovery observations
- Well construction, intake and screen depths
- Local testing and installation requirements
Common mistakes
- Calling an incompletely recovered level static.
- Combining level and flow from different times.
- Extrapolating far beyond the tested flow.
- Treating specific capacity as sustainable yield.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.