Pump Drawdown & Know the Safe Yield
Reads specific capacity
From a pumping or step-drawdown test, get the specific capacity, the well efficiency from the Jacob loss split, and the safe sustainable yield from the available drawdown — so you know if your bore is over-pumped.
Pumping / step-drawdown test
Next: run at up to about 6,667 Lpm — your target of 1,200 Lpm holds the drawdown (3.6 m) inside the safe line. Re-test annually to catch declining specific capacity early.
Specific capacity = Q ÷ drawdown. Safe yield keeps drawdown to 67% of the available column (static level → intake). Well efficiency uses the Jacob step-drawdown split s = B·Q + C·Q². Field estimate — confirm with a longer constant-rate test for design.
Well drawdown — key facts
- Specific capacity
- Q ÷ drawdown (Lpm/m)
- Step-test split
- s = B·Q + C·Q²
- Well efficiency
- B·Q ÷ (B·Q + C·Q²)
- Good efficiency
- > 70%
- Safe drawdown
- ≈ ⅔ of available column
- Safe yield
- Sc × safe drawdown
- Available column
- static level → pump intake
- Privacy
- Runs in your browser; nothing uploaded
A flow rate alone won't tell you if you're mining your own well
Knowing a borewell pumps so many litres a minute says little on its own — what matters is how far the water level has to fall to give that flow, and whether that fall leaves any reserve. Specific capacity, the yield per metre of drawdown, captures that in one number, and the drawdown your intended rate produces, compared against the column you actually have above the pump, tells you whether the bore can sustain it or whether you are slowly mining it dry. A step-drawdown test adds the why: how much of the drawdown is the aquifer simply not giving water fast enough, and how much is your own screen choking the flow.
This tool turns a pumping test into the figures that drive the decision: specific capacity, well efficiency from the Jacob s = B·Q + C·Q² split, the safe sustainable yield, and the drawdown your target rate will cause — with an over-pumping flag and a well cross-section that turns red when the pumped level crosses the safe line. Pair it with the Total Dynamic Head, NPSH Available and Solar Pump Sizing tools to match a pump to the yield you can safely take.
Specific capacity bands (indicative)
| Band | Specific capacity | What it means |
|---|---|---|
| Very low | < 5 Lpm/m | Tight/fractured rock — modest yields, drawdown sensitive. |
| Low | 5 – 20 Lpm/m | Marginal aquifer; pace pumping carefully. |
| Moderate | 20 – 60 Lpm/m | Typical productive borewell. |
| High | 60 – 150 Lpm/m | Good alluvial / sand aquifer. |
| Very high | > 150 Lpm/m | Highly transmissive aquifer. |
Indicative ranges; specific capacity varies widely with aquifer type. Source: generalised from Driscoll, “Groundwater and Wells”.
Well efficiency interpretation
| Efficiency | Class | What to do |
|---|---|---|
| > 70% | Good | Clean screen; most drawdown is real aquifer loss. |
| 50 – 70% | Fair | Some well loss; monitor and consider redevelopment. |
| < 50% | Poor | Large well loss; redevelop the screen to recover yield. |
Source: Jacob (1947); Kruseman & de Ridder, “Analysis and Evaluation of Pumping Test Data” (ILRI Pub 47).
How to use it
- 1. Enter the well geometry. The static water level (depth before pumping) and the pump intake depth.
- 2. Enter step 1. The pumping rate and the stabilised drawdown it produced.
- 3. Add step 2 (optional). A second, higher rate and its drawdown to get well efficiency from the Jacob split.
- 4. Set your target rate. The rate you want to run the pump at day to day.
- 5. Act on the verdict. Read specific capacity, the safe sustainable yield and whether your target over-pumps, then follow the next step.
Frequently Asked Questions
What is specific capacity and how is it calculated?+
Specific capacity is the well's yield per unit of drawdown: specific capacity = pumping rate Q ÷ drawdown s. If a well delivers 1,000 litres per minute at 3 metres of drawdown, the specific capacity is 333 Lpm per metre. It is the single best summary of how productive a well is, and watching it fall over the years is the clearest early warning that a well or its screen is declining.
How do I know if my borewell is over-pumped?+
Compare the drawdown your intended pumping rate produces against the safe drawdown — about two-thirds of the column between the static water level and the pump intake. If running at your target rate pulls the level below the safe line you are over-pumping, and if it reaches the intake the pump will break suction and can burn out. The tool draws both lines on a well cross-section and flags the moment the pumped level crosses the safe line.
What is the safe sustainable yield of a well?+
It is the rate the well can supply while keeping the water level above a safe drawdown, leaving a reserve so the pump never runs dry. The tool computes it as specific capacity × safe drawdown, where safe drawdown is two-thirds of the available column from the static level to the intake. For a 333 Lpm/m well with 30 m available, the safe drawdown is 20 m and the safe yield is about 6,667 Lpm.
What is a step-drawdown test and why do it?+
A step-drawdown test pumps the well at two or more increasing rates and records the drawdown at each, which lets you separate the two parts of drawdown: aquifer (formation) loss that is proportional to rate, and well loss through the screen that grows with the square of the rate. From that split you get the well efficiency. It is the test that tells you whether high drawdown is the aquifer's fault or a clogged screen's fault.
How is well efficiency calculated from the Jacob equation?+
Jacob (1947) writes drawdown as s = B·Q + C·Q², where B·Q is aquifer loss and C·Q² is well loss. Using two steps, C = (s₂/Q₂ − s₁/Q₁) ÷ (Q₂ − Q₁), then B = (s₁ − C·Q₁²) ÷ Q₁. Well efficiency is B·Q ÷ (B·Q + C·Q²). Above about 70% is good, 50–70% is fair, and below 50% means a large share of drawdown is wasted as well loss.
What does low well efficiency mean for me?+
It means much of your drawdown is turbulent loss as water forces through a partly clogged or poorly developed screen and gravel pack, not real aquifer limitation. The water is there; the well just struggles to let it in. Redeveloping the well — surging, jetting, or acidising the screen — often recovers specific capacity and lifts efficiency, which is far cheaper than drilling a new bore.
Why keep a third of the drawdown in reserve?+
Pumping all the way down to the intake leaves no margin for a dry spell, a falling regional water table, or a measurement error, and the pump can break suction and overheat if the level drops to it. Keeping drawdown to about two-thirds of the available column preserves a buffer and protects the pump. The tool uses that two-thirds rule for the safe drawdown and safe yield.
What specific capacity is good for an irrigation borewell?+
It depends entirely on the aquifer: tight fractured rock may give under 5 Lpm/m, a marginal aquifer 5–20, a typical productive bore 20–60, and a good alluvial sand aquifer 60–150 or more. The number matters less in the absolute than relative to its own history — a steady fall in specific capacity at the same drawdown signals decline regardless of the band it sits in.
Can I use a single pumping rate instead of two steps?+
Yes. One rate and its drawdown give the specific capacity and the safe sustainable yield, which is enough to judge over-pumping. You only need a second step to separate aquifer loss from well loss and report well efficiency. The tool lets you switch the second step off and still returns the yield analysis.
How accurate is this for designing a well?+
It is a sound field estimate for judging over-pumping and setting a pumping rate, using the standard specific-capacity and Jacob step-test relationships. For pump selection and long-term aquifer behaviour, a longer constant-rate test with recovery measurement and, ideally, an observation well gives transmissivity and storativity that a short test cannot. Treat the result as a working figure and confirm with a full test for design.
What is the difference between static and pumping water level?+
The static water level is the depth to water before pumping starts; the pumping (or dynamic) level is the lower level once the pump has been running and the cone of depression has formed. The difference between them is the drawdown. Specific capacity uses that drawdown, so an accurate static level and a stabilised pumping level are what make the result trustworthy.
Does specific capacity stay constant as I pump harder?+
Not exactly — because well loss grows with the square of the rate, specific capacity falls slightly as you pump harder, so the simple linear estimate of drawdown at a higher rate is a little optimistic on an inefficient well. For a clean, efficient well the drop is small. The step-test efficiency figure tells you how much the well-loss term is bending the relationship.