Pump Drawdown & Know the Safe Yield

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.