Bore Yield vs Drawdown Calculator
Specific capacity is defined as yield divided by drawdown: SC = Q ÷ s, where Q is the pumping rate (L/s) and s is the drawdown (m) at that rate.
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How to use this calculator
- Enter the steady bore yield (pumping rate) in litres per second and the drawdown (how far the water level fell below rest level) in metres, both measured at the same steady point in a pumping test.
- Read the specific capacity in L/s per m — a higher figure means a more productive bore. The tool also converts the yield to m³/h and m³/day.
- Optionally enter pumping hours per day to estimate the daily volume the bore can supply for a dewatering or water-supply plan.
How it works
Specific capacity is defined as yield divided by drawdown: SC = Q ÷ s, where Q is the pumping rate (L/s) and s is the drawdown (m) at that rate. It expresses how many litres per second the bore delivers for each metre the water level is drawn down, so it is a simple, comparable measure of bore productivity and efficiency. Yields are converted to volume rates using 1 L/s = 3.6 m³/h = 86.4 m³/day.
The calculation assumes a single well at (or near) steady state, and that yield and drawdown were read at the same pumping step. In real aquifers specific capacity is not a fixed constant — it tends to fall as the pumping rate rises (drawdown grows faster than yield) and it drifts with test duration, well losses and turbulence near the screen. Treat the result as a screening estimate for comparing bores or sizing dewatering, not as a substitute for a full pumping-test analysis (e.g. Theis/Cooper–Jacob) and a competent hydrogeologist's design.
Worked example
Bore pumping 8 L/s with 12.5 m of drawdown. A dewatering bore is pumped at a steady 8 L/s and the water level draws down 12.5 m below its rest level. Specific capacity = 8 ÷ 12.5 = 0.64 L/s per m — so every metre of drawdown 'buys' about 0.64 L/s of yield. The bore delivers 28.8 m³/h, or 691.2 m³/day if run continuously. A second bore giving the same 8 L/s at only 6 m of drawdown would have a specific capacity of 1.33 L/s per m and is the more productive (efficient) bore.
Common mistakes
- Confusing drawdown with total water-level depth. Drawdown is only the DROP from the rest (static) level to the pumping level — not the depth to water or the bore depth.
- Reading yield and drawdown from different pumping steps. Specific capacity must use the yield and drawdown measured at the same steady pumping rate, or the ratio is meaningless.
- Assuming specific capacity stays constant as you pump harder. Drawdown usually grows faster than yield, so specific capacity falls at higher rates — don't extrapolate a low-rate figure to a much larger design yield.
Frequently asked questions
What is a good specific capacity for a bore?
There is no universal 'good' value — it depends entirely on the aquifer. A highly transmissive sand or fractured-rock aquifer might give several L/s per m, while a tight formation may give well under 0.1 L/s per m. The number is most useful for comparing bores in the same area or tracking whether a bore's performance is declining (e.g. from screen clogging) over time.
Why can't drawdown be zero?
Specific capacity divides yield by drawdown, so a zero drawdown would mean dividing by zero (an infinite, meaningless result). In practice any real pumping produces some drawdown; if you measured zero, the reading is wrong or the water level had not yet responded. The tool requires a drawdown greater than zero.
Does this account for well losses or aquifer type?
No. This is the simple ratio of yield to drawdown and treats the bore as a single steady-state well. It does not separate aquifer (laminar) losses from well (turbulent) losses, nor does it fit aquifer parameters like transmissivity or storativity. For those, analyse a proper multi-rate or time-drawdown pumping test with a hydrogeologist.
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