Specific Capacity of a Well

Specific capacity is the field number that tells you whether a well is any good: the yield divided by the drawdown that produces it. A well giving 1000 m³/d with 4 metres of drawdown has a specific capacity of 250 m³/d per metre. It takes one flow measurement and one water level, which is why it appears on drillers' logs going back a century and why it is the first thing to look at when comparing wells in the same formation.

Rearrange the Thiem equation and you can see what it is really measuring: $Q/s = 2\pi T/\ln(R/r)$, so specific capacity is proportional to transmissivity with only a weak logarithmic dependence on geometry. That gives a genuinely useful field estimate, and the rough rule in US practice is that transmissivity in gpd/ft is about 2000 times the specific capacity in gpm/ft. It is an estimate rather than a measurement, because part of the drawdown at the well is well loss rather than aquifer loss.

That split is what the number is best used for over time. Aquifer loss rises in proportion to the flow, but well loss rises roughly with the square of it, so a step-drawdown test at three or four rates separates the two and tells you how efficient the well itself is. A well above 70 percent efficiency is in decent shape. The real value is the trend: specific capacity that has fallen 25 percent over a few years is the standard trigger for rehabilitation, and the cause is almost always incrustation, biofouling or sand plugging of the screen rather than the aquifer failing. Two cautions on comparing values. Specific capacity is not constant, since it declines as the drawdown deepens and is still declining while the cone expands, so quote the pumping duration alongside it. And a partially penetrating well reads low for reasons that have nothing to do with the aquifer's quality.