Well Sustainable Yield from Specific Capacity
The inverse of the well-drawdown tile: the sustainable pumping rate a well gives up without pulling the water below the pump, max_yield = specific_capacity x allowable_drawdown. A 1.0 GPM/ft well with 30 ft of usable drawdown (static to a safe level above the pump) yields 30 GPM; a marginal 0.3 GPM/ft well with 40 ft yields only 12 GPM. Answers 'how hard can I pump it' instead of the specific capacity from one test. Specific capacity declines with rate, so confirm with a constant-rate test. AWWA A100 / USGS; the well driller governs.
Formula and source
max_yield_gpm = specific_capacity_gpm_ft x allowable_drawdown_ft. The inverse of specific_capacity = discharge / drawdown.
AWWA A100 (Water Wells) standard; USGS well-testing methods (USGS Open-File Report 02-197).
Audience
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