Every water well has one number that tells you how the well itself is doing, separate from the pump, the motor and the aquifer: specific capacity. It is the yield the well gives up for each foot of water level you pull down to get it, in gallons per minute per foot (gpm/ft). It is cheap to measure, it is the earliest honest sign of screen and gravel-pack plugging, and most districts still keep it in a spreadsheet nobody opens until the well is in trouble.
The arithmetic
Specific capacity = pumping rate ÷ drawdown, where drawdown is the pumping water level minus the static water level. A well that produces 900 gpm with a static level of 120 ft and a pumping level of 165 ft has 45 ft of drawdown and a specific capacity of 20 gpm/ft. Two years later the same well makes 900 gpm from 185 ft: drawdown 65 ft, specific capacity 13.8 gpm/ft. Nothing else about that well has to change for you to know it is 30% harder to get water out of it than it was.
| Test | Rate | Static | Pumping | Drawdown | Specific capacity |
|---|---|---|---|---|---|
| As built | 900 gpm | 120 ft | 165 ft | 45 ft | 20.0 gpm/ft |
| Year 2 | 900 gpm | 120 ft | 185 ft | 65 ft | 13.8 gpm/ft |
| Year 3 | 880 gpm | 124 ft | 201 ft | 77 ft | 11.4 gpm/ft |
Compare like with like
Specific capacity is not a constant. Drawdown has a linear part (aquifer loss) and a part that grows with the square of the pumping rate (well loss through the screen, pack and formation damage), the classic s = BQ + CQ² of a step-drawdown test. Pump a well harder and its specific capacity drops even when it is perfectly clean, and it keeps dropping slowly through the first hours of a test as the cone of depression develops. So a fair comparison holds three things constant: the pumping rate, the duration before the pumping level is read, and the season, because static levels ride the aquifer’s annual cycle.
When tests were run at different rates, the step-test coefficients let you normalize them to a reference rate. When you have no step test, at least compare tests within a few percent of the same rate and note the rest as uncertainty. Waterwell IQ™ stores the baseline per well, normalizes later tests to the baseline rate where the coefficients exist, and flags the comparison as approximate where they do not.
What a falling specific capacity means
- Mineral scale — calcium carbonate and iron or manganese oxides precipitating on the screen and in the pack as pressure drops at the well face. Common in hard, oxidizing groundwater; often the largest share of decline in Southern California wells.
- Biofouling — iron- and sulfur-related bacteria building slimes that trap fines. Shows up as a fast decline after a period of stability, sometimes with odour or turbidity complaints.
- Silt, sand and clay migration — fines moving into the gravel pack, especially in wells that were under-developed or pumped far above design.
- Screen corrosion or collapse — the structural failures a video survey confirms.
A falling static level with an unchanged specific capacity is a different problem: the aquifer is declining, and no amount of rehabilitation fixes that. Keep the two numbers apart and the likely cause usually narrows to one or two — which a video survey or a step-drawdown test can then settle.
When to act
Most districts that manage this deliberately use a threshold against the well’s baseline — as-built or post-rehabilitation — somewhere between a 15% and 25% decline, and act before the decline reaches 40%, because scale that has been in place for years mineralizes and recovers less on treatment. The right threshold for a given well depends on its energy cost, its role in the system and the recovery its past rehabilitations achieved; that is exactly the calculation the lifecycle model in Waterwell IQ runs, with a probability attached rather than a rule of thumb.
How often to measure
A sounder reading of static and pumping levels takes twenty minutes and needs no contractor. Quarterly is enough to catch a plugging trend early; monthly on wells that have plugged before or that carry most of the system’s peak demand. Record the rate at the same time — a flow meter reading, not the design rate — and the number becomes trustworthy.
See specific capacity, efficiency and the rehabilitation forecast for one of your own wells. Request a walkthrough →
Sources
- Water Replenishment District of Southern California. Specific Capacity — A Measure of Well Performance, Well Problems, and Aquifer Transmissivity, Technical Bulletin TB02 (Part 1 of 2). 2005. Source for specific capacity as gpm per foot of drawdown, its decline as perforations and filter pack plug, and the 25% decline commonly used as a rehabilitation trigger.
- U.S. Geological Survey. Factors Affecting Specific-Capacity Tests and their Application, SIR 2010-5212. 2010.
- California Department of Water Resources. Water Well Standards: State of California, Bulletin 74-81. 1981.
Every link above was retrieved and read on 2026-09-09. Paid standards are named by designation so you can check them against your own copy.