Two tests get ordered on water wells more than any others, and they get confused with each other constantly. Both involve pumping the well and measuring water levels. Both produce a report full of curves. They answer completely different questions, and ordering the wrong one wastes a day of rig time and leaves the real question unanswered.
The short version: a step-drawdown test tells you about the well — how much of the water level you pull down is lost getting water through the screen and gravel pack. A constant-rate test tells you about the aquifer — how readily the formation gives up water, how far the cone of depression spreads, and whether there are boundaries out there. This note explains each, works the arithmetic on a step test, and says when to order which.
Drawdown has two parts
When a well pumps, the water level falls for two reasons. The aquifer has to deliver water toward the well, and that takes a gradient: aquifer loss. Then the water has to get through the formation right around the borehole, the gravel pack and the screen openings, fast, and that costs more head: well loss.
The two behave differently as the rate goes up. Aquifer loss grows roughly in proportion to the pumping rate. Well loss grows much faster, because flow through the screen zone is turbulent — roughly with the square of the rate. That is the relationship C. E. Jacob wrote down in 1947, and it is still how step tests are read:
s = BQ + CQ²
s is drawdown in feet, Q the pumping rate in gpm, BQ the aquifer loss and CQ² the well loss.
Divide both sides by Q and it becomes a straight line: s ÷ Q = B + CQ. Plot drawdown per gpm against the rate, and the intercept is B and the slope is C. That is the whole point of stepping the rate.
The step-drawdown test
The well is pumped at three or more increasing rates, each held long enough for the water level to settle, with drawdown measured at every step. The EPA’s procedure for aquifer tests describes it as three or more steps of increasing discharge, analyzed to estimate the well entry losses. Here is one, from a well that makes 1,000 gpm at 50 feet of drawdown:
| Step | Rate, gpm | Drawdown, ft | ft per gpm | Efficiency | gpm per ft |
|---|---|---|---|---|---|
| 1 | 500 | 21.3 | 0.0425 | 82% | 23.5 |
| 2 | 750 | 34.7 | 0.0463 | 76% | 21.6 |
| 3 | 1,000 | 50.0 | 0.0500 | 70% | 20.0 |
| 4 | 1,250 | 67.2 | 0.0538 | 65% | 18.6 |
Well efficiency is the share of the drawdown that the aquifer itself demands — BQ ÷ s. At 1,000 gpm it is 35 feet of 50, or 70%. The other 15 feet is the price of getting water through the well.
Three things fall out of that table that no other test gives you:
- How much of the drawdown is the well’s fault. Fifteen feet at design rate. That is the part rehabilitation can win back; the aquifer’s 35 feet it cannot.
- Why specific capacity depends on the rate. It falls from 23.5 to 18.6 gpm/ft across the steps with nothing wrong at all. This is exactly why specific capacity has to be compared at the same rate.
- Where the design rate should sit. Past a point, each extra gpm costs disproportionately more drawdown, more lift and more energy — and pulls the pumping level toward the top of the screen.
The same well, plugged
Now run the same step test on the same well after years of scale and biofouling have built up on the screen. The aquifer has not changed, so B is the same. The well loss coefficient has doubled:
| At 1,000 gpm | Clean | Plugged |
|---|---|---|
| Aquifer loss, BQ | 35 ft | 35 ft |
| Well loss, CQ² | 15 ft | 30 ft |
| Total drawdown | 50 ft | 65 ft |
| Well efficiency | 70% | 54% |
| Specific capacity | 20.0 gpm/ft | 15.4 gpm/ft |
This is the most useful thing a step test does for a district that already owns the well. Run one before rehabilitation and one after, and the change in C is the measured result of the work — not a before-and-after specific capacity taken at two different rates, which proves very little. Together with a static level trend, it is the cleanest way to tell a plugging well from a declining aquifer.
The constant-rate test
A constant-rate test pumps the well at one steady rate for a long time — the EPA procedure recommends at least 24 hours, and longer where possible — while water levels are measured in the pumping well and, ideally, in one or more observation wells nearby. The procedure also calls for a background period of at least a week of water levels before the test, and for recovery to be measured after the pump stops just as carefully as the drawdown.
What it tells you is about the formation, not the well:
- Transmissivity — how readily the aquifer transmits water across its thickness. It governs how the cone of depression develops and how neighboring wells interfere.
- Storativity — how much water the aquifer releases per foot of head decline. It needs an observation well; the pumping well alone cannot give it reliably.
- Boundaries and leakage — a fault, the edge of the basin, or a leaky confining layer shows up as a bend in the drawdown curve over time.
- A sustainable yield — whether the well can hold that rate without the water level still falling at the end.
That is the test for siting a new well, sizing a new pump for sustained duty, assessing interference with neighbors, or meeting a permit or a groundwater agency requirement. It is expensive in time and water: days of pumping, a place to put the water, and often a discharge permit.
Which one to order
| The question | The test |
|---|---|
| How efficient is this well, and how much would rehabilitation win back? | Step-drawdown |
| Did the rehabilitation work? | Step-drawdown, before and after |
| What rate should the new pump be designed for? | Step-drawdown, then a constant-rate test at the chosen rate |
| What are the aquifer’s properties here? Will this well interfere with its neighbors? | Constant-rate, with observation wells |
| Is the well’s decline the well or the aquifer? | Step-drawdown, plus the static level trend |
For a new production well the standard sequence is both: the step test first, to pick the rate, then the constant-rate test at that rate. For a district’s existing wells, the step test is usually the one that pays — it measures the part of the problem money can fix.
What to do with this on Monday
- Find out which of your wells have ever had a step test, and pull the B and C values if the report gives them.
- Schedule a step test before any rehabilitation you are planning, and another after it. Write the “after” test into the rehabilitation contract.
- Record the rate of every test alongside the drawdown, so specific capacities can be compared fairly.
- Save the constant-rate test for the questions only it can answer — new wells, interference, aquifer properties, permits.
Quick answers
What is a step-drawdown test?
A test in which a well is pumped at three or more increasing rates, with drawdown measured at each. Plotting drawdown per gpm against rate separates aquifer loss from well loss, which gives the well’s efficiency and shows how much of its drawdown rehabilitation could recover.
What is well efficiency?
The share of the total drawdown caused by the aquifer itself, as opposed to losses at the screen, gravel pack and borehole wall. A well with 35 feet of aquifer loss and 15 feet of well loss is 70% efficient at that rate.
How long should a constant-rate pumping test run?
The EPA’s procedure recommends at least 24 hours and longer where possible, with at least a week of background water levels before the test and recovery measured after it.
See the step tests on your own wells read into well loss, aquifer loss and efficiency. Request a walkthrough →
Sources
- U.S. Environmental Protection Agency. Suggested Operating Procedures for Aquifer Pumping Tests, EPA/540/S-93/503. 1993. Source for step-drawdown tests as three or more steps of increasing discharge analyzed for well entry losses, constant-rate tests of at least 24 hours for transmissivity, storage and leakage, at least one week of background water levels, and recovery measurements.
- Jacob, C. E. Drawdown Test to Determine Effective Radius of Artesian Well. Transactions of the American Society of Civil Engineers, vol. 112, 1947. Source for drawdown as the sum of a term proportional to the rate and a term proportional to its square.
- Driscoll, F. G. Groundwater and Wells, 2nd edition. Johnson Division, 1986. Source for well loss, aquifer loss and well efficiency as used in practice.
Every link above was retrieved and read on 2026-09-26. Paid standards are named by designation so you can check them against your own copy.