17.3 Static Level, Pumping Level, and Drawdown
Key Takeaways
- Static and pumping water levels are meaningful only with the measuring point, datum, units, pump status, pumping rate, elapsed time, and nearby-well conditions recorded.
- When values are depths below one datum, drawdown equals pumping-level depth minus static-level depth; when values are elevations, subtract pumping elevation from static elevation.
- Specific capacity is pumping rate divided by drawdown and should be trended only under comparable test and operating conditions.
- WPI gives no universal acceptable drawdown; safe operation depends on well construction, pump setting, available submergence, aquifer response, and authority-approved limits.
Two levels require one reference
The WPI Class I outline explicitly requires operators to measure static water level and pumping level in wells. A static water level is measured under a defined nonpumping condition after the well has had the specified opportunity to recover. It may still reflect nearby pumping, recharge, barometric effects, tides, or a regional trend, so record conditions rather than assuming perfect equilibrium. A pumping water level is measured while the well discharges at a stated rate and elapsed pumping time. Because drawdown changes with time, a pumping level without rate and time is incomplete.
Establish one durable measuring point (MP) and its relation to the chosen datum, such as land-surface datum or a surveyed elevation. Depth below the MP increases as water falls; water-level elevation decreases as water falls. Mixing a depth from the top of casing with an elevation referenced to another datum creates a false change.
| Record item | Why it matters |
|---|---|
| MP and datum correction | Places every measurement on the same vertical reference |
| Date, time, units, and method | Makes the result traceable and comparable |
| Pump status, rate, and elapsed time | Defines static, pumping, or recovery condition |
| Nearby-well and source conditions | Identifies interference, recharge, drought, or flood effects |
| Recent maintenance or access change | Reveals changed casing reference, airline, pump, or instrumentation |
USGS field guidance describes steel-tape measurement from an established MP and correction to the selected datum. Electric tapes, pressure transducers, and air lines may also be used under approved procedures. Each has limitations: tape can hang on equipment; electric probes can respond to moisture or conductive material; transducers can drift; and an air line depends on correct installation and pressure interpretation. Verify the device and repeat a questionable reading. Protect sanitary integrity—use approved access, clean or disinfect equipment as required, replace caps and seals, and never drop unclean tools into a potable-source well. Follow electrical, fall, confined-space, and chemical safety procedures for the site.
Calculate drawdown without a sign error
Drawdown is the decline caused by pumping under the stated conditions. When measurements are depths below the same MP or datum:
Drawdown = pumping-level depth − static-level depth
A well has a static depth of 28.0 feet and, after the stated pumping period, a pumping depth of 43.0 feet. Drawdown is 43.0 − 28.0 = 15.0 ft. In metric units, a static depth of 8.5 meters and pumping depth of 13.1 meters gives 13.1 − 8.5 = 4.6 m.
When records use water-level elevations instead, the order reverses because the pumping elevation is lower:
Drawdown = static water-level elevation − pumping water-level elevation
For example, static elevation 512.4 ft minus pumping elevation 497.4 ft is again 15.0 ft. Do not subtract a depth from an elevation until both have been converted to the same reference representation.
Specific capacity is pumping rate divided by drawdown. It is a practical performance indicator, not a universal pass/fail limit. At 420 gpm and 15.0 ft of drawdown, specific capacity is 420 ÷ 15.0 = 28 gpm/ft. At 30 L/s and 4.6 m, it is 30 ÷ 4.6 = 6.5 L/s/m after rounding. The current WPI formula table supplies the exam's conversions but does not list a dedicated specific-capacity equation; understand the rate-per-unit-drawdown relationship and use the units stated in the question.
Compare matched operating conditions
A larger drawdown is expected when pumping rate or pumping duration increases. Therefore, compare specific capacity and drawdown at similar rate, elapsed time, antecedent recovery, season, and nearby-well status. Also trend static level separately. The pattern helps select the next check:
| Observed comparable trend | Possible interpretation—not a diagnosis |
|---|---|
| Static level falls; drawdown at the same rate stays similar | Regional or seasonal water-level decline, recharge change, or interference |
| Static level similar; drawdown rises at the same rate and time | Increased well loss, screen or formation plugging, measurement error |
| Levels and specific capacity similar; delivered rate falls | Pump wear, power, valve, meter, or discharge-system issue |
| Pumping level oscillates unexpectedly | Variable rate, cascading water, control cycling, air-line or sensor problem |
USGS explains that well yield depends on the aquifer, well, and pump. These observations separate hypotheses; they do not authorize an operator to acidize, redevelop, pull a pump, or increase speed without engineering review and the approved maintenance process. Water-quality changes such as turbidity, sand, color, or conductivity should be trended with the hydraulic data.
Scenario: unchanged drawdown, less available head
A well formerly measured 25 ft static and 40 ft pumping at 400 gpm after the defined duration: 15 ft drawdown and 400 ÷ 15 = 26.7 gpm/ft. Later, comparable measurements are 30 ft static and 45 ft pumping at 400 gpm: drawdown and specific capacity are unchanged, but both levels are 5 ft deeper. The pattern points first toward a background water-level shift or interference rather than a new increase in well loss. Check season, nearby withdrawals, recovery, measurement quality, source trends, and the well's approved lowest operating level.
WPI establishes no universal safe drawdown or minimum specific capacity. The lowest practical pumping level depends on screen or open-hole position, pump intake and required submergence, well construction, aquifer behavior, water quality, and authority-approved operating criteria. If pumping level approaches a site limit, flow becomes unstable, air enters, sand increases, or water quality changes, follow the operating plan: verify rate and level, protect equipment and water quality, notify the responsible role, reduce or stop pumping when the procedure requires, and document the event.
Official source trail
A well's static level is 21.6 ft below the measuring point and its pumping level is 37.9 ft below the same point. What is drawdown?
A well pumps 360 gpm with 18 ft of drawdown under a defined test condition. What is its specific capacity?
Static water level is lower than last year, but drawdown and specific capacity at the same rate and elapsed time are nearly unchanged. What is the best interpretation?