Wells, Yield, Drawdown & Well Operation

Learn well operation fundamentals, including static and pumping water levels, drawdown, specific capacity, well yield, recovery, pump setting, well interference, overpumping, sand production, and troubleshooting.

A drinking-water well must provide adequate quantity and acceptable water quality without being pumped beyond the sustainable capability of the well and aquifer. Operators should understand water levels, drawdown, well yield, specific capacity, pump setting, recovery, and long-term performance trends.

A decline in well production does not always mean the aquifer has permanently failed. The cause may be aquifer conditions, well-screen plugging, pump wear, excessive pumping rate, interference from nearby wells, or changing groundwater levels.

Basic Well Components

A typical groundwater well may include:

well casing;
well screen or open-hole production interval;
grout and sanitary seal;
pump;
drop pipe;
water-level measurement access;
wellhead protection components.

Well Casing

The well casing provides structural support and helps isolate the well from shallow formations and surface contamination.

Well Screen

A well screen allows groundwater to enter the well while helping limit entry of formation material.

Screen performance can be affected by:

mineral scale;
biological growth;
corrosion products;
fine sediment.

Open-Hole Wells

Some rock wells obtain water directly from fractures or openings in exposed rock below the casing.

Production can depend strongly on the number and size of water-bearing fractures intersected by the well.

Static Water Level

Static water level is the water level in the well when the pump is not operating and the well has had enough time to recover toward equilibrium.

Static water level provides useful information about:

aquifer conditions;
seasonal groundwater changes;
long-term groundwater decline;
recovery after pumping.

Pumping Water Level

Pumping water level is the water level measured while the well is operating at a specified pumping rate.

It is normally lower than static water level because pumping reduces hydraulic head around the well.

Drawdown

Drawdown is the decline in water level caused by pumping.

When levels are expressed as depth below a common reference point:

Drawdown = Pumping Water Level Depth - Static Water Level Depth

Drawdown Example

Static water level is 35 feet below the measuring point.

Pumping water level is 60 feet below the same point.

Drawdown = 60 - 35

Drawdown = 25 feet

Why Drawdown Matters

Drawdown helps operators evaluate how the well and aquifer respond to pumping.

Increasing drawdown at the same pumping rate can indicate:

declining groundwater level;
well-screen plugging;
formation plugging;
interference from nearby wells;
changing aquifer conditions.

Well Yield

Well yield is the rate at which a well can produce water under specified operating conditions.

Yield may be expressed as:

gpm;
MGD;
another flow unit.

Yield Is Not Just Pump Capacity

A pump may be capable of producing more flow than the well can sustainably supply.

Safe well operation must consider:

aquifer response;
drawdown;
pump setting;
water quality;
recovery.

Pumping Rate

Pumping rate is the actual rate at which water is withdrawn from the well.

A higher pumping rate generally causes greater drawdown.

Specific Capacity

Specific capacity is a useful indicator of well productivity.

A common relationship is:

Specific Capacity = Pumping Rate ÷ Drawdown

Typical units are:

gpm per foot of drawdown

Specific Capacity Example

A well pumps 500 gpm with 25 feet of drawdown.

Specific Capacity = 500 ÷ 25

Specific Capacity = 20 gpm/ft

Interpreting Specific Capacity

If specific capacity decreases significantly over time at similar operating conditions, possible causes include:

screen plugging;
mineral deposits;
biological fouling;
aquifer decline;
changes in nearby pumping.

Trend Specific Capacity, Not Just Flow

A well may continue producing the same flow while requiring much more drawdown.

That condition can indicate declining performance even though production appears unchanged.

Example of Declining Performance

Initial condition:

flow = 600 gpm;
drawdown = 20 ft.

Specific Capacity = 600 ÷ 20 = 30 gpm/ft

Later condition:

flow = 600 gpm;
drawdown = 40 ft.

Specific Capacity = 600 ÷ 40 = 15 gpm/ft

The well still produces 600 gpm, but specific capacity has decreased by half.

Cone of Depression

Pumping lowers groundwater levels around the well and creates a cone of depression.

The largest decline occurs near the pumping well.

Well Interference

If nearby wells pump from the same aquifer, their cones of depression may overlap.

This can cause:

additional drawdown;
lower pumping water levels;
reduced well capacity.

Interference Example

A well may operate normally when nearby wells are off but experience greater drawdown when several wells run simultaneously.

This does not necessarily mean the well itself is mechanically damaged.

Pump Setting

Pump setting refers to the depth at which the pump intake is installed in the well.

The pump intake should remain adequately submerged during normal operation.

Insufficient Submergence

If pumping water level approaches the pump intake, the pump may experience:

air entry;
loss of capacity;
unstable flow;
damage.

Do Not Lower a Pump Without Understanding the Cause

Lowering a pump may provide additional submergence but does not correct:

aquifer depletion;
screen plugging;
excessive pumping rate;
well interference.

Well Recovery

Recovery is the rise in water level after pumping stops.

Recovery data can provide information about aquifer and well response.

Recovery Time

A well that recovers slowly may indicate:

limited aquifer transmissive capacity;
high previous pumping stress;
regional groundwater decline;
well impairment.

Measure Recovery Consistently

Useful recovery records include:

time pumping stopped;
water level at shutdown;
water levels at defined time intervals;
final recovered level.

Static Level Must Be Truly Static

A water-level measurement taken immediately after pump shutdown may still represent recovery rather than true static conditions.

Pumping Test

A pumping test evaluates how a well and aquifer respond to controlled pumping.

Measurements may include:

pumping rate;
water level;
drawdown;
recovery;
time.

Step-Drawdown Test

A step-drawdown test uses several increasing pumping rates.

Operators and engineers can use the results to evaluate how drawdown changes as pumping rate increases.

Constant-Rate Test

A constant-rate test pumps the well at a relatively steady flow while water levels are monitored over time.

Why Pumping Tests Matter

Pumping tests can help determine:

well capacity;
aquifer response;
appropriate operating rate;
well efficiency trends.

Overpumping

Overpumping means withdrawing water at a rate that creates unacceptable well or aquifer conditions.

Possible consequences include:

excessive drawdown;
loss of pump submergence;
sand production;
changes in water quality;
reduced long-term well performance.

Signs of Possible Overpumping

Possible indicators include:

rapidly falling pumping level;
unstable flow;
air in discharge water;
increased turbidity;
sand production;
reduced specific capacity.

Sand Production

A properly operating well should not continuously produce excessive sand.

Possible causes include:

screen damage;
improper screen size;
excessive pumping rate;
formation instability;
well-development problems.

Why Sand Is a Problem

Sand can:

damage pumps;
wear valves;
increase turbidity;
accumulate in tanks;
interfere with treatment equipment.

Sudden Turbidity Increase

If a normally clear well suddenly becomes turbid, review:

pumping rate;
recent pump changes;
well construction condition;
water-level changes;
surface flooding;
sampling and analytical accuracy.

Well-Screen Fouling

Well screens can lose capacity because of:

mineral scale;
iron deposits;
manganese deposits;
biological growth;
fine sediment.

Common Signs of Screen Fouling

At the same pumping rate, operators may observe:

greater drawdown;
lower specific capacity;
longer recovery time.

Well Rehabilitation

Depending on the cause and well design, rehabilitation may involve specialized mechanical or chemical methods.

Rehabilitation should follow appropriate engineering and facility procedures.

Pump Wear Versus Well Decline

Reduced discharge flow can result from:

pump wear;
motor problems;
valve problems;
well or aquifer decline.

Water-level data help distinguish these causes.

Example: Pump Problem

If:

well water levels are normal;
drawdown is normal;
flow has decreased;

the pump, motor, discharge piping, or flow measurement may deserve closer inspection.

Example: Well or Aquifer Problem

If:

flow remains similar;
pumping water level is much lower;
drawdown has increased;

well or aquifer performance may have declined.

Static Level Decline

If both static and pumping water levels decline over time, possible causes include:

drought;
regional groundwater decline;
increased aquifer pumping;
reduced recharge.

Pumping Level Declines but Static Level Is Stable

If static level remains similar while pumping level becomes deeper at the same flow, possible causes include:

well-screen plugging;
near-well formation plugging;
well efficiency loss.

Well Cycling

Frequent pump starts and stops can:

increase mechanical wear;
increase electrical stress;
create unstable system pressure.

Long Pump Runs

Long pump runs may be normal under high demand, but unexpected increases in run time can indicate:

lower well flow;
higher system demand;
storage problems;
pump deterioration.

Flow Meter Verification

Well-performance calculations depend on accurate flow measurement.

An incorrect flow meter can produce misleading:

yield;
specific capacity;
production records.

Water-Level Measurement

Water-level measurements should use a consistent reference point.

Changing reference points can create false trends.

Record the Measurement Reference

Useful records include:

reference elevation or measuring point;
date and time;
pump status;
pumping rate;
water level.

Well Production

Total daily production can be calculated from flow and operating time.

For constant flow:

Volume = Flow Rate × Time

Production Example

A well pumps 500 gpm for 8 hours.

Convert time:

8 hr × 60 min/hr = 480 min

Then:

Volume = 500 gpm × 480 min

Volume = 240,000 gallons

Average Daily Well Production

Production records help operators evaluate:

source contribution;
pump run time;
seasonal demand;
changes in well capacity.

Multiple Wells

Systems with multiple wells should compare:

flow;
drawdown;
specific capacity;
energy use;
water quality;
recovery.

Rotate Wells When Appropriate

Where system design allows, operating different wells can:

balance source use;
provide recovery time;
reduce excessive stress on one well.

Do Not Rotate Wells Blindly

Wells may differ in:

water quality;
capacity;
treatment requirements;
energy efficiency.

Well Interference During Multiple-Well Operation

When several wells operate simultaneously, compare water levels with single-well operation.

Additional drawdown may indicate hydraulic interference.

Wellhead Inspection

Routine well inspection should include:

casing condition;
sanitary seal;
vents;
drainage;
electrical components;
leaks;
security.

Surface Drainage

Water should not routinely pond around the wellhead.

Surface drainage should reduce the opportunity for contaminated water to move toward the well casing.

Flooding

Flooding around a well can create contamination risk and should be handled according to applicable facility procedures.

Water-Quality Changes During Pumping

Longer or higher-rate pumping can sometimes change:

iron;
manganese;
conductivity;
turbidity;
temperature.

Why Water Quality Can Change

Changes may occur because pumping alters:

the contributing portion of the aquifer;
fracture flow;
groundwater flow direction;
mixing of groundwater zones.

Energy Use as a Performance Indicator

Increasing energy use per unit water produced can indicate:

greater pumping lift;
pump wear;
hydraulic restrictions;
declining well performance.

Trend Energy with Well Data

Useful comparisons include:

kWh;
gallons produced;
pumping water level;
flow rate.

Unexpected Well Shutoff

If a well shuts down unexpectedly, review:

power supply;
motor overload;
low-water protection;
pressure controls;
level controls;
pump condition.

Low-Water Protection

Some systems automatically stop a well pump before water level reaches an unsafe point.

Repeated low-water shutdowns should be investigated rather than simply reset.

Common Well Operation Mistakes

Confusing static water level with pumping water level.
Ignoring drawdown trends.
Using pump capacity as if it were sustainable well yield.
Looking only at flow and ignoring specific capacity.
Ignoring interference from nearby wells.
Lowering a pump without investigating why water level declined.
Ignoring sand production.
Using inconsistent water-level reference points.
Ignoring flow-meter accuracy.
Assuming lower production always means aquifer failure.
Ignoring changes in well water quality during pumping.

A Practical Well Performance Review

Measure static water level after adequate recovery.
Operate the well at a known flow.
Measure pumping water level.
Calculate drawdown.
Calculate specific capacity.
Compare with historical values.
Review recovery after shutdown.
Review turbidity and other water-quality indicators.
Review pump and motor performance.
Document operating conditions.

A Practical Declining-Yield Review

Verify the flow measurement.
Review static water level.
Review pumping water level and drawdown.
Calculate specific capacity.
Compare nearby well operation.
Review pump amperage and energy use.
Inspect for sand or turbidity.
Review well-screen or formation fouling history.
Determine whether the problem is pump-related, well-related, or aquifer-related.

A Practical Excessive-Drawdown Review

Verify water-level measurement.
Verify pumping rate.
Compare current static level with historical values.
Check whether nearby wells are operating.
Review recent drought and recharge conditions.
Review specific-capacity trend.
Reduce pumping rate if required by approved operating procedures.
Investigate well rehabilitation or aquifer conditions if the problem persists.

What to Remember for the Exam

Static water level is measured when the well is not pumping and has recovered toward equilibrium.
Pumping water level is measured while the well operates.
Drawdown is the decline from static water level to pumping water level.
When depths below the same reference point are used, drawdown equals pumping-level depth minus static-level depth.
Well yield is the rate at which a well can produce water under specified conditions.
Pump capacity is not necessarily the same as sustainable well yield.
Specific Capacity = Pumping Rate ÷ Drawdown.
Specific capacity is commonly expressed as gpm per foot of drawdown.
A declining specific capacity can indicate well fouling, aquifer decline, or interference.
Pumping creates a cone of depression around a well.
Nearby wells can interfere with one another when their cones of depression overlap.
The pump intake must remain adequately submerged during operation.
Recovery is the rise in water level after pumping stops.
Overpumping can cause excessive drawdown, sand production, loss of submergence, and reduced well performance.
Sand production can indicate screen, formation, or pumping-rate problems.
If static level is stable but drawdown increases at the same flow, well efficiency may be declining.
If both static and pumping levels decline, broader aquifer or recharge conditions may be involved.
Flow and water-level measurements must use accurate instruments and consistent reference points.
Production volume equals flow rate multiplied by operating time.
Good well operation requires trending flow, static level, pumping level, drawdown, specific capacity, recovery, water quality, and pump performance together.