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    WELL DRAWDOWN

 

What is a drawdown?

Drawdown is the drop in the level of water in a well when water is being pumped. Drawdown is usually
measured in feet or meters.

Why should drawdown be measured?

One of the most important reasons for measuring drawdown is to make sure that the source water is
adequate and not being depleted. The data that is collected to calculate drawdown can indicate if the water
supply is slowly declining. Early detection can give the system time to explore alternative sources, establish
conservation measures or obtain any special funding that may be needed to get a new water source.

Drawdown measurements give important information about the performance and efficiency of wells.
Drawdown data can be combined with well yield to evaluate the efficiency and performance of a well.

Drawdown measurements can also help detect some other problems in their early stages. For example
accurate drawdown measurements can be used with well yield data to detect the plugging of a well screen
by encrustation or if a pump needs adjustment.

How is drawdown measured?

Accurate drawdown measurements depend on finding the distance from the surface to the water level in a
well.

How often is drawdown measured?

Drawdown should be measured annually. During drought years the drawdown should be measured during
the dry time to see if the well is keeping with the demand.

SEVERAL KEY TERMS ASSOCIATED WITH WELL DRAWDOWN

Static level – Static level is the level of water in a well when no water is being taken from the well pumps.
It is usually expressed as the distance in feet or meters from the ground surface to the water level.

Pumping level – Pumping level is the level of water in the well during pumping. This too, is usually
expressed as the distance in feet or meters from the ground surface to the water level.

Drawdown – Drawdown is the drop in level of water in a well when water is being pumped. Drawdown
measurements record the difference (in feet or meters) between the static level and the pumping level.

Well yield – Well yield is the volume of water per unit of time that is produced from the well pumping.
Usually, well yield is measured in terms of gallons per minute (gpm) or gallons per hour (gph). Sometimes,
large flows are measured in cubic feet per second (cfs).

Specific capacity
– Specific capacity is expressed as the well yield per unit of drawdown. For example, if
the well yield is 100 gpm and the drawdown is 10 ft, the specific capacity of the well is 10 gpm per foot of
drawdown.

METHODS FOR MEASURING DRAWDOWN

There are four different methods for measuring drawdown.
1.) Transducers
2.) Electric sounder method
3.) Air line method
4.) Wetted tape method

1.) Transducers

A more recent application of technology to well testing has been the development of transducers for the
measuring depth to water. A transducer is a device that converts input energy of one form into output
energy of another.
Transducers work by sending information to equipment above ground level about the pressure of the
column of water over them. If the exact depth of the transducer is known the information can be used to
find the water level at any time.

Acoustic well sounder

An acoustic well sounder uses sound waves to measure the depth to water level. This device operates by
bouncing sound waves off the surface of the water.
Advantages of the acoustic well sounder are that it is easy to operate, gives instant data and cannot
contaminate the well (that is, no measuring devices touch the water and there are no probes or wires to
hang up).

2.) Electric sounder method

The electric sounder method is an accurate method of determining the depth to water level. A basic sounder
consist of an electrode, a dry-cell battery, a pair of insulated wires marked at regular intervals (usually
every 5 ft), weights and a milliampere meter (or some other device to show the flow of current such as a
bell or light.)

One end of the pair of insulated wires is connected to a drycell battery and a milliampere meter. The
electrode contains the exposed ends of the wires separated by an air gap. Several manufacturers sell electric
sounders.

Operating an electric sounder

An electric sounder works by lowering the electrode into water. When the electrode contacts water, a
circuit is completed and the current flow registers on a light, bell or a meter.

Slowly lower the electrode into the well until the milliampere meter detects a steady current. Lower the line
another 10 feet to see if the meter continues to register. If it doesn’t, or if it fluctuates, the electrode may
not have reached the water level. (Something else, such as cascading water from the well screen may be
interfering.)

If the milliampere meter does not fluctuate after you have lowered it another 10 feet, pull the wire back to
the first point where the current registered. This is the water level.

To find the depth to water:
1. Hold the place on the wire where the current was first detected.
2. Pull the wire out of the well until a regular a regular marker is seen.
3. Record the reading on the marker.
4. Measure the distance from the marker to place being held on the wire.
5. Add this measurement to the reading on the marker to find the depth of the water.

3.) Air line method

Another method for measuring well drawdown is the air line method. If one end of an open tube is put
under water, the water will rise in the tube until it equals the water level outside the tube. If enough air
pressure is supplied to the tube, water can be pushed back out of the tube. The amount of air pressure
needed to do this is expressed in feet of water.

If the length of tube is known, and the amount of pressure needed to push the water out is determined, the
depth of the water can be calculated. Simply subtract the pressure from the length of the tube.

For example, a tube is 100 ft long, and the pressure to clear the tube is 20 ft of water. To find the distance
from the top of the tube to the water level, subtract 20 ft from 100 ft. In this case the distance from the top
of the tube to the water level is 80 ft.

The equipment needed to measure the depth to water:
1. An air line of known length
2. A tee in the line with attachments for a pressure gauge and an air pump
3. A pump
4. A pressure gauge

Some wells are equipped with a small air line tube that extends from the top of the well to several feet
below the anticipated level of water during prolonged pumping. These lines are usually made of plastic
(PVC), copper or brass, and are 1/8” to ¼” in diameter. In order for the air line method to work correctly,
the tube must be airtight.

Finding the depth to water

Attach an air pump (such as a tire pump) and pressure gauge to the tee in the air line. Fill the line with air
until the gauge pressure is constant. Record the pressure. Subtract this from the length of the air line to find
the depth to water.

Example:
Air line = 150 feet
Gauge reading = 80 feet
Static water level = 70 feet

If the gauge is calibrated in pounds per square inch (psi), the readings can be converted to feet of water by
multiplying the number or pounds by 2.31.

Example:
45 psi x 2.31 = 104 feet

Length of air line tube unknown

For the air line method to be accurate the length of the air line tube must be known. This should be in the
original well plans. If the length of the air line tube is not known, it can be determined in the following
steps:
1. Pump air into the line until the pressure stabilizes.
2. Find the static level with an electric sounder or by some other method.
3. Add the static level and gauge reading to find the length of the air line tube.

Example:
Static level = 145 feet
Gauge reading = 16 psi
16 psi x 2.31 = 37 feet
145 ft + 37 ft = 182 feet (The length of the air line tube)

4.) Wetted tape method

An old-fashioned way to measure the depth to water is the wetted tape method. It is typically used for
depths up to 80 or 90 feet. In order to use this method the depth of water must already be known within a
few feet.

Materials needed to do the wetted tape method:
1. A steel measuring tape
2. A weight
3. Carpenter’s chalk

To measure the depth of water, attach the weight to the end of the measuring tape. Coat the lower 3 to 4 ft.
of the tape with carpenter’s chalk. Lower the tape into the well until the lower part of the tape is under the
water, then lower the tape a little more until the next foot marker is at the surface level.

Record the number of feet indicated. Remove the tape from the well and record the length of tape that was
under water. (The chalk, which was under water, will be wet or washed away.) The difference in these two
measurements is the depth to water.

Example:
Tape reading at surface level: 65 feet
Tape under water: 2 feet
Water level: 65 – 2 = 63 feet

Disadvantages of the wetted tape method

In some cases this method can be very inaccurate. If, for example, the water level is below the well screen,
water flowing into the well can wash the chalk off of the tape. Heavy condensation on the tape may also
wash away some chalk.

Another disadvantage of this method is that it is difficult to use for wells more than 100 ft deep. For deeper
wells, the long tape is difficult to handle.

CALCULATING WELL DRAWDOWN

Step 1: Find the static level

1. Manually turn off the pump.
2. When the well has fully recovered, measure the depth to water. This is the static level.

Well Recovery

The recovery time of a well is the time required for the aquifer to stabilize at the static water level once
pumping has stopped. The rate of the wells recovery determines how much time should pass before
measuring the static level. For some wells, 30 minutes is enough time. For others it takes a longer time.

Step 2: Find the pumping level

1. Turn on the pump. Allow it to pump until the well reaches a constant pumping rate.
2. Measure the depth to water at regular intervals until the water level stops dropping. This is the
pumping level.
3. Reset the pump control to the original pump setting.

Step 3: Calculate the drawdown

Calculate the drawdown by subtracting the static level from the pumping level.
Example:
Static level: 65 feet
Pumping level: 72 feet
Drawdown: 7 feet

RECORDING AND USING WELL DATA DRAWDOWN

Records

Make sure a file is setup for each well in the system. This file should contain such information as:
1. Operating records
2. Maintenance records
3. Initial design plans
4. Construction records
5. Well acceptance test
6. Pump data
7. Well abandonment records

The data you gather from drawdown measurements can give you valuable information about the condition
of your wells. To make the best use of drawdown data, however, you also need to know your well’s yields.
With this information, you can evaluate the efficiency and performance of your pumps, wells and aquifer.
If you need any technical assistance on this or other issues, pleases feel free to call 1-877-474-5755

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