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How to Calculate Drawdown in a Well
How to Calculate Drawdown in a Well. Water well drawdown occurs during groundwater withdrawal through the well screen. The natural water level in the aquifer depresses from the stress of the pumping in the aquifer and removal of the water. You can measure the water level in the well at different times to determine the drawdown that occurs during...
Water well drawdown occurs during groundwater withdrawal through the well screen. The natural water level in the aquifer depresses from the stress of the pumping in the aquifer and removal of the water. You can measure the water level in the well at different times to determine the drawdown that occurs during pumping activities. Some aquifers recharge more quickly than others; therefore, the drawdown in identical wells in different aquifers may not be the same. A few quick measurements will help you determine how much drawdown your well will exhibit over time.
Things You'll Need
Well with submersible pump
Electronic water level indicator
Notebook
Pen
Calculator
Insert the probe of the electronic water level into the well when the pump is at rest. Read the water level depth from the graduated tape when the electronic meter sounds a solid tone. This measurement is the static water level, which is the natural water level in the well.
Record the static water level in your notebook.
Turn the pump on and allow it to run.
Measure the water level in the well at period intervals of five to 10 minutes and record the data in your notebook. As the pump runs, record both the time and value of the measurement. Continue to collect water level data until minimal change in the water level measurements exists.
Subtract the measurements you collect as the pump is running from the static water level. The difference is the drawdown. For example, if the static water level is 1 foot below the top of the casing and the water level after one hour is 3 feet below the top of the casing, the drawdown at one hour of pumping is 2 feet. You can perform this calculation for any time after the start of pumping.
Tips & Warnings
You can install an electronic water level inside the well that constantly reads the water table elevation so that you do not have to measure the water level manually.
The biggest change in water level should be after you initially turn on the pump; therefore, you should collect more readings immediately after you turn on the pump and can space the measurements further apart as the pumping continues.
Measure the water level from the same spot on the casing each time to ensure accuracy.
If you plot the drawdown versus time on a graph, generally you will see a steep initial drawdown that levels over time.
Before you measure the static water level, ensure that the pump has not run for approximately 24 hours; this safeguard ensures you obtain the most accurate results.
The subtracted drawdown will be a negative number because you are recording measurements below the land surface; however, the negative number does not affect the results of the drawdown.
You can convert the data so that the drawdown is positive by converting the well data to elevations above mean sea level. This conversion will require a land survey or the assumption of a well elevation.
