Aquifer Drawdown Calculator

About the Aquifer Drawdown Calculator

When water is pumped from a well, the water level in the well drops below the static (non-pumping) level. This decline is called drawdown. The amount of drawdown depends on the pumping rate and the aquifer's specific capacity — the number of gallons per minute the well produces per foot of drawdown.

Excessive drawdown can expose the pump intake, reduce pump efficiency, and even damage the aquifer by compacting sediments. Monitoring and predicting drawdown helps you plan pumping schedules and avoid operating beyond safe limits.

This page turns pumping rate and specific capacity into an estimated pumping water level so you can check clearance to the pump and expected seasonal stress.

When This Page Helps

Drawdown matters when it changes pumping strategy or reveals that the well is being pushed too hard. This page makes that risk easier to see.

How to Use the Inputs

1. Enter the static water level (depth to water when not pumping) in feet.
2. Enter the pumping rate in GPM.
3. Enter the well's specific capacity in GPM per foot of drawdown.
4. Read the estimated drawdown and pumping water level.
5. Compare the pumping water level to your pump intake depth.

Example Calculation

Tips & Best Practices

• Specific capacity varies with pumping rate; it decreases at higher rates in unconfined aquifers.
• Keep at least 20–30 ft of submergence above the pump intake.
• Record drawdown at the start and end of each season to track aquifer trends.
• Interference from nearby wells increases drawdown; account for this in multi-well systems.
• Declining specific capacity over years suggests well deterioration or aquifer decline.
• A pump test at multiple rates gives the most accurate specific capacity data.

Understanding Drawdown Dynamics

When pumping begins, drawdown increases rapidly at first, then gradually stabilizes as the cone of depression reaches equilibrium with aquifer recharge. The time to stabilization depends on aquifer transmissivity and storativity. In highly transmissive sand-and-gravel aquifers, equilibrium may take hours; in low-permeability formations, it may take days.

Well Interference

If multiple wells pump from the same aquifer within 500–2,000 ft, their cones of depression overlap, increasing drawdown at each well. Account for interference by adding drawdowns from each pumping well or by reducing per-well pumping rates to stay within total aquifer capacity.

Long-Term Aquifer Decline

In many regions, aquifer levels decline 0.5–2 ft per year due to regional over-pumping. This raises the static water level measurement each year, increasing energy cost and drawdown. Tracking static levels over decades informs water policy and individual farm planning.

Frequently Asked Questions

• What is specific capacity?

  Specific capacity (SC) is the well's productivity ratio: GPM per foot of drawdown. A well yielding 800 GPM with 40 ft of drawdown has SC = 20 GPM/ft. Higher SC means a more productive well.

• Is the drawdown-GPM relationship linear?

  It is approximately linear for confined aquifers and low pumping rates. In unconfined aquifers at high rates, drawdown increases faster than linearly due to dewatering effects. This calculator uses the linear assumption; for precision, use a step-drawdown test.

• What causes excessive drawdown?

  Pumping faster than the aquifer recharges, well screen plugging (reducing specific capacity), or interference from neighboring wells. All three can be diagnosed with monitoring data.

• How deep should the pump be set?

  Set the pump at least 20–30 ft below the expected pumping water level at peak season rates. This accounts for seasonal aquifer decline and prevents air entrainment.

• What happens if the pump breaks suction?

  If drawdown drops to the pump intake, the pump takes in air, loses prime, and can burn out. Modern pumps have low-water shutoffs, but damage can still occur. Prevent this by ensuring adequate submergence.

• How do I measure static water level?

  Use an electric water level indicator (e-tape) lowered into the well before the pump starts. Measure from the wellhead to the water surface. Record this before each irrigation season.