Well Yield Calculator

About the Well Yield Calculator

Well yield is the rate at which a well can sustainably produce water, typically measured in gallons per minute (GPM). It is determined by a pumping test where a known volume of water is extracted over a measured time period, and drawdown is monitored to ensure stability.

The basic yield calculation is straightforward: volume divided by time. However, interpreting whether that yield is sustainable requires observing drawdown stabilization. If the water level continues dropping throughout the test, the pumping rate may exceed the aquifer's ability to recharge.

This page turns a pumping test into a working yield estimate and daily capacity number so irrigation plans stay inside the well's likely limit.

When This Page Helps

Yield is only useful when it tells you whether the well can support the acres and set times being planned. This page answers that more directly.

How to Use the Inputs

1. Enter the volume of water pumped during the test.
2. Select the volume unit (gallons, cubic feet, or liters).
3. Enter the duration of the pumping test in minutes.
4. Read the yield in GPM.
5. Compare yield to your irrigation system's GPM requirement.

Example Calculation

Tips & Best Practices

• A well test should run at least 4–8 hours (24+ hours preferred) to assess sustainability.
• Monitor drawdown during the test; if it stabilizes, the rate is sustainable.
• Seasonal yield may be less than test yield if the aquifer's recharge rate is low.
• Well yield often declines over years as wells age or aquifer levels drop.
• Sand or turbidity during pumping indicates the well needs development or redevelopment.
• Step-drawdown tests at multiple rates give a complete picture of well performance.

Pumping Test Procedures

A standard constant-rate pumping test sets the pump at the design flow rate and measures drawdown over 24+ hours. Observation wells at known distances can provide aquifer transmissivity and storativity values used for long-term yield predictions.

Step-Drawdown Tests

A step-drawdown test pumps at 3–5 increasing rates (e.g., 200, 400, 600, 800 GPM), each for 1–2 hours. Plotting drawdown vs flow rate reveals well efficiency and the maximum practical yield where additional pumping produces diminishing returns.

Seasonal vs Instantaneous Yield

A well may yield 800 GPM during a spring test but only 600 GPM by late August due to declining aquifer levels. Plan for seasonal decline by either building in margin or monitoring drawdown monthly and adjusting irrigation strategy.

Frequently Asked Questions

• How long should a pumping test last?

  For irrigation wells, a 24-hour constant-rate test is ideal. Shorter tests (4–8 hours) can be useful for screening, but they may miss long-term aquifer effects that reduce sustainable yield.

• What is a good yield for irrigation?

  Needs depend on acreage and crop. A quarter-section pivot needs 700–900 GPM. A 50-acre drip system might need only 100–200 GPM. Compare yield to your system's required GPM.

• How often should I test my well?

  Annually, typically at the start of the irrigation season. A simple flow meter reading at operating speed gives year-over-year comparison. Formal pump tests every 3–5 years.

• Can I increase well yield?

  Well redevelopment (surging, jetting, chemical treatment) can restore yield lost to encrustation or plugging. Drilling deeper or adding perforations may access additional water-bearing zones. Hydrofracturing can help in hard-rock aquifers.

• What causes yield to decline?

  Common causes: aquifer depletion (over-pumping regionally), well screen clogging (mineral or biological), sand bridging, and pump degradation. Monitoring drawdown trends identifies the cause.

• What is specific capacity?

  Specific capacity = GPM / feet of drawdown. It measures well productivity per unit of drawdown and helps compare wells and track performance over time. A declining specific capacity signals well or aquifer problems.