Calculate well yield in GPM from a pumping test using volume pumped and elapsed time. Determine sustainable yield for irrigation planning.
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 calculator computes yield from volume and time, converts to common units, and estimates daily and seasonal capacity to help you plan irrigation operations within your well's limits. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process.
Over-pumping a well causes excessive drawdown, reduced pump efficiency, and potentially permanent aquifer damage. Knowing your well's sustainable yield prevents these problems and ensures reliable water through the season. Having a precise figure at your fingertips empowers better planning and more confident decisions. Manual calculations are error-prone and time-consuming; this tool delivers verified results in seconds so you can focus on strategy.
Yield (GPM) = Volume (gallons) / Time (minutes) Daily Capacity (gal) = GPM × 60 × Hours/Day Seasonal Capacity (ac-ft) = GPM × 60 × Hours/Day × Days / 325,851
Result: Yield = 667 GPM
Yield = 40,000 gallons / 60 minutes = 667 GPM. Running 20 hrs/day for 90 days, that's 667 × 60 × 20 × 90 = 72 million gallons = 221 ac-ft of seasonal capacity.
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.
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.
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.
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.
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.
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.
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.
Common causes: aquifer depletion (over-pumping regionally), well screen clogging (mineral or biological), sand bridging, and pump degradation. Monitoring drawdown trends identifies the cause.
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.