Can my well keep up with my crop?
A well only has to do one job on the worst day of the season: supply water at least as fast as the crop is using it. That comparison is between two flow rates, in gallons per minute (GPM) or liters per second, not between two volumes. A well that comfortably fills a stock tank overnight can still fall short of what 200 acres of corn needs on a 100°F afternoon, because the comparison that matters is rate, not total amount pumped.
The flow a field needs, often called gross system capacity, depends on four things: how fast the crop is using water at its peak, how large the field is, how efficiently the irrigation system delivers water to the root zone, and how many hours a day the well can run. University of Nebraska-Lincoln Extension frames this as the difference between net capacity (what the crop root zone needs) and gross capacity (what must be pumped, after accounting for application losses).
| Soil texture | Region 1 (gpm/ac) | Region 2 (gpm/ac) |
|---|---|---|
| Loam, silt loam over silt loam subsoil | 3.85 | 4.62 |
| Silty clay loam, clay loam | 4.24 | 5.07 |
| Silty clay, clay | 4.36–4.48 | 5.13–5.19 |
| Sandy loam, loamy sand | 4.83 | 5.42 |
These net figures still need dividing by your system's application efficiency to get the gross flow a pump must deliver, which is exactly what the calculator above does with the values you enter.
Check local water rights first. Many states and groundwater districts cap how much a well can pump, regardless of what a crop could use. A calculated requirement above your permitted rate means you need more acres' worth of wells, not a bigger permit.
How this well capacity calculator works
The calculator first works out the flow needed per acre from your peak water use, efficiency and pumping hours. It then multiplies that by your acreage (if you entered one) to get the flow your field needs, and divides your well yield by the same per-acre figure to get the maximum acres your well can supply.
1. Flow needed per acre (gross system capacity)
US units
GPM per acre = (453 × Peak ETc) ÷ (Pumping hours × Efficiency)
453 converts acre-inches per hour to gallons per minute: one acre-inch is 27,154 gallons, and 27,154 ÷ 60 minutes ≈ 453. Efficiency is entered as a percentage and used as a decimal (80% → 0.80). This is the same gross-capacity formula University of Nebraska-Lincoln Extension uses for center pivot design.
Metric units
L/s per hectare = (2.778 × Peak ETc) ÷ (Pumping hours × Efficiency)
2.778 converts hectare-millimeters per hour to liters per second: one hectare-millimeter is 10,000 liters, and 10,000 ÷ 3,600 seconds ≈ 2.778.
2. Flow your field needs
Required flow
Required flow = Flow per acre × Acres
Only calculated if you entered an acreage. Compared directly against the well yield you entered.
3. Maximum acres your well can supply
Maximum acres
Maximum acres = Well yield ÷ Flow per acre
The same relationship rearranged: instead of asking whether a given acreage is covered, this asks how large a field your measured well yield can cover at these settings.
Results are rounded to a sensible level, such as the nearest 10 GPM for a large system, because peak water use and efficiency are themselves estimates. For a flow figure without the acreage question, use the GPM per acre calculator. To convert a target depth straight to acre-inches or gallons, use the acre-inches calculator.
When this calculator isn't the right tool
It's built for comparing a well's flow against crop water use and acreage. Use a different tool for:
- Measuring your well's actual yield, which this calculator takes as a given input. See the bucket test calculator.
- Finding your crop's peak water use rate in the first place. See the crop evapotranspiration calculator.
- Sizing the pipe or mainline to carry your well's output without excess friction loss. See the irrigation pipe size calculator.
- Designing a specific center pivot's nozzle package and pressure, which needs more than a flow comparison. See the center pivot calculator.
- Household or livestock well sizing, which follows fixture counts and herd size rather than acres and crop water use.
For other agricultural water questions, browse the full set of agriculture calculators.
Well capacity calculation example
A grower wants to run a 130-acre center pivot. Peak crop water use for the field is 0.32 inches per day, the pivot's application efficiency is 85%, and the well can realistically pump 22 hours a day. Their well test reported 1,200 gpm.
- Flow per acre: (453 × 0.32) ÷ (22 × 0.85) = 144.96 ÷ 18.7 ≈ 7.75 gpm/acre.
- Flow needed for 130 acres: 7.75 × 130 = 1,007.7 gpm, shown as about 1,010 gpm.
- Compare to the well: 1,200 gpm available vs. about 1,010 gpm needed — the well meets the demand, with roughly 190 gpm to spare.
- Maximum acres for this well: 1,200 ÷ 7.75 ≈ 154.8, shown as about 155 acres — more than the 130 acres planned, consistent with the surplus above.
The same calculation in metric
A 20-hectare block with peak water use of 8 mm/day, an 80% efficient system, 20 pumping hours a day, and a well tested at 25 L/s:
- Flow per hectare: (2.778 × 8) ÷ (20 × 0.80) = 22.224 ÷ 16 ≈ 1.389 L/s per hectare.
- Flow needed for 20 ha: 1.389 × 20 = 27.78 L/s, shown as about 28 L/s.
- Compare to the well: 25 L/s available vs. about 28 L/s needed — the well falls short by roughly 2.8 L/s.
- Maximum area for this well: 25 ÷ 1.389 ≈ 18.0, shown as about 18 ha — less than the 20 ha planned, matching the shortfall above.
The two examples show why the comparison matters more than either number alone: the same well that comfortably covers one field can fall short on a slightly larger one, or with a less efficient system.
Measuring your inputs and applying the result
Well yield
Use a real pump test, a driller's log, or a flow meter reading rather than a guess. University of Minnesota Extension describes a simple field method: fill a container of known volume and time it, or record the volume delivered over a fixed number of minutes with a flow meter, then divide volume by time. Test more than once and at different times of year if you can, since yield can shift with the season.
Typical efficiency by system
Application efficiency is the share of pumped water that actually reaches the crop root zone rather than being lost to runoff, deep percolation, drift or evaporation. The ranges below, for well-designed and well-managed systems, come from a University of Wyoming Extension and University of Nebraska-Lincoln Extension irrigation series. Pick the low end of a range for an older or poorly maintained system.
| System | Typical efficiency |
|---|---|
| Subsurface drip | above 95% |
| Surface drip | 85–95% |
| Micro-spray, bubbler | 80–90% |
| Center pivot, linear move | 75–85% |
| Solid-set, hand-move, side roll | 65–85% |
| Traveling gun | 65–75% |
| Furrow, conventional | 45–65% |
If you've measured your own system's efficiency, for example with a catch-can test, use that figure instead of a table value.
Applying the result
If the calculator shows your well meets a planned acreage, keep in mind that's a peak-day comparison: it doesn't leave margin for a well that's already declining, a pump nearing the end of its service life, or expanding acreage later. Many designers build in a buffer above the strict minimum for exactly that reason. If the well falls short, the practical options are reducing the acreage served, raising application efficiency (for example, moving from a traveling gun to a pivot), adding pumping hours where the water right and power supply allow, or developing additional well capacity.
Common mistakes
- Using average or seasonal water use instead of peak. A well sized to the season's average will fall behind during the hottest stretch.
- Assuming 24 hours of pumping. Maintenance, moves and power scheduling almost always cut into that.
- Skipping the efficiency step. Comparing well yield directly to the crop's net water need, without dividing by efficiency, understates the flow actually required from the pump.
- Treating one well test as permanent. Yield can decline as an aquifer draws down over a season or over years.
Factors that change the result
The calculator only knows what you enter. These are the main reasons the real-world answer can differ from the calculated one.
| Factor | Effect | What to adjust |
|---|---|---|
| Crop growth stage | Peak water use rises through the season and falls after canopy closure ends or near harvest. | Use the single highest daily rate the crop reaches, not an average. |
| Heat waves | Short spells can push daily crop water use above the seasonal peak used for design. | Consider a peak from a hot year, not just a typical one, if your area is prone to heat spells. |
| System age and maintenance | Worn nozzles, leaks and uneven pressure lower real efficiency below the table values. | Use the low end of the efficiency range, or measure your own with a catch-can test. |
| Aquifer drawdown | Well yield can fall during a dry season or over years of heavy regional pumping. | Re-test yield periodically; use a conservative figure if yield has been declining. |
| Power or scheduling limits | Off-peak electricity rates or shared well time reduce available pumping hours. | Enter the hours you can actually count on, not the full 24. |
| Multiple wells or storage | Combined wells or a buffering pond can supply more than one well's steady-state yield. | Add well yields together first; this calculator doesn't model storage. |
Frequently asked questions
How do I find my well's flow rate (GPM)?
The most reliable source is a pump or well test done when the well was drilled or serviced, often recorded on the driller's log as gallons per minute. If you don't have that record, time how long it takes your pump to fill a container of a known size, such as a 55-gallon drum, then divide the volume by the time. A flow meter on the discharge line gives a continuous reading and is the most accurate option if your system has one.
How many acres can my well irrigate?
It depends on your well's flow rate, your crop's peak daily water use, your system's application efficiency and how many hours a day you can pump. A higher-yielding well, a lower peak water use rate, a more efficient system or more daily pumping hours all raise the acres a well can cover. There's no single acres-per-well number that applies everywhere, which is why this calculator asks for those four values directly.
What GPM per acre do I need?
It ranges widely by crop, climate and system efficiency, typically from about 3 to 10 gallons per minute per acre. University of Nebraska-Lincoln Extension lists minimum net capacities from about 3.85 to 6.60 gpm per acre depending on soil and region, before dividing by system efficiency. University of Minnesota Extension notes that the sandy loam soils common in Minnesota often need around 6 gpm per acre for full-season irrigation. Calculate your own figure instead of using a generic number, since peak crop water use and efficiency vary by location.
Why does the calculator ask for pumping hours per day instead of assuming 24?
Few operations pump a well continuously. Time is lost to system moves, maintenance, power interruptions or off-peak electricity schedules. University of Nebraska-Lincoln Extension's own worked example for center pivot design capacity uses 22 hours per day rather than 24, to leave room for that downtime. Enter the hours you can realistically count on, not the full 24, or the calculator will overstate how many acres your well can cover.
Does well yield stay the same all season?
Not necessarily. A well's yield can drop as the water table or aquifer level falls during a dry season, as nearby wells draw from the same aquifer, or as a pump or screen ages. A single test result is a snapshot, not a guarantee. If your area has experienced declining water levels, retest periodically and consider using a conservative, lower yield figure in this calculator rather than a best-case number.
How is peak crop water use different from seasonal water use?
Peak crop water use, sometimes written as peak ET or peak ETc, is the highest daily water use rate the crop reaches, usually during the hottest, sunniest, most actively growing part of the season. Seasonal water use is the total over the whole growing season and is much less useful for sizing a well, because a well only has to keep up with the worst single stretch, not the average. Use a peak-day or peak-week figure here, not a seasonal total.
See the crop water requirement calculator for seasonal totals instead.
Where do I find my crop's peak water use rate?
A state or regional agricultural weather network, a university extension crop water use guide, or a local NRCS office can usually supply a peak ETc figure for your crop and area. It's calculated as reference evapotranspiration multiplied by a crop coefficient that changes through the growing season.
Does this calculator account for multiple wells or a storage pond?
No. It treats the well yield you enter as the full available supply, pumped for the hours you specify. If you draw from more than one well, add their yields together first. If you fill a storage pond or reservoir that lets you draw faster than the well recharges during short peak periods, this calculator will understate your effective capacity, since it doesn't model storage.
Can I use this for a household or livestock well instead of a crop?
Not directly. This calculator is built around crop peak water use and irrigated acreage. Household and livestock water demand follow a different pattern, based on fixtures, herd size and daily use schedules rather than acres and evapotranspiration, so a different sizing approach applies.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formula, unit conversions and both worked examples verified by hand against the sources below.
- The US gross-capacity formula was checked against University of Nebraska-Lincoln Extension's own worked example.
- Efficiency ranges follow the University of Wyoming / University of Nebraska-Lincoln Extension irrigation efficiency series.
Sources
- Kranz, W.L., Martin, D.L., Irmak, S., van Donk, S.J., Yonts, C.D. University of Nebraska-Lincoln Extension. G1851: Minimum Center Pivot Design Capacities in Nebraska.
- Rudnick, D.R., Irmak, S. (edited by M. Geiger). University of Wyoming Extension, in partnership with University of Nebraska-Lincoln. System Performance and Efficiency (B-1264 | E3A-IE.4).
- Hoidal, N., Sharma, V. University of Minnesota Extension. Digging or Expanding a Well.
- USDA Natural Resources Conservation Service. National Engineering Handbook, Part 652, Irrigation Guide. [VERIFY URL]