Calculators
Irrigation Water Requirement Calculator
Estimate crop water use from reference ET and crop coefficient, net and gross irrigation after rain and efficiency, total gallons and pump run time.
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The tool
Enter your numbers below. Results update as you type, and nothing leaves your browser.
Everything you enter stays on your own machine. Nothing is sent to us or to anyone else, which you can confirm in your browser network tab.
What people use it for
- Irrigation scheduling
- Pump and well sizing
- Water allocation planning
- Energy cost estimates
About this tool
What the Irrigation Water Requirement Calculator does
Crop water use is reference evapotranspiration (ET₀) times the crop coefficient (Kc) for its growth stage. Effective rainfall reduces what irrigation must supply, and system efficiency means more water must be pumped than the crop uses. One acre-inch is about 27,154 gallons.
Formula
- ETc = ET₀ × Kc × days
- Net = ETc − effective rain
- Gross = net ÷ efficiency
- Gallons = gross inches × acres × 27,154
Worked example
Using the starting values in the calculator above:
| Input | Value |
|---|---|
| Reference ET (ET₀) | 0.25 in/day |
| Crop coefficient (Kc) | 1.1 |
| Period | 7 days |
| Effective rainfall in the period | 0.4 in |
| Irrigation efficiency | 80% |
| Irrigated acres | 120 |
| Pump flow rate | 900 gpm |
| Result | Value |
|---|---|
| Gross irrigation to apply (in) | 1.91 |
| Pump run time | 115 hours |
| Crop water use ETc (in) | 1.93 |
| Net irrigation need (in) | 1.53 |
| Total water (gallons) | 6,211,478 |
| Total water (acre-feet) | 19.06 |
How to read the result
Kc changes through the season, low at emergence and highest at full canopy; update it by growth stage.
Soil moisture sensors or an ET-based checkbook help avoid over- and under-watering.
Estimates for planning. Use soil tests, local extension guidance and your agronomist’s recommendations for field decisions.
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Questions
Questions about this tool
The things clients actually want to know — costs, timelines, ownership and what happens after launch.
Reference ET times the crop coefficient for its growth stage, less effective rainfall.
About 27,154 gallons.
Local weather networks and extension services publish daily reference ET.
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