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Crop Water Requirement Calculator
Estimate crop water requirement from reference evapotranspiration and crop coefficient, and convert it into an irrigation depth and volume.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Math & Science
Crop Water Requirement Calculator
Frontend preview — no upload or external service.
Water requirement
ET₀ 5.2 mm/day with a mid-season maize Kc of 1.20 gives 6.24 mm/day. Over 1.2 ha at 75% efficiency that is about 100 m³ of water per day.
How the Crop Water Requirement Calculator works
- Enter reference evapotranspiration for your location, from a weather station or a regional service.
- Select the crop and its growth stage, which sets the crop coefficient.
- Divide by irrigation efficiency to get the volume actually applied, since not all of it reaches the root zone.
The method
Crop water requirement is reference evapotranspiration scaled by a crop coefficient that varies with growth stage, then divided by system efficiency.
ETc = ET₀ x Kc; applied volume = ETc x area / efficiency
5.2 mm/day x 1.20 = 6.24 mm/day. Over 12,000 m² that is 74.9 m³, and at 75% efficiency about 100 m³ must be delivered.
FAQ
Where do I get ET₀?
From a local meteorological service, an agricultural extension office, or an on-farm weather station. Estimating it from temperature alone is possible but noticeably less accurate.
Why does the crop coefficient change during the season?
Because a seedling covering little ground transpires far less than a full canopy. Kc typically rises from about 0.3 at establishment to well over 1.0 at mid-season and falls again at maturity.
Does rainfall count toward this?
Effective rainfall does — the portion that infiltrates and stays in the root zone. Subtract it from the requirement rather than the total rainfall figure.
How we compare
| Feature | Online Tool Store | A graphing calculator | A stats package |
|---|---|---|---|
| Stage-specific crop coefficients | ✓ | ✗ | ✓ |
| Includes system efficiency | ✓ | Manual | ✓ |
| No subscription | ✓ | ✓ | ✗ |
| Local weather data | ✗ | ✗ | ✓ |
Crop Water Requirement Calculator follows the standard ET₀ × Kc method and depends entirely on a local ET₀ figure being accurate.