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Irrigation Scheduling with ETo

Also known as: ETo-based irrigation, Water balance scheduling, Evapotranspiration scheduling

OriginatorRichard G. Allen, Luis S. Pereira, FAOYear1998Sources2Related methods13

Irrigation Scheduling with ETo is a water balance pipeline for determining when and how much to irrigate based on reference evapotranspiration (ETo), soil properties, and crop water demand. Standardized by the FAO in the Penman-Monteith equation and widely adopted globally, this method enables efficient water use in irrigated agriculture.

Key highlights

  • Standardized methodology (FAO) ensures consistency across regions and facilitates technology transfer.
  • Simple to implement with minimal data (temperature, rainfall) using approximate ETo methods.
  • Direct link between weather, crop demand, and irrigation action enables real-time decision support.
  • Water-saving compared to fixed-schedule or soil-sensor-only methods when properly tuned to soil and crop.

Intuition

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How it works

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When to use it

Use this method for annual crops with predictable water demand in warm climates with reliable weather data. It is most effective in regions with distinct growing seasons and good weather station coverage. Ideal for pressurized systems (drip, sprinkler) where irrigation timing is flexible. Less suitable for rainfed agriculture, perennial crops with changing canopy cover, or regions with erratic rainfall patterns.

Strengths & limitations

Strengths
  • Standardized methodology (FAO) ensures consistency across regions and facilitates technology transfer.
  • Simple to implement with minimal data (temperature, rainfall) using approximate ETo methods.
  • Direct link between weather, crop demand, and irrigation action enables real-time decision support.
  • Water-saving compared to fixed-schedule or soil-sensor-only methods when properly tuned to soil and crop.
Limitations
  • Assumes homogeneous soil and uniform field conditions; does not account for spatial variability in water availability.
  • Crop coefficient (Kc) values are generic; local calibration needed for high precision.
  • Requires daily weather data, which may be unavailable in remote or poorly instrumented areas.
  • Does not optimize for crop yield or quality; scheduled irrigation may not align with phenologically sensitive stages.

Common pitfalls

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Applications

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Frequently asked

What is the difference between ETo and ETc?

ETo is reference evapotranspiration from a well-watered short grass surface under optimal conditions; it depends only on weather. ETc is actual crop evapotranspiration, calculated as ETc = ETo × Kc, where Kc is a crop-specific coefficient (0.3–1.2) that accounts for crop height, ground cover, and growth stage.

Which ETo equation should I use if I only have temperature data?

The Hargreaves equation (1985) requires only maximum/minimum temperature and latitude, making it suitable for data-scarce regions. For better accuracy, use Penman-Monteith if relative humidity and wind speed are available. Simpler methods like Blaney-Criddle are less reliable.

How often should I irrigate?

Frequency depends on soil water-holding capacity, root zone depth, and allowable depletion fraction (MAD). Sandy soils may require daily or every-other-day irrigation; clay soils can go 1–2 weeks. Use the formula: Days to irrigation = (FC − WP) × MAD × depth / (ETc). For example, a 1-meter rooted crop in loam with 25% MAD might be irrigated every 4–7 days.

Can I use soil moisture sensors instead of this method?

Soil sensors measure actual moisture and can trigger irrigation automatically, bypassing ETo calculations. However, they require calibration, placement at representative depths, and account for only point measurements. Combined use—ETo for scheduling frequency, sensors for spatial variability—often gives best results.

Sources

  1. 1.
    Allen, R. G., Pereira, L. S., Raes, D., Smith, M., & Higgins, R. B. (1998). Crop evapotranspiration: Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56, Rome.
    https://www.fao.org/3/x0490e/x0490e00.htm
  2. 2.
    Hargreaves, G. H., & Samani, Z. A. (1985). Reference crop evapotranspiration from temperature. Applied engineering in agriculture, 1(2), 96-99.
    DOI 10.13031/2013.26773

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ScholarGate. (2026, June 3). Irrigation Scheduling with ETo. ScholarGate. https://scholargate.app/agronomy/irrigation-scheduling-etref