What Is Reference Evapotranspiration (ET0)?
Reference evapotranspiration (ET0) is a key concept in precision agriculture and efficient water management. It is defined as the amount of water lost from a reference surface (typically a 12 cm tall grass crop, actively growing, and with adequate soil moisture) due to two combined processes: direct evaporation from the soil and transpiration from plants.
In simple terms, ET0 represents the atmospheric demand for water. It depends neither on the specific crop nor on soil conditions, but solely on climatic factors: solar radiation, temperature, relative humidity, and wind speed.
Why Is ET0 Important for Agriculture?
Knowing ET0 is essential for optimizing irrigation and avoiding two common problems: over-irrigation (which wastes water, leaches nutrients, and can cause root diseases) and under-irrigation (which stresses the crop, reduces yield, and affects harvest quality).
By combining ET0 with the crop coefficient (Kc), we obtain the crop evapotranspiration (ETc), which is the actual amount of water a crop needs at a specific growth stage. The formula is: ETc = ET0 × Kc.
Practical Example
Imagine a corn field in central Chile. During a week of high temperatures and moderate winds, the calculated ET0 is 35 mm. If the Kc for corn at that stage is 1.2, then the ETc is 42 mm. This means the crop needs 42 mm of water (equivalent to 420 m³/ha) to avoid water stress.
Without this calculation, the farmer might irrigate by habit, applying only 30 mm (insufficient) or 50 mm (excess). Both scenarios have negative consequences for yield and costs.
Factors Affecting ET0
- Solar radiation: The primary factor. More radiation means more energy available to evaporate water.
- Air temperature: Indirect influence; warm air can hold more water vapor, speeding up evaporation.
- Relative humidity: The drier the air, the higher the evapotranspiration rate.
- Wind speed: Wind renews the humid air near leaves, maintaining the vapor gradient and promoting transpiration.
How Is ET0 Measured or Calculated?
There are two main methods:
1. Direct Method: Lysimeters
These are buried tanks where grass is grown, and water loss is measured by continuous weighing. They are accurate but expensive and impractical for daily field use.
2. Indirect Methods: Empirical Equations
The most widely used and FAO-recommended method is the Penman-Monteith equation. It requires data on temperature, humidity, radiation, and wind. Fortunately, today there are automatic weather stations and apps (like Contingencias) that calculate ET0 automatically from local data.
Practical Tips for Using ET0 in Irrigation
- Install a weather station on your farm or use data from the nearest station. The accuracy of the calculation depends on data quality.
- Update crop coefficients (Kc) according to the phenological stage. Don't use the same Kc for seedling and flowering stages.
- Monitor ET0 daily during the irrigation season. Sudden weather changes (heat wave, strong wind) can double water demand in just a few days.
- Combine ET0 with soil moisture measurement (tensiometers, capacitive sensors) to fine-tune irrigation with millimeter precision.
- Use ET0 to plan drip irrigation: know how long you need to run the system to replenish consumed water.
Benefits of Managing Irrigation with ET0
- Water savings: Up to 30% less consumption without affecting yield.
- Greater energy efficiency: Fewer pumping hours, lower electricity or fuel costs.
- Healthier crops: Avoiding excess moisture reduces fungi and root diseases.
- Better harvest quality: More uniform fruits, with higher sugar content and better texture.
- Sustainability: You contribute to water resource conservation in your region.
Conclusion
Reference evapotranspiration (ET0) is not just a technical number; it's a practical tool that transforms how we irrigate. In a context of climate change and growing water scarcity in South America, measuring and applying ET0 marks the difference between a farmer who reacts and one who plans.
From Contingencias, we recommend integrating ET0 into your irrigation management routine. With accurate and up-to-date data, you can make informed decisions that benefit your wallet, your crops, and the environment.