Optimizing Irrigation: Using Crop Evapotranspiration (ETc) to Precisely Manage Soil Water Content

We've previously explored how to calculate the daily water consumption of plants, known as evapotranspiration (ET) (See “Understanding Evapotranspiration: What It Is and How We Measure It”). Now, the crucial question arises: how do we translate this information into effective irrigation practices?

This article delves into the "soil water budget" model, a powerful tool for determining soil water deficit (or content), enabling us to precisely gauge irrigation needs and maintain optimal soil moisture levels.

Understanding the Soil Water Budget

The soil water budget is a simple yet effective accounting system that tracks all water inputs and outputs in the soil. By carefully monitoring these factors, we can accurately estimate the soil's current water content and determine how much water is needed to replenish it.

Key Components of the Soil Water Budget

• Inputs: 

  • Irrigation

  • Rainfall

• Outputs: 

  • Crop Evapotranspiration (ETc)

  • Drainage

  • Runoff

For a deeper dive into soil water budgets, refer to our post "Mastering the Soil Water Balance: Your Irrigation Blueprint."

Integrating ETc and Soil Moisture Monitoring

To effectively utilize the soil water budget, a reliable starting point for stored soil water content is essential, and periodic validation is crucial. This is where the synergy between ETc calculations and soil moisture sensors becomes invaluable.

• ETc's Role: ETc provides a quantitative measure of the water consumed by the crop, indicating the amount of water that needs to be replenished through irrigation.

  
  

• Soil Moisture Sensors' Role: Soil moisture sensors provide real-time data on the soil's water content, allowing us to verify if irrigation has effectively restored the soil to field capacity.

By combining these two methods, we can ensure that irrigation applications precisely match the crop's water requirements, preventing both over- and under-watering.

Calculating Irrigation Run Time

Knowing the ETc and the irrigation application rate allows us to calculate the precise irrigation run time needed to replace the water lost. This ensures efficient water use and optimal plant health.

Challenges in Indoor and Containerized Environments

Accurately calculating ETc for indoor and containerized plants poses unique challenges. Unlike field crops, indoor plants and those grown in pots often have complex microclimates and soil/substrate conditions.

Crop Coefficients (Kc)

• ETc is typically calculated by multiplying the reference evapotranspiration (ETo) by a crop coefficient (Kc), which accounts for the specific water use characteristics of the plant species, its growth stage, and the soil/substrate evaporation.

  
  

• Ideally, Kc should encompass all relevant factors, including plant species, growth stage, and management practices.

  
  

• However, developing accurate Kc values for containerized and indoor plants is complex due to the numerous parameters involved.

  
  

• Kc is well developed for field crops.

An Alternative Method for Estimating ETc in Pots

For plants grown in pots, an alternative method for estimating ETc involves:

• Weighing the plants periodically to measure the change in water content.

  
  

• Dividing the change in water content by the surface area of the container to obtain the ETc value.

  
  

This method provides a practical approach for estimating water use in these challenging environments.

Conclusion

By effectively integrating ETc calculations with soil moisture monitoring, we can optimize irrigation practices and ensure efficient water use. The soil water budget model provides a framework for managing soil water content, while ETc and soil moisture sensors offer the data needed for precise irrigation decisions.

While calculating ETc for indoor and containerized plants can be complex, alternative methods, such as weighing plants, can provide valuable insights.