Efficient irrigation is key to successful vegetable farming, especially in North Carolina, where weather conditions can vary significantly. A critical concept for managing water effectively is evapotranspiration (ET). In this guide, we’ll explain what evapotranspiration is, its different types, its role in agriculture, and how to use it to improve irrigation practices. We’ll also discuss a key factor called the crop coefficient (Kc) and why it’s vital for your farm.

Overhead irrigation in potatoes

Photo by Alexis Suero

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Overhead irrigation in potatoes

Photo by Alexis Suero

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Evapotranspiration is the combined process of water loss from the soil (evaporation) and water used by plants (transpiration).

• Evaporation is when water from the soil or plant surfaces turns into vapor and rises into the air.
• Transpiration happens when plants pull water from their roots and release it as vapor through their leaves.

Together, these processes determine how much water a crop needs to stay healthy.

There are two main types of evapotranspiration farmers should know:

1. Potential Evapotranspiration (ET₀):
   • This is the amount of water that would be lost if a well-watered plant grew under ideal conditions with no limits on water.
   • ET₀ is mainly influenced by weather factors like temperature, sunlight, wind, and humidity.
2. Actual Evapotranspiration (ETa):
   • This is the real amount of water lost by the soil and plant. It depends on how much water is available in the soil and how well the plant is watered.

Evapotranspiration is essential because it tells you how much water your vegetables need to grow. If you don’t replace the water lost through ET, your crops can become stressed, reducing yield and quality.

Farmers use ET information to schedule irrigation effectively. By knowing how much water your vegetables are losing, you can:

• Avoid overwatering, which wastes water and causes root problems.
• Prevent underwatering, which leads to plant stress and lower yields.
• Save money on water and energy costs.

The crop coefficient (Kc) is a simple number that adjusts ET₀ to fit a specific crop. Different crops have different water needs, and their water requirements also change as they grow.

• How it works:
  • Multiply ET₀ by the Kc to calculate your crop’s specific water needs.
  • For example, if ET₀ is 0.2 inches/day and the Kc for tomatoes is 0.8, the crop ET is 0.16 inches/day.
• Why it matters:
  • Early-stage crops with small leaves (like young lettuce) have a low Kc because they use less water.
  • Mature crops with lots of leaves (like fruiting tomatoes) have a high Kc because they transpire more.

Using Kc ensures you’re not under- or overestimating your crop’s water needs, helping you irrigate more accurately.

ETc refers to the crop evapotranspiration, which is the actual water demand of a specific crop under specific field conditions. ETc is an estimate, while ETa is the real, measured amount of evapotranspiration (often only used in research). ETc is calculated by multiplying the potential evapotranspiration (ET₀) by the crop coefficient (Kc):

ETc = ETo × Kc

• ET₀ (Potential ET): The baseline water loss under ideal conditions.
• Kc (Crop Coefficient): Adjusts ET₀ to reflect the unique water needs of a specific crop at a particular growth stage.

1. Monitor weather and ET₀: Use local weather stations or apps that calculate ET₀ for your area.
2. Know your crop’s Kc: Get the Kc values for your vegetables at different growth stages (these are often available through extension services or online).
3. Calculate water needs: Multiply ET₀ by the Kc to find out how much water your crops need per day.
4. Irrigate accordingly: Use this information to schedule irrigation. Consider your soil type and system efficiency to ensure water reaches the roots.

Knowing about evapotranspiration and Kc helps you become more efficient with water. In a state like North Carolina, where both droughts and wet spells can occur, using water wisely is critical for:

• Sustaining high crop yields.
• Protecting the environment by conserving water.
• Reducing farming costs.

Understanding ET and Kc is not just for scientists—it’s a practical tool that every vegetable farmer can use to improve their operation.

• Evapotranspiration (ET) measures water loss from soil and plants and helps determine irrigation needs.
• ET₀ is the potential water loss, while actual ET (ETa) depends on real conditions.
• The crop coefficient (Kc) adjusts ET₀ for specific crops and growth stages.
• Farmers who use ET and Kc can improve irrigation efficiency, save water, and boost crop performance.

For help getting local ET and Kc values, contact your North Carolina Cooperative Extension center or visit their website for resources.